Physics Notes

4th Form Physics notes (AG)

LightStatement: Light travels in straight lines. This is known as ‘rectilinear propagation’.Evidence: The formation of shadows provides evidence that this is the case.

Light and shadowsLight travels out from a light source as millions of rays. In the diagram below, some of the rays are prevented from reaching the screen by the opaque object and a shadow is formed.

The pinhole cameraThis provides more evidence for rectilinear propagation. The image is inverted. A small pinhole produces a sharp image, but also a dim one. A larger pinhole produces a brighter, less sharp image. Multiple pinholes produce multiple images.

ReflectionMirrors reflect light rays. We can use a ray box to investigate what happens to a ray when it reflects.

NORMAL – Line drawn at 90o to the mirrorANGLE OF INCIDENCE – angle between normal and incident rayANGLE OF REFLECTION – angle between normal and reflected ray

Object Shadow

LAWS OF REFLECTION:

1. Angle of incidence = angle of reflection2. Normal, incident, and reflected rays are in the same plane

Remember to label the direction of light with arrows.

Refraction Light travels at different speeds in different MEDIA. This can cause it to change direction. When light enters an ‘optically denser medium’, it slows down and turns towards the normal. When light enters a less dense medium, it speeds up and turns away from the normal. When the incident ray is along the normal, it does not change direction – but it does change speed. Glass, Perspex, and water are optically denser than air.

Snell’s lawThe REFRACTIVE INDEX (n) is a measure of the optical density of a material. The refractive index of a vacuum or air is 1. The greater the value of n, the slower the light travels in the material.

For light travelling from air into another material, the relationship between refractive index,

angle of incidence and angle of reflection is: sin i

sin r . . n is constant for light passing from air to

a given medium. This is known as Snell’s Law.

Total Internal ReflectionFor light passing from glass to air, there is a strong reflected ray, and a weak reflected ray. As the angle of incidence increase, the angle of refraction increases until the refracted ray travels along the boundary. This angle of incidence is called the CRITICAL ANGLE. If the angle of incidence is greater than the critical angle, ALL the light is refracted.

The refractive index is related to the critical angle according to the following equation:

Sin c = 1n

Optical fibresOptical fibres are fine strands of glass. The fibre is covered with ‘cladding’ that is less optically dense than the fibre to ensure Total Internal Reflection (TIR) takes place. There is usually a thin protective coating around the cladding.

Glass fibres can be used to transmit data using digital signals. Light entering the end of the fibre undergoes TIR.

No (or very little) energy is lost so as much light leaves the fibre as enters it. Optical fibres are also used in ENDOSCOPY. One bundle of fibres takes light inside the body to illuminate it. A second bundle carries the image out to the surgeon.

ImagesIn a real image, rays from the object pass through it; hence it can be projected onto a screen. In a virtual image, no rays from the object pass through it; hence it cannot be projected onto a screen.

Properties of the image in a plane mirror Virtual – no rays pass through it (so it can NOT be projected onto a screen) Same size as the object Laterally inverted – right is left, but top is still top As far behind the mirror as the object is in front (and a line joining the object and

image crosses the mirror at 90o)

Volume and densityThe quantity of space occupied by an object is called its VOLUME. The SI unit for volume is m3. This is a large volume so we often use cubic centimetres (cc) or litres (1000 cc) instead. The volume and mass of an object are related by the property called density.

Density = Mass

Volum e

In symbols: D = MV

Density is measured in kg/m3, mass is measured in kg, and Volume is measured in m3.

Forces – prep school information1. A force can be thought of as a push or pull of one body on another2. There are various types of force (e.g. gravitational, electrostatic, etc.)3. We use arrows to show the size and direction of forces.4. Force is measured in Newtons (N)

Forces – new information5. If the forces of an object are balanced, it does not accelerate or decelerate. The

RESULTANT force is zero. 6. If the forces are NOT balanced, it will accelerate or decelerate. The RESULTANT

force is NOT zero.

Weight, mass, and gravityThe earth’s gravitational field acts on all objects close to the earth. The resulting force towards the centre of the earth is called the WEIGHT of that object.

