Physics: Ideas to Implementation PP

This module is about:Understanding the history, and nature of Physics.Its applications & uses.Its implications for society & the environment. The current research & developments in Physics.

When an electric charge moves into a magnetic field it experiences a force.

F = qvB Sin Өq = charge in coulombsv = velocity in m/sB = Magnetic field intensity in TeslaF = Force on particle in Newtons The 3 vectors F, B, v are at right angles to each other. The

direction of the force is determined using the right hand palm rule, (for a positive charge). Where the: - fingers point = in the direction of the

………………..................... field, - thumb points = in the direction of the

………………..................... of particle. - palm points = in the direction of the

………………........................ on particle.Activity: For the particle in the diagram work out the direction of the force on it. (Note: For a negative particle use the left hand.)Solution: Using left hand the force is out of the board.

Force Produced on Charged Particles by Magnetic Fields

The magnetic force will cause the direction of the velocity of the electric charge to change to a new

direction as it moves. The result will be a continually changing path that forms a ……….................… motion.

i.e. this magnetic force equals the centripetal force. For a negative particle use your left hand.

For a positive particle use your right hand.

Draw the magnetic field into the board and draw in the path a negative particle would make. Include vector arrows for Fc and v.

When qvB mv2/r, then a perfect …………….. occurs When qvB mv2/r, there isn't enough magnetic force, and the

charged particle ………………………... When qvB mv2/r, there's too much magnetic force, and the

charged particle …………………………...

Activity: For each diagram below write in the equation above it the sign that matches the orbit the charged particle is undergoing.

When qvB = mv2/r, then a perfect orbit occurs When qvB < mv2/r, there isn't enough magnetic force, and the

charged particle flies out of orbit. When qvB > mv2/r, there's too much magnetic force, and the

charged particle spirals inward.

Activity: For each diagram below write in the equation above it the sign that matches the orbit the charged particle is undergoing.

Problem 1:A proton travelling at 5.0 x 104 m s-1 enters a magnetic field of

strength 1.0 Tesla at 90°. Determine the magnitude of the force experienced by the proton.

Problem 2:The path of a helium nucleus, travelling at 3.0 x 103 m s-1, makes an

angle of 90° to a magnetic field. The helium nucleus experiences a force of 1.2 x 10-15 N while in the

field. Calculate the strength of the field.

Problem 3:Determine which particle below is which undergoing the paths in this magnetic field. Alpha particle Beta particle Neutron Positron

A1.

A2. A3.

Q1. A charge of +5.25mC moving with a velocity of 3 x 10² m/s due East enters a uniform magnetic field of

0.310T directed vertically downwards into the board. a) Find the magnetic force on the charge.b) direction of the force.

Q2. The diagram shows a beam of electrons about to enter a magnetic field. The direction of the field is into the page. What will be the direction of the deflection, if any, as the beam passes through the field?a) Towards the bottom of the page b) Towards the top of the page c) No deflection d)  Out of the page

Q3. An ion of mass 5 x 10ˉ²4 kg, carrying a charge of -3.2 x 10ˉ19 C, is placed in a uniform electric field of strength 5000V/m. Find:-

a) The Force on the ion.b) The acceleration that this force would produce.c) The work done by the field to move the ion through a distance of

10cm.d) The velocity of the charge after it travels 10cm, assuming it

started from rest.

Questions:

Q4. An alpha particle (mass = 6.68 x 10-² kg, q = +2e) staring from rest is accelerated through a potential

difference of 1 x 10³ V. It then enters a uniform magnetic field B = 0.02T, perpendicular to its direction of

motion. Find:-a) Radius of its pathb) Periodc) Frequencyd) Draw a diagram of magnetic field & alpha particles movement in field.

Q5. An electron moving at 20 000m/s enters a B of 0.1T, from the left at right angles to the field which is

directed into the page. q = -1.6 x 10-19C, m = 9.1 x 10-³¹ kg. Find:-a) Magnitude of the Force on the charge in the field.b) Direction of the Force on the charge just after it enters the field & just

before it leaves the field. c) The position where the electron leaves the field.

Q6. An electron enters a B = 1T with a velocity of 1 x 10 m/s at an angle of 45º to the field. What size of

force acts on the electron & in what direction?

If you know the potential difference between two parallel plates, you can calculate the electric field

strength between the plates. As long as you’re not near the edge of the plates, the electric field is constant

between the plates, and its strength is given by:

Units for the electric field strength are volts per meter [V/m]. According to the formula, the smaller you make the distance between

the plates, the stronger the electric field becomes if the potential difference is held constant.

A pair of flat parallel metal plates, A and B, was set so that the plates were 4.00 cm apart. An electric

potential of +250 V was applied to plate A while an electric potential of –150 V was applied to plate B.

Find:a) the electric field strength between A and B;b) the electric potential at a point P, 3.00 cm from plate B.

Solution:a) ΔV  =  +250 V  -  (-150 V) =   400 VSo, |E|  =  ΔV/d    = 400 V/0.04m =   1.00 x 104 V/m

So, the electric field at every point between the plates is  1.00 x 104 V/m  or  1.00 x 104 N/C.

So, V  = E x d = 1 x 104 x 0.03) V = 300V V = -150 V  +  300 V  =  1.50 x 102 V

Q1.  Two parallel flat metal plates lie 2.0 cm apart.  A p.d. of 1.5 x 102 V is applied across the plates.  What is the electric field intensity between them?

Q2.  If the field between two flat parallel plates is 4.00 x 103 N/C, what is the force on a charge of 1.00 x 10-6 C?

Q3. Two horizontal metal plates are held 4.0 cm apart.  The lower plate has a potential of 16 V and the upper plate has a potential of 48 V. What is the electric potential at A, 1.0 cm, B, 2.0 cm, and C, 3.0 cm above the lower plate.  What is the intensity at each of these points?

Q4. Two horizontal metal plates are 15 cm apart.  The lower and upper plates have respective potentials of 1.0 x 102 V and 4.0 x 102 V.  Find the potential and electric field at points 5.0 cm and 10 cm above the lower plate.

Q5. Two parallel plates having a potential difference of 1000V are separated by a gap of 0.02m. What is the strength of the electric field between the plates?

Q6. A charge of +6pC enters an electric field of 50 000 N/C acting between 2 parallel plates. What is the size of the force acting on the charge.

Q1.  Two parallel flat metal plates lie 2.0 cm apart.  A p.d. of 1.5 x 102 V is applied across the plates.  What is the electric field intensity between them?

Q2.  If the field between two flat parallel plates is 4.00 x 103 N/C, what is the force on a charge of 1.00 x 10-6 C?

Q3. Two horizontal metal plates are held 4.0 cm apart.  The lower plate has a potential of 16 V and the upper plate has a potential of 48 V. What is the electric potential at A, 1.0 cm, B, 2.0 cm, and C, 3.0 cm above the lower plate.  What is the intensity at each of these points?

The intensity at each of the points is the same, since the field is uniform.

Q4. Two horizontal metal plates are 15 cm apart.  The lower and upper plates have respective potentials of 1.0 x 102 V and 4.0 x 102 V.  Find the potential and electric field at points 5.0 cm and 10 cm above the lower plate.

Q5. Two parallel plates having a potential difference of 1000V are separated by a gap of 0.02m. What is the strength of the electric field between the plates?

Q6. A charge of +6pC enters an electric field of 50 000 N/C acting between 2 parallel plates. What is the size of the force acting on the charge.

Q7. Two parallel metal plates are separated by a distance of 10.0cm. The potential difference between the plates is 20.0V.a)Calculate the electric field between the plates, assuming it to be uniform. b)A charge of +2.0 x 10¯³C is placed in the electric field. Calculate the electric force on the charge.

A1. |E|  =  1.5 x102 V/0.02m = 0.75 V/m

A2. E|  =  F/q  So, F  =  4.00 x 103 N/C  x  1.00 x 10-6 C =  4.00 x 10-3 N

A3. The intensity at each of the points is the same, since the field is uniform.

|E| = [48 V - 16 V]/4.0 cm =  8.0 V/cm Thus, the potential increases by 8.0 V for every centimeter

moved from the lower plate. Potential at A  =  16 V  +  8.0 V =  24 V Potential at B  =  16 V  +  [2.0 cm  x  8.0 V/cm] =  32 V Potential at C  =  16 V  +  [3.0 cm  x  8.0 V/cm] =  40 V

A4. Electric field is t he same at all points between the plates. |E|  =  3.0 x 102 V/15 cm =  20 V/cm Potential at 5.0 cm from the lower plate  =  1.0 x 102 V  + 

[5.0 cm  x  20 V/cm] =  2.0 x 102 V Potential at 10 cm from the lower plate  =  1.0 x 102 V  + 

[10 cm  x  20 V/cm] =  3.0 x 102 V   

A5. E = V/d = 1000/0.02 = 50000V/m

A6. F = qE = +6 x 10¯9 x 50000 = 3 x 10-4 N

A7. a) E = V/d = 20/ 0.10 = 200 V/m b) F = qE = +2.0 x 10¯³ x 200 = 0.4 N

This work will be stored in the charge as electric potential energy. The potential energy of the charge on reaching the positive plate  =  qΔV. By the principle of conservation of energy: W = PE F d  =  q Δ V

Normally this positive charge will be attracted to the negative plate [1] and repelled from the positive plate [2].The work required to move the positive charge from the lower to the upper plate is given by; W  =  F d.