The weight of an object depends on its MASS, and also on the gravitational field strength (g). Smaller planets have weaker gravitational fields so things weigh less on smaller planets, even though the mass is the same.

Mass is the same everywhere in the universe, but weight is not.

Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)

W = m x g

‘g’ is the symbol for gravitational field strength. On earth, g = 9.8 N/kg (9.81…)

Pressure

Pressure = ForceArea

Pressure can be measured in N/m2 or Pascals. A force distributed over a large area creates lower pressure than the same force acting

on a small surface area. If objects are said to be ‘blunt’, they really have a larger surface area than a ‘sharper’

object with a smaller surface area.

In Physics, we usually assume that forces act at points. In real life, this never happens. Forces are always spread over areas. The force divided by the area is PRESSURE. The unit of pressure is N/m2 or Pascal. Pressure (N/m2) equals force (N) divided by area (m2).

P = FA

A high pressure is applied when a force is concentrated on a small area. A lower pressure is applied when a force is spread out over a large area.

Stretching springsWhen a force is applied to a (helical) spring, its length increases. For most materials, the change in length is PROPORTIONAL to the force. This means that:

If you double the load, the extension is doubled The graph is a straight line through the origin

When the force is removed, the spring returns to its original length. This is ELASTIC BEHAVIOUR.

Eventually, the spring (or length of wire or rubber band) reaches its elastic limit and equal increases in force produce reducing increases in length. This is INELASTIC (PLASTIC) BEHAVIOUR. Up to the elastic limit:

F = kx

Where F = Force, x = extension (=current length - original length). k is the spring constant1 and equals the gradient of the F-x graph.

This is Hooke’s Law for an elastic object [springs and wires]: Load is proportional to extension.

Brownian motionQ: What causes pollen grains to move?A:

Water is made up of particles (atoms and molecules) Water molecules are too small to see They are in RANDOM motion They collide with the pollen grains These are larger and visible This causes BROWNIAN MOTION

Structure of matterThere are three “states” of matter. These are SOLID, LIQUID, and GAS. The KINETIC THEORY helps us understand how solids, liquids, and gases behave. This states that matter is made up of tiny particles in constant motion {only at Absolute Zero will particles not be in motion}.

Changes of stateSolid to liquid – meltingLiquid to gas – evaporation or boiling

Solids and liquids – similarities and differencesParticles in a liquid have a random motion within a close-packed structure. Particles in a solid vibrate about fixed positions within a close-packed regular structure.

Types of motionVIBRATION / OSCILLATION – Motion to and fro about a fixed positionTRANSLATION – motion between two distinct positions

1 This is essentially the stiffness of the spring. It is measured in N/m, i.e. how many Newtons of force it takes to extend the spring in question by a metre.

ROTATION – CIRCULAR MOTION about a NOMINAL position.

Static ElectricityElectrical conductors allow electrons to move about freely within them. Metals are good conductors. Insulators do not allow electrons to move about freely. Plastics are insulators. Insulating materials can be charged by FRICTION. Positive means there are fewer electrons than positive charges. The material has LOST electrons. Neutral means there are equal numbers of electrons and positive charges (earth is neutral). Negative means there are more electrons than positive charges. The material has GAINED electrons. LIKE charges repel, ad UNLIKE charges attract. REMEMBER – FOR THE TOPIC OF STATIC ELECTRICITY, ONLY ELECTRONS MOVE (in solids – unlike liquids in electrolyte where positive and negative charge).

Practical uses of electrostatic chargeCharge can be supplied by the electricity supply, instead of by friction. Practical uses include:

1) Photocopying [aka xerography]

Toner (powdered ink) is attracted to charged areas on a drum This is then transferred to paper and heated

2) Electrostatic precipitators

Ash receives negative charge (from negative wires) as it rises It is attracted to positive plates

3) Inkjet printing4) Paint spraying

Dangers of electrostatic chargeA very large charge may cause sparks and this can be dangerous, e.g. when refuelling an aircraft. Charge can build up due to friction between the fuel and the pipe so the aircraft and the tanker must be earthed2 during fuelling to avoid sparks.

Shuttling ball experimentA ping-pong ball is coated in conducting paint. It carries negative charges (free electrons) from the cathode (left-hand plate) to the anode (right-hand plate). The ball then shuttles to and fro at a steady rate. If this rate is big enough, the microammeter shows a steady current reading. The higher the rate of shuttling, the larger the current, as more charge is transferred

2 Connected with a large neutral body such as the Earth to neutralise it.

per second. This experiment shows that an electric current (I) is the rate of flow of charges (Q). Hence, current can be defined as the rate of flow of charge.

Current (I) = C harge(Q)

time(t )

Charge (Q) = Current (I) x time (t)

Units

Current – ampere – remember 1A = 1000 mACharge – CoulombsTime – seconds

Measuring current and voltageCurrent is measured by an ammeter in series. An ideal ammeter has no resistance. Voltage is measured using a voltmeter in parallel. An ideal voltmeter has infinite resistance. Which is a) easier to measure and b) why? [It is therefore easier to measure voltage because, since it is measured in parallel, it is not necessary to disconnect anything.]

Current rule (Kirchhoff’s first law)The total CURRENT flowing into any JUNCTION in a circuit is equal to the total current flowing out of the junction. This explains why the current is the same at all points in a series circuit.

Voltage rule (Kirchhoff’s second law)The battery voltage is equal to the sum of the other voltages around any LOOP / ROUTE / PATH in a circuit. This explains why:

Voltages around a series circuit add up to the battery voltage Voltage is the same across components in parallel

5th Form Physics Notes (AG)

Calculating Speed

Average speed = Totaldistance (m)

Total time (s)

Speed = Distance

Time

Speed is a SCALAR quantity because it only has magnitude. Velocity is a VECTOR quantity because it has magnitude and direction.

Mass is scalar. Weight, as all forces, is vector.

AccelerationAcceleration is a vector quantity – it has magnitude and direction. Deceleration (or retardation) is a negative acceleration.

Avge acceleration = C h ange∈velocity

time taken

We sometimes simplify this to: Acceleration = C h ange∈speed

time taken

a = v−u

t

V = FINAL VELOCITYU = INITIAL VELOCITY

[Don’t use‘s’ for speed because it actually represents ‘displacement’]

Unit of acceleration is: m/s/s (not technically correct but ok in exam) OR m/s2 OR m/s-2.

Displacement-time graphs Straight line means constant velocity Slope (i.e. gradient) = velocity Curve means acceleration (or deceleration) Instantaneous velocity is the slope of the tangent at that point

Velocity-time graphs Straight line means constant acceleration Slope = acceleration Curve means acceleration is changing Area under curve/line = distance travelled

N.B. Displacement (s) = distance as a vector quantity.

Newton’s 2nd Law of MotionWhat are the 2 things that affect the (rate of) acceleration of an object? Resultant/unbalanced force and mass.

Force (N) = Mass (kg) x Acceleration (m/s/s)

F = ma

If the UNBALANCED (i.e. resultant) force is zero, then the object will NOT be accelerating. If the unbalanced force is in the direction of motion, it will accelerate. If the unbalanced force is in the opposite direction to the motion, it will decelerate. Why does a ball decelerate on the way up and accelerate on the way down? Because the resultant force (i.e. weight and some not very significant air resistance) is in the opposite (going up) / same (going down) direction to the motion.

For a constant mass, (unbalanced) Force is proportional to acceleration. The gradient of a force-acceleration graph is equal to the mass.

If the unbalanced force is constant, mass is inversely proportional to acceleration.

Stopping distance for carsStopping Distance = Thinking Distance + Braking Distance.

Thinking distance is the distance travelled during the reaction time (i.e. before the brakes are applied). It is affected by speed and also by age, drugs, alcohol3, distractions, tiredness, etc. Braking distance is the distance travelled after the brakes are applied. It is affected by mass and speed and also by road conditions, design and maintenance of the brakes, tyres, etc.