F = Electric Force (N)d = distance charge is moved (m)q = charge in (C)V = voltage in (V)W = Work in (J)

Question 1: A charge of 2x10-3 C is moved through a potential difference of 10 volts in an electric field. How much work, in electron-volts, was required to move this charge?

Question 2: A potential difference of 40.0 volts exists between two points, A and B, within an electric field. What is the magnitude of charge that requires 2.0 × 10–2 joule of work to move it from A to B?

Solution: A1. W = Fd = qV = 2 x 10-3 x 10 = 2 x 10-² J

A2. W = Fd = qV 2.0 x 10¯² = q x 40 q =

In a cathode ray tube, a strong electric field tears electrons away from their atoms in a vacuum. Free electrons in the tube are affected by electric & magnetic fields. The basic features include: Electron gun A deflection system A fluorescent or phosphorescent screen

Cathode ray tubes are used in electron microscopes computer monitors radar displays oscilloscopes televisions

where a beam of free electrons forms a picture on the screen. Our ability to accelerate & guide electrons has impacted on technology particularly in the fields of communications

& electronics.

The Cathode Ray Tube

Defection system

Fluorescent screen

Electron Gun

Create a table of the parts of the CRT. Use the diagram above.

Electron Gun

Heated filament

Create a table of the parts of the CRT. Use the diagram above.

Electron Gun

Heated filament

Part FunctionElectron GunCathodeAnodesDeflection Plates or coilsFocusing CoilFluorescent or Phosphor screenHeated Filament

Part FunctionElectron Gun Produces a narrow beam of electronsCathode Encloses a filament at a negative potentialAnodes Accelerate the electrons in their directionDeflection Plates or coils

These plates deflect the beam in the horizontal (x) and vertical (y) directions in oscilloscopes and electromagnetic coils are used in TV’s.

Focusing Coil Focuses the beam as the negative e-’s are repelled from all sides by the negative cylinder

Fluorescent or Phosphor screen

Coated with zinc sulfide or a similar material that fluoresces with a blue-green colour when hit by energetic electrons.

Heated Filament

Electrons are produced by thermionic emission from a hot wire filament

In 1885, Sir William Crookes carried out a series of investigations into the behaviour of metals heated in a vacuum. The experiment of Crookes and others showed that a heated cathode produced a stream of radiation, which could cause gases at low pressure to glow and which made other substances emit light. The radiation emitted from the cathode was given the name 'Cathode rays'. By mid-nineties it was known that these rays could be deflected by a magnetic field and they carried a negative charge. Some scientists felt that these rays were waves and others were inclined to think they were particles.

In 1897, J. J. Thomson showed that the stream of radiation were indeed particles called electrons. He conducted the famous discharge tube experiment, by passing electricity at high voltage through a gas at low pressure. A common discharge tube is a long glass tube having two metal plates, sealed at its two ends as electrodes. It has a side tube through which air can be pumped out by using a vacuum pump, so that experiments can be performed at low pressure.

When the pressure of air in the discharge tube is reduced to .001 mm of mercury and a high voltage is applied to the electrodes, the emission of light by air stops. But the phenomenon of fluorescence is observed in which the walls of the discharge tube at the end opposite to the cathode begin to glow with a greenish light. It is now deduced that some invisible rays were formed at the cathode, which on striking the glass tube emitted a green light. Since they are formed at the cathode they are known as cathode rays.

Question: Q1. Who showed that a stream of radiation was created in a vacuum tube

when a cathode was heated? And when? Q2. Who worked out that cathode rays were particles? And when?

This consists of 2 sets of parallel plates. Signal voltages applied to these plates deflect the beam

in the horizontal (x) and vertical (y) directions. In many cathode ray tubes the deflection is caused

by magnetic coils while in others it consists of parallel plates that produce an electric field.

Label the plates generating electric fields so the first lot of cathode rays (left diagram) go up & the second lot (right diagram) go into the page. Draw the resulting cathode beam directions as well.Q1. A beam of electrons passes through a television tube to the screen. Why may the beam be deflected by a strong bar magnet? a) The magnet magnetises the sensitive coating of the screen. b) A beam of electrons behave as an electric current in a magnetic field. c) The North pole of a magnet repels all negatively charged particles. d)  The magnet neutralises the charge on the electrons.

Deflection System

The electron beam strikes the front of the tube which is coated with zinc sulfide or a similar

material that fluoresces with a blue-green colour when hit by energetic electrons.

Q1. Why is the fluorescing screen so important in the modern

cathode ray tube used such as the CRO or television? Q2. What are the 3 colours that a colour TV screen fluoresce with?Q3. Why when a TV is finally turned off at night the screen

appears to have a soft glow for a short time?

A1. The screen converts the kinetic energy of the electron beam into light energy that is easy to detect with the human eye.

A2. Red, green, blue.A3. The absorbed energy from earlier exposure to the beam of

electrons is re-emitted later as light from the phosphor material.

Fluorescent Screen

The electron gun produces a narrow beam of electrons.

The anodes accelerate the electrons.

The deflection plates or coils establish an electric field that controls the deflection of the electron beam from side to side and up and down.

The fluorescent screen is coated with a material that emits light when struck by electrons in the cathode ray. This allows the position of the beam to be seen where it strikes the screen.

In television sets, and …………………….. monitors, the entire front area of the tube is scanned ………………………. and systematically in a fixed pattern called a raster. An image is produced by controlling the …………………….of each of the three electron ……………… one for each primary color (…………, ……….... and ………….). In all modern CRT monitors and televisions, the beams are bent by …………………….... deflection, a varying magnetic field generated by coils and driven by electronic circuits around the neck of the tube.

Word Bank: beams, blue, computer , green, intensity, magnetic, red, repetitively

In television sets, and computer monitors, the entire front area of the tube is scanned repetitively and systematically in a fixed pattern called a raster. An image is produced by controlling the intensity of each of the three electron beams, one for each primary color (red, green, and blue). In all modern CRT monitors and televisions, the beams are bent by magnetic deflection, a varying magnetic field generated by coils and driven by electronic circuits around the neck of the tube.

In oscilloscope CRTs, electrostatic deflection ........ used, rather than the magnetic deflection commonly used with television …………. other large CRTs. The beam is deflected horizontally by applying an electric field between a ……….. of plates to its

left and right, and vertically by applying ……… electric field to plates above and below. Oscilloscopes use electrostatic rather than magnetic deflection because the inductive reactance of the magnetic coils would limit …….. frequency response of the Instrument.

In electronic systems, reactance is the opposition of a circuit element to a change of electric current or voltage, due to that element's inductance or capacitance. A built-up electric field resists the change of voltage on the element, while a magnetic field resists the change of current. The notion of reactance is similar to electrical resistance, but they differ in several respects.

Inductance are inherent properties of an element, just like resistance. Reactive effects are not exhibited under constant direct current, but only when the conditions in the circuit change. Thus, the reactance differs with the rate of change, and is a constant only for circuits under alternating current of constant frequency

In oscilloscope CRTs, electrostatic deflection is used, rather than the magnetic deflection commonly used with television and other large CRTs. The beam is deflected horizontally by applying an electric field between a pair of plates to its

left and right, and vertically by applying an electric field to plates above and below. Oscilloscopes use electrostatic rather than magnetic deflection because the inductive reactance of the magnetic coils would limit the frequency response of

the instrument

From an understanding of the way electron’s interact with magnetic fields, society has for ever

been changed. As seen in developments of:

Our experience in accelerating electrons to high energies has also led to the development of

‘atom smashers’.

Electrons & Magnetic fields & Modern Society

Device Used for

mobiles, ipods, mp3, satellites

research, producing positrons etc

Magnetron

flights in aeroplanes

Radar work, school, i.e. internet

microwave ovens and mobile phonesnon invasive medical procedures

Computers Communication devices Microwaves

computers, mobiles, ipods, TV’s etcMRI

Semiconductor devices

From an understanding of the way electron’s interact with magnetic fields, society has for ever

been changed. As seen in developments of:

Magnetron - research, producing positrons etc Radar - flights in aeroplanes Microwaves - microwave ovens and mobile

phones MRI - non invasive medical procedures Semiconductor devices - computers, mobiles, ipods, TV’s

etc Computers - work, school, i.e. internet Communication devices - mobiles, ipods, mp3, satellites

Our experience in accelerating electrons to high energies has also led to the development of

‘atom smashers’.

Electrons & Magnetic fields & Modern Society

Jean Perrin collected cathode rays on an electrode attached to a gold leaf electrometer.

The electrometer was initially charged negative & the leaves spread apart under mutual

repulsion. When the cathode ray beam was focused on the electrode the leaves spread further

apart, indicating that the charge on the cathode rays was negative.

Eugen Goldstein demonstrated cathode rays could be deflected by electric fields. The beam

bent towards the positive electrode consistent with the rays being particles carrying a

negative charge.

Q1. Describe the two experiments that indicated the charge of the cathode ray.

Indentifying the Charge of Cathode Rays

Lower frequency, longer wavelength, less energyRadioMicrowa

veInfrared

Visible

Ultraviolet

X-raysGamma

raysHigher frequency, shorter wavelength, more energy

Causes electronic oscillations in the antenna

Causes molecular rotation

Causes molecular vibration

Can excite electrons to orbits of higher energy. Visible light ranges from 400-700 nm. 400ish being violet, 700ish being red.

Can break bonds and excite electrons so much as to eject them, which is why UV is considered ionizing radiation. Photoelectric effect.

Ionizing radiation & can penetrate the human body.

Even more energetic than X-rays & can penetrate many materials

Lower frequency, longer wavelength, less energyRadio Causes electronic oscillations in the antennaMicrowa

ve Causes molecular rotation

Infrared Causes molecular vibration

VisibleCan excite electrons to orbits of higher energy.

Visible light ranges from 400-700 nm. 400ish being violet, 700ish being red.

Ultraviolet

Can break bonds and excite electrons so much as to eject them, which is why

UV is considered ionizing radiation. Photoelectric effect.

X-rays Ionizing radiation & can penetrate the human body.

Gamma rays

Even more energetic than X-rays & can penetrate many materials

Higher frequency, shorter wavelength, more energy

In 1845, Michael Faraday discovered that the angle of polarization of a beam of light as it passed through a

polarizing material could be altered by a magnetic field. This was the first evidence that light was related to

electromagnetism. Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which

could propagate even in the absence of a medium such as the ether. Faraday's work inspired James Maxwell to study electromagnetic radiation and light.

Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed,

which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation: he first

stated this result in 1862. Maxwell asserted that light is an electromagnetic oscillation. The significance of Maxwell’s analysis explained the propagation of electromagnetic waves &

also predicted that these waves could have a full range of frequencies. i.e. an electromagnetic spectrum.

Maxwell also theoretically calculated the velocity of Electromagnetic radiation as 3 x 10^8 m/s. In 1887 Heinrich Hertz confirmed Maxwell's theory experimentally by generating and

detecting radio waves in the laboratory, and demonstrating that these waves behaved exactly like visible light, exhibiting

properties such as reflection, refraction, diffraction, and interference.

Faraday Maxwell HertzIn 1845, Maxwell discovered Hertz confirmed

Proposed in 1847 In 1862 He generated

Maxwell’s analysis He demonstrated

Calculated the

Q1. Maxwell's theory and Hertz's experiments led directly to the development of?.

Faraday Maxwell HertzIn 1845, Faraday discovered that the angle of polarization of a beam of light as it passed through a polarizing material could be altered by a magnetic field. The first evidence that light was related to electromagnetism.

Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light.

Hertz confirmed Maxwell's theory

Experimentally.

Proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the ether.

In 1862 Maxwell concluded that light was a form of electromagnetic radiation

He generated and detected radio waves in the laboratory.

Maxwell’s analysis explained the propagation of electromagnetic waves & also predicted that these waves could have a full range of frequencies

He demonstrated that these waves behaved exactly like visible light, exhibiting properties such as reflection, refraction, diffraction, and interference.

Calculated the velocity of Electromagnetic radiation as 3 x 10^8 m/s.

Q1. Maxwell's theory and Hertz's experiments led directly to the development of?A1. modern radio, radar, television, electromagnetic imaging, and wireless communications.

Antennae (or aerials) are devices for radiating or receiving electromagnetic waves. There is

little difference between antennae for transmitting and receiving radio waves. Often the same

antenna is used for both purposes eg. Mobile phones. The antenna has the best reception when it has a size of ½ the wavelength (or some

value of n/2 times) of the wave it is receiving. This length enables the signal to produce a

resonating AC Effect in the aerial & reduce the interference with itself. The electromagnetic signal broadcast from the antenna is produced by an AC. i.e.

electrons in the antenna are moving backward and forward. When the electrons move in the

antenna an electromagnetic field is created around the antenna that travels in all directions at

………… m/s. The generated electromagnetic signal travels until it hits a receiving antenna where

the electrons in the metal of the receiving antenna vibrate at the same frequency as the original AC.

Thisproduces an electric current in the receiving antenna. (Hertz’s experiment). A device such as a mobile phone, radio or TV uses amplifier circuits to enhance the

signal.

How Radio Antennae Work

Questions: On AntennaeQ1. Explain why aerials for modern mobile phones are not

visible from the outside of the phone?

Q2. For TV station SBS which transmits at a frequency of 648 MHz, calculate the wavelength of its carrier wave & determine the length of the antenna required to broadcast this signal.

Q3. If the wave has a frequency of 600 000Hz, the electrons in the wire are moving back and forth ……………………. times a second.

Solutions: A1. Mobile phones uses microwaves as a carrier wave & they

are shorter than radiowaves.A2. λ = v / f = 300 000 000 / 648 000 000 = 0.46m aerial L=

0.23mA3. 600 000

The electromagnetic signal broadcast from the antenna is produced by an AC. i.e. electrons in the antenna are moving backward and forward.

When the electrons move in the antenna an electromagnetic field is created around the antenna that travels in all directions

The generated electromagnetic signal travels through the air until it hits a receiving antenna

Electrons in the receiving antenna are set vibrating at the same frequency as the original AC.

Where it produces an electric current in the receiving antenna

At the turn of the 20th century light was thought to be a wave in nature because of the seemingly

conclusive experimental results of Thomas Young and Augustin Fresnel.

Interference of light waves Diffraction of light waves Refraction of light waves Reflection of light waves Polarisation of light waves The concept of light as a wave came to be known as the classical

wave theory of light. After around ½ a century of accepting the classical wave theory of

light as the ultimate answer some unexplained phenomena began to crop up that could not be

explained by the classical wave theory. This dealt with the way light is produced. Q1. List examples of how light is produced?

A1. In a discharge tube , from the surface of the Sun, candle burning, bioluminescence, fluorescence, from the head of a match stick, a CRT for a TV.

Waves v Quanta

Light behaves both as a wave and a particle. Select from the following list and place in the correct

column:-Reflection, refraction, diffraction, polarisation,

interference, gravitational bending of light, photoelectric effect, radiation pressure. You can use the words more than once.Evidence of Particle Nature Evidence of Wave Nature

Q1. What is meant by wave- particle duality?Q2. Explain the significance of ‘matter waves’ in atomic theory. Q3. Explain what gravitational bending of light is? http://www.youtube.com/watch?

v=shEYww1W4Lw&feature=relmfu

Evidence of Particle Nature Evidence of Wave NatureReflection Reflection

Refraction RefractionPhotoelectric effect InterferenceRadiation Pressure DiffractionGravitational Bending of light PolarizationA1. Light behaves as both a wave and a particle. i.e. it has both properties.A2. This explains how matter like an electron behaves like a wave.A3. When light enters a strong gravitational field it’s path is affected and curves. i.e. Light from distant stars is curved as it passes out own sun.

An idealised model radiator was used to explain the behaviour of materials in terms of the

radiation emitted at different temperatures. A perfect blackbody absorbs all radiation that falls on it,

then radiates energy as a result of its temperature & the interaction of the energy its atoms

absorb from the original radiation falling upon the body.

Q1. Draw and label the model cavity radiator or black body.Q2. What material is the experimental models of ideal

blackbodies constructed of? Q3. What is the emitted radiation dpendent on?

Solution: A2. A metal block with an internal cavity with a very small

opening.A3. The temperature of the block.

When substances are heated they emit energy including light. When this emitted energy is analysed it

shows radiation at various wavelengths. The radiation emitted at the highest intensity is represented by

the peak. The peak doesn’t fall in the visible spectrum unless the temperature over 3700K. A hotter black

body emits typical radiation with shorter wavelengths. This explains why black bodies at higher temperatures are blue, and those at lower temperatures are red.

Q1. Describe how the shapes of the curves change with temperature.