N.B. Note spelling of BRAKING.

Gravitational Potential Energy (gPE)The change of gPE of an object depends on its mass (m), the gravitational field strength (g), and the change in height (∆h): ∆gPE = mg∆h (Joules)

Kinetic EnergyThe Kinetic Energy of an object depends on its mass (m) and on its velocity (v): KE = ½.m.v2

Energy Transfers and the Principle of Conservation of EnergyEnergy cannot be created or destroyed, but it can be transferred into other forms. There are many situations where KE is transferred into gPE and vice versa. If the amount of one form of energy reduces, then the amount of other types of energy must change by an equal amount. This is because the total amount of energy remains constant.

Calculations involving KE/PE transfersIn problems involving gPE/KE transfers, the key principle is that:

The total energy remains constant

3Although technically alcohol is a drug

If the only energy transfers are between gPE and KE:Reduction in gPE = gain in KEOrreduction in KE = gain in gPEor generally∆gPE = ∆KE

Worked ExampleA pendulum of mass 0.1kg is released from a height of 0.1m above its lowest possible point.

a) What is the reduction in gPE between release and its lowest position?

∆gPE = mass x g x ∆h = 0.1 x 10 x 0.1 = 0.1 Joules

b) What is the maximum speed?

KE is max when gPE is min∆KE = ∆gPEKEmax is when reduction in gPE is greatestKEmax = 0.1 JoulesSpeed is max when KE is maxKEmax = ½m v2

0.1 = 0.5 x 0.1 x v2

v = 1.4m/s

c) What is the speed when the height is 0.05m below the high point?

∆gPE = ∆KE = 0.5 x 0.1 = 0.05 Joules = ½m v2

0.05 = 0.5 x 0.1 x v2

v = 1m/s

WORK – GET SOME DONE!Work is done whenever energy is transferred. In fact work done is EQUAL to the energy transferred.

Work done (Joules) = Energy transferred (Joules)

GCSE calculations involve MECHANICAL work done. Mechanical work is done when a force moves.

Work done (J) = Force (N) x distance (m)W = F.dKE is the work done when…Change in gPE is the work done when…

The KE of a body is the work done when the body is accelerated to a velocity v (from rest). The increase in gPE of a body is the work done in raising the height of a body.

POWER - …but don’t take all day!The faster a car climbs a hill, the more power it consumes. Power is the rate of doing work. It is also the rate at which energy is transferred.

Power = Work donetime taken

= Energy transferred

time taken Energy transferred/time taken

Unit of Power is the Watt (W), 1W = 1J/s

[The equation can be written: P = Wt

. This is OK for GCSE but should really be written in

full].

Work, Energy, and Stopping DistancesThe work done by the brakes is equal to the Kinetic energy transferred (to heat). Work done = Braking force (F) x braking distance (s)KE transferred = Initial KE – Final E = ½mv2 – 0F.s = ½mv2

Efficiency

Efficiency = (useful) Energy output

(total ) Energy inputOr

Work outputWork input

Or

Power outputPower input

Efficiency has no units but can be expressed as a %.

It must be less than 1 (or 100%).

Centre of gravity, stability, and “tipping” and “toppling”The weight of a body acts through its centre of gravity. The stability of an object is related to the angle through which it is “tipped” before it “topples”. Increasing the size of the base and/or reducing the height of the centre of gravity will make an object more stable (because

this increases the angle). An object will not “topple” (i.e. is stable) providing the centre of gravity is above a point on the base.

MOMENTSDefinition:The moment of a force about a point depends on the force and the perpendicular distance from the force to that point.

Moment of a force (N.m) = force (N) x perpendicular distance to point (m)

Principle of Moments:If an object is balanced (i.e. in equilibrium)…Sum of clockwise moments = sum of anticlockwise moments.This is the principle of moments.

Moments calculations – presentationIs the body in equilibrium?If so, sum of the clockwise moments = sum of anticlockwise moments.Where are you taking moments about?

IN EQUILIBRIUM

MOMENTS = MOMENTS MOMENTS ABOUT X

E.g.

In eqbm. M = M

(F1 x d1) = (F2 x d2)

Current and Voltage in circuitsCurrent is the rate of flow of charge:

Perpendicular distance

I = Qt

(I is in Amps, Q in Coulombs, t in seconds).Voltage is the energy transferred per coulomb of charge:

V = EQ

(V in volts, E in joules, Q in coulombs. I = Current in Amp(ere)s (A)

Current rule (Kirchhoff’s 1st Law)The total current flowing into a junction is equal to the total current flowing out of the junction.

Voltage rule (Kirchhoff’s 2nd Law)The supply voltage is equal to the sum of all the other voltages around any loop in a circuit.

ResistanceResistance reduces the flow of charge in a circuit. The greater the Resistance, the less the current (for a particular voltage). Resistance in a metal wire is caused by collisions between moving electrons and stationary atoms. These collisions are the means of energy transfer (or power consumption). Without resistance, no energy is transferred. Longer wires have more resistance than shorter wires. Thicker wires have less resistance than thinner wires. The resistance increases if the temperature increases. The units of resistance are OHMS. We calculate resistance from the formula:

R () = V (V )I ( A)

R

V

I

A

V

Bulbs, wires, resistors and diodesIn a wire at constant temperature (or a resistor), current is proportional to voltage and the resistance is constant. A thinner, longer wire has more resistance. In a filament lamp, the current is not proportional to voltage – resistance increases as voltage increases. Diodes allow current to flow in one direction only (+ to -).

LEDs and Thermistors Semiconductor devices. More energy on device reduces Resistance (increases current).

V

I

V

Filament bulb Resistor Diode

R

T

R

L

II

VV

PowerPower is the rate of energy transfer. Electrical Power is:

Power = Voltage x CurrentP (W) = V (V) x I (A)

Household ElectricityA direct current is always in the same direction but an alternating current changes direction. The electricity in our homes is AC with a frequency of 50 Hz.What do we pay for in our electricity bills?The answer is ENERGY. Electricity companies measure energy in Kilowatt hours (kWh).Energy = Power x timeEnergy in Joules = Power in Watts x Time in secondsEnergy in kW.h = Power in kW x Time in hours

A

V

E.g. I = 2A V = 3V

R = VI

= 1.5

P = V.I = 6W

1 UNIT of energy is 1kWh. It is the energy converted when a 1kW appliance operates for 1 hour. The cost of electricity is simply the number of units consumed x the cost per unit.E = V.I.t

Electrical SafetyAppliances are fitted with FUSES as safety devices. The “size” of the fuse (in Amps) should be just above the normal operating current. Why is this?

Energy is supplied into our homes using a LIVE and a NEUTRAL wire. The live wire varies from +230 Volts to -230 Volts 50 times every second. The neutral is maintained constantly at zero Volts. Circuits are completed when the live and neutral wires are connected to appliances; current flows and energy is transferred. Why must the live and neutral wire be insulated from each other?A third wire is connected to “earth”. Usually, no current passes through this wire.Why do we have an earth wire which normally carries no current?

FUSES and CIRCUIT BREAKERS prevent FIRE due to electrical faults. Circuits can overheat when too much current flows. A fuse is designed to “blow” before overheating of cables, etc. can occur. A circuit breaker uses a simple electromagnet to switch off the current when it is too high. It can be reset.The earth wire together with a fuse (or circuit breaker) prevents ELECTROCUTION. If a metal casing becomes “live”, a very high current flows “to earth”. This blows the fuse and the appliance stops working.

DOUBLE INSULATED appliances don’t need an earth wire because any metal parts are completely surrounded by an insulating polymer.

Heat TransferWhen heat energy is transferred, the temperature of an object may change. Heat energy can be transferred by CONDUCTION, CONVECTION, RADIATION or EVAPORATION.

ConductionHeat causes atoms to vibrate and pass on their energy to neighbouring atoms. If there are free electrons, these can transfer the energy more rapidly by bypassing immediate neighbours.Why are metals good conductors and how are they different from insulators?