A1. The curve peaks shift to shorter wavelengths at higher temperatures. The emitted radiation peaks are higher at higher temperatures.

http://www.youtube.com/watch?v=l_t8dn4c6_g

Black body/ nhf/ photoelectric

In 1893 Wien proved that the radiant intensity emitted from the hole in the blackbody is a function of

the temperature of the solid & of the frequency of the emitted radiation. Wien’s Law stated that as

temperature of the solid increases, the peak intensity of radiation emitted & therefore the peak of the

curve is displaced to shorter wavelength (higher frequency) electromagnetic radiation. In 1896 Wien

produced his law that was based on evidence of emissions at visible wavelengths (400-700nm).

In 1900 Rayleigh observed that Wien’s radiation law made no sense at low frequencies. It didn’t

fit experimental data. So Rayleigh and Jeans produced their version of a radiation law. Their law

correctly predicted the behaviour of the experimental data at low frequency radiation but produced

data that didn’t fit the experimental data at high frequencies. This failure to fit the experimental data

at short wavelengths became known as the UV catastrophe!!!!!!!!!!! #@*% In a cavity radiator the radiation emitted from the walls of the cavity could be expected to

be constantly reabsorbed by the walls. As such the radiation in the cavity would become shorter

& shorter in wavelength as time goes on. This is a problem because shorter wavelength radiation

is of higher energy. The energy emitted from the hole in the cavity would become infinitely shorter wavelength & energetic that it would destroy the universe with a blast of high energy radiation every time

such an object was heated!!!!!!!!!!!!!!!!!!!!!!!!

Q1. What is the UV catastrophe?A1. Failure of the radiation law to explain the emitted radiation at high frequencies (short wavelengths).

In the early 1900's, German physicist E. Planck noticed a fatal flaw in physics by demonstrating

that the electron in orbit around the nucleus accelerates. Acceleration means a changing electric

field (as the electron has charge), which means photons should be emitted. But, then the electron

would lose energy and fall into the nucleus. Therefore, atoms shouldn't exist!!!!!!!!!!!!!!

To resolve this problem, Planck made a wild assumption that energy, at the sub-atomic level, can only be transferred in small units, called quanta. Due to his insight, we call this unit Planck's constant (h). The word quantum derives from quantity and refers to a small packet of action or process, the smallest unit that can be associated with a single event in the microscopic world.

Changes of energy, such as the transition of an electron from one orbit to another around the nucleus of an atom, is done in discrete quanta. Quanta are not divisible and the term quantum leap refers to the abrupt movement from one discrete energy level to another, with no smooth transition. There is no “inbetween''.

The quantization, or “jumpiness'' of action as depicted in quantum physics differs sharply from classical physics which represented motion as smooth, continuous change. Quantization limits the energy to be transferred to photons and resolves the UV catastrophe problem.

In 1900 Planck announced his radiation law. His law predicted the experimental results for radiation emitted at all wavelengths from cavity radiators. It ushered in a new explanation of the nature of light. and matter.

Planck next looked for a model of the atomic processes taking place in the walls of the cavity that would explain the data. He assumed that each atom behaves as an electromagnetic oscillator that is essentially a small antenna. Each small atomic antenna has a characteristic

frequency of oscillation that emits electromagnetic radiation into the cavity but, like a real

antenna, could also receive electromagnetic radiation. Q1. What are quanta? Q2. How did Planck describe what is happening with the atoms in the

walls of the cavities of black bodies? Q3. Planck was not confident with his explanation & law. Discuss why.

A3. Planck thought of ‘quanta’ as a sort of fudge factor, & that all he had done was create a mathematical trick that makes his equation describe the experimental data. Planck still believed electromagnetic radiation was a wave phenomena and waves were not created from little bursts of energy.

1. These oscillators can only have energies given by: E = nhf 2. The oscillators energies were quantised. i.e. energy

released or absorbed had to be in integer whole number multiples of ‘energy packets’ (quanta). Not in continuously variable amounts.

3. These quanta are emitted or absorbed when an oscillator changes from one quantised energy state to another quantised energy state.

4. If an oscillator remains in one of its quantised energy states, then it will neither emit or absorb energy.

E = nhf E = energy per photon H = Planck’s constant ……………………………….. Js F = frequency of the electromagnetic wave in Hz n = 0,1,2…….

Questions: Q1. Determine the energy of a quantum of radio radiation of wavelength 1000m. Q2. What is the quantum of energy of a photon of red light with a wavelength of 700nm? Q3. What was revolutionary about Planck’s idea about the radiation from cavity radiators?

Solution: A1. E = hf = hc / λ = 6.624 x 10ˉ³ x 3 x 10 /1000 = 2 x 10ˉ² JA2. E = hf = hc / λ = 6.624 x 10ˉ³ x 3 x 10 / 700 x 10ˉ = A3. It required an entire new way of representing light & matter. It changed Physics from a classical point of view to a quantum point of view.

http://www.youtube.com/watch?v=lTnpb9YPrQs&feature=related

Explaining the formula + questions and answers

In 1886 & 1887, Hertz discovered that UV light can cause electrons to be ejected from a metal

surface. In classical theory if light could cause the emission of

electrons, a greater light intensity should cause the electrons on the metal to be ejected from the

surface with a greater KE. However experimental results found:- KE of the electrons ejected depends on the f…………………… of

the light falling on the metal not the intensity. Higher intensities of light caused the ejection of

m………………….. electrons from the metal surface but did not c……………………. the KE with which the electrons were ejected.

If the frequency of the light hitting the metal was not of a threshold f……………………… no electrons were r……………………….. at all.

Questions:

Q1. If the intensity of the light was increased what happens?Q2. If the frequency of the light was increased what would happen?Q3. Is the type of metal important for the photoelectric effect?Q4. Did Hertz investigate the photoelectric effect? Q5. How do we use the photoelectric effect in society?

Clickview: Photoelectric effect

In 1886 & 1887, Hertz discovered that UV light can cause electrons to be ejected from a metal

surface. In classical theory if light could cause the emission of

electrons, a greater light intensity should cause the electrons on the metal to be ejected from the

surface with a greater KE. However experimental results found:- KE of the electrons ejected depends on the frequency of the

light falling on the metal not the intensity. Higher intensities of light caused the ejection of more

electrons from the metal surface but did not change the KE with which the electrons were ejected.

If the frequency of the light hitting the metal was not of a threshold frequency no electrons were released at all.

Questions:

A1. Nothing unless the light frequency is of UV frequency or above. If it is, then more electrons can be ejected but the KE does not alter. A2. The ejection of electrons would occur and the KE of each electron would be greater.A3. Yes as some metals hold onto their electrons stronger then others.A4. NoA5. Security motion sensors, photocells, solar cells, automatic door openers

The device to illustrate the photoelectric effect was a plate and wire inside a vacuum t…………. The battery is arranged so that a negative voltage is applied to the plate. No current is flowing because the voltage is too low to cause the electrons to break away from the plate.

When v……………. light shines on the plate, there is still ………… current. The classical view of light was that energy from the light would accumulate and eventually eject the electrons, but that ………………………. happened.

When ultraviolet light shines on the plate, …………………… are ………………….. from the surface and current …………………... Electrons go to the other terminal and flow through the circuit. Other electrons leave the negative side of the ……………………… and make it to the plate. If UV light continues, current will continue.

At the time this effect could be demonstrated no one knew why UV light worked and visible light didn't.

The device to illustrate the photoelectric effect was a plate and wire inside a vacuum tube. The battery is arranged so that a negative voltage is applied to the plate. No current is flowing because the voltage is too low to cause the electrons to break away from the plate.

When visible light shines on the plate, there is still no current. The classical view of light was that energy from the light would accumulate and eventually eject the electrons, but that never happened.

When ultraviolet light shines on the plate, electrons are ejected from the surface and current flows. Electrons go to the other terminal and flow through the circuit. Other electrons leave the negative side of the battery and make it to the plate. If UV light continues, current will continue.

At the time this effect could be demonstrated no one knew why UV light worked and visible light didn't.

Term DefinitionPhoton

Photocathode

Photoelectron

Photocurrent

Stopping Potential

Threshold FrequencyWork Function

Term DefinitionPhoton Quantised packets of light energy that carry

energy, hf, into the photoelectric surface.Photocathode

Metal plate acting as a cathode that electrons are liberated from because of light falling on it.

Photoelectron

Emitted electrons that are attracted to a positive electrode when light falls on the photocathode.

Photocurrent

A current generated via the photoelectric effect.

Stopping Potential

When the reverse potential is large enough to make the current zero. When this happens it means the photoelectrons are doing work when they move against the electric field. Vo = Stopping Potential

Threshold Frequency

The minimum frequency (which determines the energy) the light must have to cause the photoelectric effect in a metal is called the threshold frequency for that metal. The cut off frequency below which no photoelectrons will be emitted.

Work Function

The work function is defined to be the minimum energy required to free electrons from the metal surface, and is different for different metals.If the photon has just enough energy to eject the photoelectrons from the material & no more to convert to KE then that energy value is called the work function = .

The work function is defined to be the minimum energy required to free electrons from the metal surface, and is different for different metals. The wavelength cut off of the photoelectric effect is therefore in fact an energy cut off. (i.e. E=hf = hc/λ) If the photons have an energy greater then the work function energy, then the extra energy goes into providing KE to the photoelectrons. The KE of the photoelectrons is given by: KE = hf - work function The kinetic energy of the photoelectrons released is determined by the difference between the energy of the photon (hf ) and the work function (W) of the metal.

Example: No electrons were emitted when using a dull red, low frequency light on a particular material. Even a bright red light produced no photocurrent. However if a high frequency violet light, even a faint one were used a small photocurrent was produced increasing with the brightness of the light.

When light with a frequency greater than the threshold frequency shines on a photoelectric cathode embedded in an evacuated tube, electrons will be emitted as expected. These electrons will have a specific amount of maximum kinetic energy and in this case will move away from the cathode across the vacuum tube to reach the collector. This causes a photoelectric current to flow. One way of actually measuring the kinetic energy of the photoelectrons experimentally is by making the collector negative enough to repel the photoelectrons to prevent them from reaching the collector. This can be done by inserting a power source into the circuit with the correct orientation, The consequence of this is that the photocurrent in this circuit drops to zero. The minimum voltage the power source needs to supply in order to make this happen is known as the stopping voltage. The stopping voltage correlates directly with the kinetic energy of the photoelectrons; a larger stopping voltage means more work needs to be done to stop the photoelectrons reaching the collector, which in turn reflects higher kinetic energies. KE = Work done = qV

Q1. Suppose the incident EMR has a wavelength of 350 nm and the photoelectric cathode has a work function of 4.53 × 10–19 J. (a) Calculate the maximum kinetic energy for the photoelectrons released. (b) Find the maximum velocity of these photoelectrons.(c) How big should the stopping voltage be in this case?

Q2. Light of wavelength 300nm is incident on a metallic plate. If the stopping potential for the photoelectric effect is 2V find: (a) The maximum energy of the emitted electrons. KE = work done = qV(b) The work function of the material KE = nhf – work function(c) The cut off wavelength. KE = nh(c/λ) – work function

A1. a) KE = nhf – work function = nh (c/λ) – work function = 6.626 x 10-34 x (3 x10 ^8 / 350 x 10-9) – 4.53 x 10-19 = 1.15 x 10-19 J

b) KE = ½ mv² 1.15 x 10-19 = ½ x 9.109 x 10-31 x v² v = 5.02 x 10^5 m/s

c) In order to stop the photoelectrons reaching the collector, the power source must apply an opposing energy (work) that is at least equal to the kinetic energy of these photoelectrons. The work done by any electric system can be defined as the product of the V and q, ie.(W = qV ). Hence: KE = work done = qV 1.15 x 10-19 = 1.602 x10-19 x V V = 0.72V

A2. a) KE = work done = qV = 1.6 x 10-19 x 2 = 3.2x10-19J

b) KE = hf - work function = hc/λ – work function = (6.626x10-34 x 3x10^8 / 300x 10-9 ) - work function = 6.623x10-19 - work function 3.2 x 10-19 = 6.626 x 10-19 – work function work function = 3.43 x 10-19 J

c) The cut off wavelength occurs when the KE of the photoelectrons is zero.

0 = hf – work functionwork function = hc/λ 3.43 x 10-19 = 6.626 x 10-34 x 3x10^8 / λ λ = 579nm

A photon carries energy hf into the metal surface.

Part of the energy is used to get an electron to escape from the metal surface.

If the electron does not lose all its energy by internal collisions within the material as it escapes it will have excess energy as KE. (This is the remaining photon energy).

If the intensity of the incident light is increased, the number of photoelectrons ejected increases to a maximum value.

If the photon has just enough energy to eject the photoelectrons from the material & no more to convert to KE then that energy value is called the work function (Eo ).

For each statement determine whether it is true or false

Statement T/Fa The intensity of the light is proportional to the

number of photons arriving on the metal atoms to release an electron.

b When red light shines on the plate, electrons are ejected from the surface and current flows.

c The higher the frequency, the more energy these photons would have.

d If the frequency of the light hitting the metal is below the threshold frequency electrons are still released.

e Higher intensities of light caused the ejection of more electrons from the metal surface but did not change the KE with which the electrons were ejected.

f There is only one model theory for light.g Quanta are emitted or absorbed when an

oscillator changes from one quantised energy state to another quantised energy state.

h Einstein used photons & quanta to explain the photoelectric effect

i Einstein termed the phrase quanta.

For each statement determine whether it is true or false

Statement T/Fa The intensity of the light is proportional to the

number of photons arriving on the metal atoms to release an electron.

T

b When red light shines on the plate, electrons are ejected from the surface and current flows.

F

c The higher the frequency, the more energy these photons would have.

T

d If the frequency of the light hitting the metal is below the threshold frequency electrons are still released.

F

e Higher intensities of light caused the ejection of more electrons from the metal surface but did not change the KE with which the electrons were ejected.

T

f There is only one model theory for light. Fg Quanta are emitted or absorbed when an

oscillator changes from one quantised energy state to another quantised energy state.

T

h Einstein used photons & quanta to explain the photoelectric effect

T

i Einstein termed the phrase quanta. F

The crucial insight into these observations came from Einstein. He explained the photoelectric effect as one in which light consisted of a stream of particles or photons. Each photon carries a fixed amount of energy, which is related to the frequency of the incident light. E = hf = hc/λEinstein also showed that the emission of photoelectrons could be considered in terms of the absorption of photons, each carrying a discrete energy and each being absorbed as whole units only. Light comes in quantised units, the photon. Einstein proposed that light was not a continuous wave but a packet or particle of energy (later called photons). Energy could only transfer to an electron by being hit by one photon of light similar to the way one particle hits another particle. He knew that the higher the frequency, the more energy these photons would have. Only when the photon’s energy was greater than the binding energy of the electron to the metal would an electron be ejected. Light had been thought of as a continuous wave with energy spread over the entire beam. Now with Einstein's explanation of the photoelectric effect, light needed to be thought of as packets of energy or particles of light (photons). The view of the universe changed because if energy was in packets, that meant that time, distance, and about everything else was broken down into packets.

Q, Identify Einstein’s contribution to quantum theory and its relation to black body radiation.

What was the explanation of light before EinsteinHow did Einstein explain light

How has this new view altered the Scientific view of how black body radiation & the photoelectric effect work

Photoelectric effect:

Black Body Effect:

How has society benefitted from this understanding of the photoelectric effect

Q, Identify Einstein’s contribution to quantum theory and its relation to black body radiationWhat was the explanation of light before Einstein

Light had been thought of as a continuous wave with energy spread over the entire beam

How did Einstein explain light

Einstein proposed that light was not a continuous wave but a packet or particle of energy. Light comes in quantised units, the photon.

How has this new view altered the Scientific view of how black body radiation & the photoelectric effect work

Photoelectric effect: Energy could only transfer to an electron by being hit by one photon of light similar to the way one particle hits another particle. The higher the frequency, the more energy these photons would have. And only when the photon’s energy was greater than the binding energy of the electron to the metal would an electron be ejected. Black body Radiation: If energy was in packets, then energy arriving on the metal atoms is transferred in small units, called quanta. And released in quanta.

How has society benefitted from this understanding of the photoelectric effect

New devices utilizing the photoelectric effect. Examples: Semiconductors (transistors), photocells (sensors for alarms, automatic door openers), solar cells (solar panels).

Came from an upper-class German family.

His famous quantum theory made him the authority in German physics. He continued his physics research at the University of Berlin under the Nazi regime during World War II.

He was not as politically active and focused on his physics research even during the WWII.

He was not amoral. He did go to Adolf Hitler in an attempt to stop his racial policies. It could be argued that this was an act guided by his moral values; it could also be argued he did this simply to preserve the development of German physics, with no intention of influencing political decisions

He viewed science as separate from politics.

Came from a Jewish working-class family that lived in Germany.

He was a strong believer in pacifism—he opposed wars and violence. Indeed he spent time preaching pacifism.

His famous equation E = mc² made him inseparable from society and politics. He was a politically active man and openly criticised

German militarism during WWI.

After he emigrated to the US (at the beginning of WW II), he wrote to the US president, Franklin Roosevelt, to convince him to set up the project of making nuclear bombs (known as the ‘Manhattan Project’), which led to the deaths of tens of thousands of people in Japan. His rationale was his fear that the Germans were developing nuclear technology and might build nuclear bombs first

He viewed science as NOT separate from politics and Scientists are responsible for how their work is used. His physics resulted in the creation of the most powerful and deadly weapon ever known to humankind. This also served as the basis for the development and implementation of nuclear power stations.

Einstein and Planck had differing views on whether Science research is removed from social and political forces. Below in the table summarize their opposite views. Include examples.

Scientist Planck Einstein

Nationality / socio-economic backgroundsPersonalities

Politics

During WWII .

Views

Einstein and Planck had differing views on whether Science research is removed from social and political forces. Below in the table summarize their opposite views. Include examples.

Scientist

Planck Einstein

Nationality / socio-economic backgrounds

Planck came from an upper-class German family.

Einstein came from a Jewish working-class family that lived in Germany.

Personalities His famous quantum theory made him the authority in German physics. Planck continued his physics research at the University of Berlin under the Nazi regime during World War II.

He was a strong believer in pacifism—he opposed wars and violence. Indeed he spent time preaching pacifism.

Politics He was not as politically active as Einstein and focused on his physics research even during the WWII.

Einstein’s famous equation E = mc² made him inseparable from society and politics. Einstein was a politically active man and openly criticised German militarism during WWI.

During WWII Planck was not amoral. He did go to Adolf Hitler in an attempt to stop his racialpolicies. It could be argued that this was an act guided by his moral values; it could also be argued he did this simply to preserve the development of German physics, with no intention of influencing political decisions.

After he emigrated to the US (at the beginning of WW II), he wrote to the US president, Franklin Roosevelt, to convince him to set up the project of making nuclear bombs (known as the ‘Manhattan Project’), which led to the deaths of tens of thousands of people in Japan. His rationale was his fear that the Germans were developing nuclear technology and might build nuclear bombs first.

Views Planck viewed science as separate from politics.

Einstein viewed science as NOT separate from politics and Scientists are responsible for how their work is used. However his physics resulted in the creation of the most powerful and deadly weapon ever known to humankind. This also served as the basis for the development and implementation of nuclear power stations.

Solar cells & Photocells apply the use of the photoelectric effect.

They can be used to detect light. Instead of a sheet of metal,

semiconductors can be used. With small semiconductors as

light detectors, one can make video cameras.

By using solar panels placed on the surface, of traffic lights a natural resource is used instead to power the traffic light instead of electric energy from a power grid.

The flash of a camera uses the photoelectric effect

Photocells are used in medicine.

Photocells are used in garage door openers.

A photocell converts the change in the intensity of light into a change in the electric c………………..

The diagram shows a photocell circuit. The c………………… is made of photosensitive material.

A beam of light is allowed to fall on it. A…………….. is kept in front of the cathode

and a battery is connected between the cathode and the anode as shown in the figure.

The battery creates an electric field from anode to cathode.

As the light falls on the cathode, photoelectrons are e…………….. out and are attracted by the anode. Thereby sending a current through the circuit that is measured by the g………………………………connected in the circuit. The current ranges of the order of microampere.

Photocells are devices used to measure the intensity of light in photographic exposure meters, astronomical photometers and film densitometers. Their popular uses included switching on street lights at dusk, in m……………………. sensors for alarms, automatic door openers.

http://www.youtube.com/watch?v=v5h3h2E4z2Q&feature=related

A photocell converts the change in the intensity of light into a change in the electric current.

The diagram shows a photocell circuit. The cathode is made of photosensitive material.

A beam of light is allowed to fall on it. Anode is kept in front of the cathode and a

battery is connected between the cathode and the anode as shown in the figure.

The battery creates an electric field from anode to cathode.

As the light falls on the cathode, photoelectrons are ejected out and are attracted by the anode. Thereby sending a current through the circuit that is measured by the galvanometer connected in the circuit. The current ranges of the order of microampere.

Photocells are devices used to measure the intensity of light in photographic exposure meters, astronomical photometers and film densitometers. Their popular uses included switching on street lights at dusk, in motion sensors for alarms, automatic door openers.

http://www.youtube.com/watch?v=v5h3h2E4z2Q&feature=related

See other power point filehttp://www.youtube.com/watch?v=xLGOagKiXqg&feature=related

In 1913 the physics community was puzzled by the emission of radiation given out as specific

spectral lines by elements when heated. Bohr adapted quantum theory to explain the spectral

lines produced by hydrogen gas when the hydrogen atom was excited by heating gas in a

discharge tube. Bohr proposed that electrons moving around the nucleus of an atom existed in

stable energy levels or orbitals. The implications of the Bohr Model is that there were only stable orbital levels

out from the nucleus where electrons had a high probability of being located. Atoms of different elements have their stable orbitals at different energy

levels. To move to a vacancy in a lower energy state (orbital) in the Bohr model, the electron has to give out energy in the form of electromagnetic radiation in a single quantum of energy. To move to a higher energy state an electron has to gain energy by absorbing a quanta of energy in the form of a photon of Electromagnetic energy.

hf

http://www.youtube.com/watch?v=wCCz20JOXXk&feature=related

This image shows the energy jumps from the electron that produces these 3 colors. The highest frequency (dark blue) is produced when an electron drops from

orbit 5 (n=5) down to orbit 2 (n=2). The turquoise blue (middle) is one were the electron drops from orbit 4 (n=4)

down to orbit 2. The lower energy light (red light) happens when the electron jumps from orbit

n=3 to orbit n=2.Q1. What if an electron dropped from n=5 down to n=1?Answer: Even higher energy is produced. i.e. Maybe u.v.

http://www.youtube.com/watch?v=Ic8OnvRonb0&feature=related

http://www.youtube.com/watch?v=8TJ2GlWSPxI&feature=related

Explains electron jumps (No audio)

Explains electron jumps (with audio)

The Rutherford-Bohr model provided the first really useful view of the atom. It matched what scientist knew about chemical reactions and the way atoms behaved. It led to some predictions that were later proven correct.

Bohr had corrected a serious flaw by recognizing that electrons had to be in orbits (energy states). But his analysis of the energy given off when an electron dropped from a higher energy orbit to a lower energy orbit didn't hold up for atoms bigger than hydrogen (the simplest atom, with only one proton and no neutrons). More work needed to be done on the model.

Questions:Q1. Describe what line emission spectra is.Q2. What was the major success of Bohr’s model of the atom?Q3. What was the problem with the Bohr model. What didn’t it

explain?Q4. When an electron jumps from a lower energy orbital to a higher

one does it gain or emit energy? Q5. When an electron jumps from a higher energy orbital to a lower

one does it gain or emit energy?

Answers: A1. When an element is excited, the element emits light of

definite wavelengths giving rise to a series of lines corresponding to specific wavelengths (colours). These lines are like a fingerprint.

A2. Bohr was able to derive two expressions from his model.

The first relates the radius of the electron orbit to the quantum number n and the second expression relates the energy of the orbit to the quantum number.

A3. The problem with the Bohr model was how to explain why the negatively charged electrons had stable orbitals & simply didn’t spiral into the positive nucleus of the atom because of attraction.

A4. Gain energyA5. Emit energy

Improving Rutherford-Bohr Model A German scientist, Erwin Schrodinger, thought the problem might be in confining the electrons to specific orbits. Other scientists had developed the idea that electromagnetic energy acted like a wave sometimes and like a particle at other times. Schrodinger thought that electrons might work the same way. How useful was Schrodinger's idea?Schrodinger's idea, and the equations he used to predict where electrons would be, solved problems that Bohr's couldn't. It also gave scientists a better understanding of the electron and how it behaves in chemical reactions. Schrodinger's understanding of the nature of electrons also led to research in semiconductors and other technologies on an atomic scale. Despite its technical flaws however, the Rutherford-Bohr model is still useful because it is simple and helps people understand atomic structure.

Schrodinger Model

http://www.youtube.com/watch?v=njGz69B_pUg&feature=related

History of the atom model (9min)

In 1924 Louis De Broglie explained that electrons behave with a wave-like character in their passage

around the nucleus. He hypothesised the electron was in a stable orbital when their passage around

nucleus was in a pattern like a standing wave. The standing wave pattern meant that the electron’s

orbital path about the nucleus had to be a whole number of wavelengths long. At intermediate

distances between orbitals the electrons in their wave like pattern were able to interfere & so were unstable

in that position in a condition like interference of any other type of wave out of phase. Only the standing wave orbitals were stable with respect

to the nucleus. De Broglie referred to these waves as matter waves.

http://www.youtube.com/watch?v=lDYMuzo40LU&feature=mfu_in_order&list=UL

Answer: The model of the De Broglie atom. Note that the number of wavelengths in an orbit is equal to the quantum number n. In the third energy level there are three wavelengths. (The electron doesn’t actually follow the wavy line).

Which diagram does not show a stable orbital for an electron?

State whether each statement is true or false.

Statement T/FA Each orbit around the nucleus represents an energy

level. B Electrons can exist in between orbits. c If energy is added, an electron can be "excited" to jump

to a higher energy level-an orbit closer to the nucleus. d An electron will return to its original state, and the atom

will absorb energy equal to the difference between the two orbits.

e In some materials, energy is given off as X-rays; other materials produce specific colors of visible light, or other types of electromagnetic energy.

f Each orbit can hold only a certain number of electrons. g The lower-energy orbits must fill up first, if the atom is

to be at its "ground" state. h Louis De Broglie explained that electrons behave with a

cloud-like character in their passage around the nucleus.

i Orbits closer to the nucleus have lower energy. j The standing wave pattern meant that the electron’s

orbital path about the nucleus had to be a whole number of wavelengths long.

State whether each statement is true or false.

Statement T/FA Each orbit around the nucleus represents an energy

level. T

B Electrons can exist in between orbits. Fc If energy is added, an electron can be "excited" to jump

to a higher energy level-an orbit closer to the nucleus. F

d An electron will return to its original state, and the atom will absorb energy equal to the difference between the two orbits.

F

e In some materials, energy is given off as X-rays; other materials produce specific colors of visible light, or other types of electromagnetic energy.

T

f Each orbit can hold only a certain number of electrons. Tg The lower-energy orbits must fill up first, if the atom is

to be at its "ground" state. T

h Louis De Broglie explained that electrons behave with a cloud-like character in their passage around the nucleus.

F

i Orbits closer to the nucleus have lower energy. Tj The standing wave pattern meant that the electron’s

orbital path about the nucleus had to be a whole number of wavelengths long.

T

Crystals are composed of an orderly three dimensional arrangement of atoms. In each

type of crystal structure a certain fundamental grouping of atoms is repeated indefinitely in

three dimensions. This is called a unit cell. There are several types of crystals:

Molecular crystals Infinite arrays – metallic crystals, - ionic crystals, - continuous covalent crystals.

Q1. Give 3 examples of each of the above types of crystals.Molecular Metallic Ionic Continuous

covalent

Molecular Metallic Ionic Continuous covalent

C6H12O6 Cu NaCl NylonBenzene Fe KI polymers

Sn MgSO4 Diamond, graphite

Metals are crystalline structures in which the atoms are arranged in a closely packed ordered array.

One way to explore crystal structure is to use diffraction of light. Australian born Lawrence

Bragg and his father William Bragg experimented with X-ray diffraction on crystals.

They proved the theory that the diffraction angle is related to the distance between the planes in a

crystal and the wavelength.

nλ = d sin Ө (students don’t need to know formula)

X-Ray diffraction is an important tool in crystallography though today neutrons and gamma rays are

used as well as light. In metallic crystals atoms are separated by distances of 10ˉ¹º m so waves with a

wavelength of this order are used.

Q1. Why can’t electrons be used to explore crystal structure? Q2. The technique of x-ray diffraction is able to measure what?Q3. Describe the phenomenon of diffraction.Q4. What is a crystal lattice?

Answers: A1. Electrons would interact with the electrons and nuclei

of the atoms making up the crystals.

A2. It measures the interatomic distances in the crystalline lattice & to analyse the geometrical arrangement of atoms in simple crystals. The X-rays penetrate several atomic layers of the crystal, and are reflected (scattered) from each layer, at certain angles.

A3. Diffraction is a wave characteristic where waves bend around objects. Bragg diffraction differs from ordinary optical diffraction from a grating in that the crystal is three dimensional and consists of successive layers of crystal planes separated by the interatomic spacing d.

A4. Consists of a regular arrangement of positive ions that are able to vibrate whilst keeping their positions.

Superconductors are diamagnetic materials

with unique properties caused by Macroscopic Quantum effects, i.e. quantum phenomena at ordinary length scales. The materials can conduct an electrical current without any losses and expel an external magnetic field, making a

permanent magnet levitate in free space above the surface of a superconductor. These unique properties can be used in a large number of applications, like for

example: laboratory magnets levitating trains magnetic resonance imaging energy storage transformers motors sensitive measurement instruments, fast electronics

The first superconductor was discovered in 1911 by Onnes. He cooled Hg with liquid He to 4K & found that

Hg lost all electrical resistance. He then continued research on other metals to discover their behaviour

at low temperatures. Onnes observed superconductivity in Pb & Sn (also at 4K) in

1913. Which was exciting because these metals could be drawn into wires & be used to produce intense

magnetic fields without the need to use electromagnets with iron cores. Below is the resistance curve for

Hg as it approaches 4K. Q1. What is occurring at -269ºC?

Temperature (°C)-269 -267-271-273

Res

ista

nce

Hg has lost its resistance

However these 2 metals couldn’t carry the required electrical currents. 50 years passed before a

niobium-3-tin or niobium titanium alloy were made that could carry the current size. However the expense of using a 4k cooling temperature still was a problem. The search

begun for materials that would operate at higher temperatures. He being the coolant of choice however may run out as it is extracted from natural gas wells

& once in the atmosphere it can escape the earth’s gravitational pull & ends up in space. In 1986 Bednorz & Muller at IBM labs discovered a group of ceramics that were superconductors at much higher temperatures. In 1988 superconductors composed of thallium, barium, calcium, copper & oxygen that operate at even higher temperatures was discovered.

Q1. What is superconductivity?Q2. What was the first metal to be discovered that demonstrated this phenomenon? Q3. Why is it desirable to make superconductors that operate at room temperature? Q4. Superconductors allow electrons to flow ....................... Q5. Define diamagnetic. Q6. Give examples of elements that are diamagnetic?

A1. Superconductivity is the phenomenon exhibited by certain conductors where they have no resistance to current. A2. MercuryA3. So energy is not used to reduce the temperature to such low values. A4. unimpeded. A5. Diamagnetism is the property of an object or material which causes it to create a magnetic field in opposition to an externally applied magnetic field.Unlike a ferromagnets, a diamagnet is not a permanent magnet. Diamagnetism is believed to be due to quantum mechanics and occurs because the external field alters the orbital velocity of electrons around their nuclei, thus changing the magnetic dipole moment. Superconductors are diamagnetic.

Discoveries made recently have raised superconducting temperatures to a much higher value. Scientists at the University of Houston

first synthesised a ceramic compound

containing Yttrium, Barium, Copper and Oxygen

which becomes superconducting at 93K [-

180°C].Its chemical formula is YBa2Cu3O7. The diagram shows the sudden disappearance of the resistivity of

YBa2Cu3O7 on cooling the sample. Other ceramic compounds containing copper also give

high transition temperatures. HgBa2Ca2Cu3O8+d discovered in

1993 shows superconductivity at 160K [-

110°C].

Liquid Nitrogen can be used for freezing and

preserving blood and food. Nitrogen is a gas

that does not burn and is used as a blanket to

prevent fires. Helium boils at 4K and is used to

reduce materials like Niobium and Tin to 4K

where they have no electrical resistance and can

carry a large current without getting hot. Liquid He is 10x more expensive to

produce then liquid N & is a finite resource.

Q1. State a disadvantage of using Superconductivity.

Boiling Point (ºC)

Boiling Point (ºK)

Oxygen

-183 90

Argon -186 87Nitrogen

-196 77

Neon -246 27Hydrogen

-253 20

Helium

-269 4

A1. Expensive to produce the liquid He necessary to cool the superconducting materials. And He is finite.

Type 1 Superconductors

Critical Temperature

(k)AluminumLeadMercury 4TinTitaniumTungstenZinc 0.85

Type II Superconductors

Critical Temperature

(K)Bi2Sr2CuO6 0 - 110KNb3GeLa2BaCuO 20 – 40Nb3AlNb3GaNb3SnNbTiHBaCuOTlTl2Ba2Cu2O5 80 - 125

Superconductors are generally divided into two categories; Low-Temperature Superconductors (LTS or type I), and High-Temperature Superconductors (HTS or type II). LTS must be cooled to just above absolute zero (-269°C) and can be utilised as storage devices that provide power conditioning and power backup and are used by some electricity utilities. HTS only have to be cooled to -173°C & are preferred to LTS technologies as they do not have problems of maintaining such low temperatures.

Examples of Superconductors Type I & II

Type 1 Superconductors

Critical Temperature

(k)

Aluminum 1.2Lead 7Mercury 4Tin 4Titanium 0.4Tungsten 0.015Zinc 0.85

Type II Superconducto

rs

Critical Temperature

(K)Bi2Sr2CuO6 0 - 110KNb3Ge 23.2La2BaCuO 20 – 40Nb3Al 18.7Nb3Ga 20Nb3Sn 18NbTi 10HBaCuOTl 90Tl2Ba2Cu2O5 80 - 125

Answers: Superconductors Type I and II

Q1. How are Type I & II different from each other? (Contrast)A1. Type I are common elements whereas type II developed in 1980’s are compound class based on copper oxide also called intermetallic or ceramic superconductors. Type I need extremely low temperatures in order to superconduct whereas Type II critical temperatures are higher.

Definition: The temperature at which superconducting material loses its resistance to the flow of electrical current.

Activity: Complete the questions in surfing book on page 44-45

Question: Which coloured line represents a normal metal conductivity and which a superconductor?

The lattice structure of a superconductive material stops it’s vibrating at below critical temperature. When this happens the

passage of the free electron’s is unhindered. But when electrons travel through the

crystal it induces vibrations of the ions in the lattice

as it passes. The positive ions move towards the

negative electrons as they pass. But because the

electrons move so fast they are gone before the

heavier ions respond. This results in a momentary

concentration of positive charge behind the first electron.

This attracts the approaching electron causing it to

accelerate forward. Q1. What does BCS stand for?Q2. The electrons travel through the crystal

in …….. called …………………..Q3. What happens to the positive ions in the

lattice as the electrons zoom past? Q4. What are Phonons?

A1. A theory named after John Bardeen, Leon Cooper, Robert Schrieffer who proposed an explanation in 1957 of why materials lose all resistance & become superconductors at their critical temperatures. They shared the Nobel prize in Physics in 1972. The theory states that single electrons do not carry an electric current in a superconductor, but paired electrons do.

A2. In pairs called Cooper pairs. The lattice vibrations force the electrons to pair up into teams that pass all of the obstacles that cause resistance in conductors at normal conditions.

A3. Phonons are packets of inaudible sound waves (vibration energy). This vibration passes from ion to ion in the crystal lattice until the other electron of the pair absorbs the vibration. The force exerted by the phonons overcomes the electrons repulsion. This means the 1st electron of the pair emits a phonon & the second absorbs that phonon. This exchange of energy keeps the paired electrons together & in the same quantum state.

Complete the diagram of the BCS model of how electrons are moved through a superconductor in

pairs. Label positive ions, electrons, area of distortion.

Activity: Complete questions in surfing book on page 46

A very interesting property of superconductors is their

ability to "push" magnetic flux out of itself.

Contrary to popular belief, Faraday's Law of induction

alone does not explain magnetic repulsion by a superconductor.

At a temperature below its Critical Temperature, Tc, a

superconductor will not allow any magnetic field to freely

enter it. This is because microscopic magnetic dipoles

are induced in the superconductor that oppose the

applied field. This induced field then repels the source of

the applied field, and will consequently repel the magnet

associated with that field.

This implies that if a magnet was placed on top of the

superconductor when it was above its Critical Temperature, then it was cooled down to below

Tc, the Superconductor would then exclude the

magnetic field of the magnet. This can be seen quite clearly as

the magnet is repelled, and thus levitates above the

superconductor.

This magnetic repulsion phenomena is called the Meissner Effect and is named after the person who first discovered it in 1933. It remains today as the most unique and dramatic demonstration of the phenomena of superconductivity.

Activity: Complete questions in Surfing book on page 49

This diagram demonstrates The Meissner Effect acting. The magnetic field lines are shown for a superconductor (the circle) at two different temperatures (T>Tc and T<Tc)

Magnet placed on superconductor

Superconductor Temperature taken below Tc

Microscopic magnetic dipoles are induced in the superconductor that oppose the applied field (from permanent magnet). The Superconductor exudes a magnetic field around it.

This induced field then repels the source of the applied field. The magnetic fields exclude one another.

This allows the small magnet to levitate above a superconductor

The applications of superconductors are diverse. They are being used to improve the efficiency of motors, generators, transmission lines, transformers, and energy storage technologies.

Using an integrated power system incorporating superconductor technologies can reduce the amount of power that is needed to be generated to supply the same demand, as well as allow many new applications, such as magnetic levitating trains.

Superconductors can be used in various ways:

1. Power Transmission …. superconductors can reduce energy losses. 2. Transformers …. superconductors can produce very strong

electromagnets.3. In computers … superconductors produce increased operating

speeds. 4. Electric Motors … superconductors can reduce the size of the motor. 5. Levitate Trains …. superconductor can levitate trains above the

tracks. Activity 1: Complete tableActivity 2: Complete questions in surfing book on page 51

1. In power transmission superconductors can reduce energy losses. A current once initiated in a loop of superconductive material will run

without any applied voltage for years. Lower resistance of the super conducting wire would mean the length of the

coil could be extremely long with no energy losses. The result would be a coil with more windings hence a greater magnetic effect.

2. In transformers superconductors can produce very strong electromagnets.

Having a current so large in the conductor that there would be no need for an iron core to magnify the magnetic effect, as the magnetic effect of the current alone would be able to produce a big enough magnetic field.

3. In computers superconductors produce increased operating speeds. Since the electrons undergo no resistance the speed they travel through

the wire is very fast.

4. In electric motors superconductors can reduce the size of the motor. Less windings are needed to carry the same current & electromagnets are

unnecessary.5. Levitate Trains Using the Meissner effect.

Advantages of Using Superconductors

Activity 2: Complete the questions from surfing book on page 53

Application Benefits Explanation

Application

Benefits Explanation

In power transmission

Superconductors can reduce energy losses.

Lower resistance of the super conducting wire would mean the length of the wire could be extremely long with no energy losses. A current once initiated in a loop of superconductive material will run without any applied voltage for years.

In Transformers

Superconductors can produce very strong electromagnets.

Having a current so large in the conductor that there would be no need for an iron core to magnify the magnetic effect, as the magnetic effect of the current alone would be able to produce a big enough magnetic field. Transformers become compact, quiet, and use no cooling oil, so they are much more environmentally acceptable for utility substations located in high-density urban areas. They can even be installed within large commercial buildings.

In computers

Superconductors produce increased operating speeds.

Since the electrons undergo no resistance the speed they travel through the wire is very fast.

In Motors & Generators

Superconductors can reduce the size and weight of the motor & generator & increase efficiency.

Wire is made of superconductor material therefore less windings are needed to carry the same current & electromagnets are unnecessary. So motor will be more efficient.Generators will use superconducting wire in place of iron magnets, making them smaller and lighter. They may also get more power from less fuel.

Levitate Trains

Less wear & tear of wheels, fast speeds.

Using the Meissner effect can make train hover above the rails.

In trains the superconductor can levitate

trains above the tracks. These trains are called MAGLEV TRAINS. Huge currents are passed through superconducting coils to create the electromagnets that

interact with the magnets in the track. This allows high speeds of 500km/h as there is no friction between tracks and the wheels.

The Maglev doesn’t even have wheels.

Q1. How is a Maglev Train stopped?

Q2. Draw a diagram & explain how the train hovers above the rail system & how the train is prevented from smashing into the sides of the rail system.

Q3. Discuss the efficiency of the Maglev Train.

Maglev Trains

http://www.youtube.com/watch?v=iaElPV0FWJ0&feature=related

http://www.youtube.com/watch?v=n_yXUcPbyss&feature=related

Answer Q2: How do Maglev’s stay on the Rails?

The train itself is equipped with several superconductors, while a series of electromagnetic coils run along the length of the track. When the train approaches these coils, the superconductors induce a current in them that works to both levitate the train several centimeters above the track and to center it between the guide rails.

A second series of electromagnetic coils, which run alongside the

levitation/guidance coils. After the train reaches a certain speed, these propulsion coils kick into gear. They receive a constantly alternating electric current that changes the polarity of the coils in such a way that they are always arranged to push or to pull the onboard superconducting magnets of the passing train. In essence it’s a motor – not a circular one, like the one in your car, but linear, running the length of the entire track. The beauty, though, is that only the coils that are in the vicinity of the moving train at any point in time need be engaged.

The beauty of maglevs is that they travel on air. The consequent elimination of friction means much greater efficiency. Just as electrons move more efficiently through a superconducting wire because there is no resistance, so, too, does a maglev travel more efficiently than a regular train because there is no friction between the wheels and the track, thanks to the Meissner Effect.

Superconducting Magnetic Energy Storage (SMES) is a new technology that is used to regulate power fluctuations and maintain the stability of the grid when large changes in load occur.

SMES systems store energy in a magnetic field created by the flow of direct current in a coil of superconducting material that has been cryogenically cooled.

A superconducting material enhances storage capacity. In low-temperature superconducting materials, electric currents encounter almost no resistance. The challenge is to maintain that characteristic without having to keep the systems quite so cold.

(Superconducting Quantum Interference Device) is a very sensitive magnetometer used to measure extremely weak magnetic fields, based on superconducting loops containing Josephson junctions, SQUID is useful in wide area of research from biology to experimental physics.