ConvectionWhen a FLUID is heated, it expands, becomes less DENSE and rises. Colder, denser fluid sinks to take its place. The process continues as convection currents are established.Why doesn’t convection happen in solids?

RadiationAll objects radiate heat energy (even very cold ones!) but the power radiated depends on the temperature.Radiated heat travels as electromagnetic waves (just like light) – it travels at the speed of light through a vacuum and can be reflected and focussed. Dull, black surfaces are good emitters and absorbers of heat. Shiny, white surfaces are poor emitters and absorbers.

What do the following facts tell us about heat radiation?

A) There is life on EarthB) Thermal imaging cameras can be used in the Arctic or the Sahara

Describe experiments to show that:

A) Heat can be reflected and focussed B) A white surface emits less heat than a black one

WavesAll wave motion involves OSCILLATION. Waves transfer energy but without any flow of material.

There are two types of waves:

TRANSVERSE waves have oscillations perpendicular to the motion of the wave (e.g. water [surface], light).

LONGITUDINAL waves have oscillations in the direction of motion of the wave (e.g. sound).

Describing wavesWAVELENGTH is the distance between any point on a wave and its equivalent point on the next wave.

AMPLITUDE is the maximum distance that a point moves from its resting position when a wave passes.

FREQUENCY is the number of waves passing any point each second. It is measured in Hertz (Hz). It is also the number of complete oscillations per second by a particle in the wave.

The amplitude depends on the energy of the wave.

The PERIOD of a wave (T) is the time for one complete wave to pass measured in seconds. It

is also the time for one complete oscillation (T = 1f

; f = 1T

)

Key Points from the MMSS ExerciseWavefronts are always at 90 degrees to the direction of movement of the waves (e.g. the rays for light waves).

The frequency of a wave is NOT affected by reflection or refraction.

The Wave Equationv = f

v is speed in m/s. f is the frequency in Hz and is wavelength in m. This equation works for all typed of wave.

Derive the wave equation starting with the formula for speed.

SoundSound travels as longitudinal waves through a medium – it cannot travel in a vacuum. Echoes are caused by the reflection of sound. Sounds travel faster in solids than in liquids or gases because the particles are closer together.Sound can be reflected, refracted and diffracted [not on syllabus] (just like any other wave) and it obeys the wave equation. The frequency range for human hearing is 20 Hz – 20 000 Hz.

How could you measure the speed of sound?

By measuring the time taken for a sound to travel a known distance, the speed can be

calculated (speed = distance

ti me). This can be applied to echoes (reflections of sound).

The Electromagnetic spectrum The different types of electromagnetic waves form a continuous spectrum with a

range of wavelength and frequency. They transfer energy at the same speed in free space [i.e. in a vacuum] They are all transverse waves which can be reflected, refracted and diffracted.

Electromagnetic waves – DangersElectromagnetic Waves carry energy and cause heating when absorbed. They can also be ionising and this can damage living tissue and cause cancer in the case of Ultraviolet, x-rays and gamma rays (which are very penetrating).Infra red and microwaves are more likely to cause burns when absorbed. Radio waves carry very small amounts of energy and are relatively safe.

Discuss/explain the differences between the harm caused by:a) UV – gamma b) Ir – microwaves

Some uses of e-m wavesX-ray machines use wavelengths that penetrate tissue but do not penetrate bone so “x-ray photographs” show broken bones and fractures. Concentrated beams of X rays and gamma rays can be used to treat cancer by destroying abnormal cells. Gamma rays can be used as “tracers” in medicine. They can also be used to sterilise food and medical equipment.

Why is it dangerous to spend too long in the sun?Why is darker skin less likely to be damaged?Why are very long wavelengths needed for long distance communication?How do microwaves get around this problem?What safety precautions should be taken when using X-rays and Gamma rays?

Radiowaves:

Frequency (Hz): < 109

Wavelength (m): > 0.3Size scale: Mountains, buildingUses:

Transmit Radio and TV programmes between different places. The longer wavelengths radiowaves are reflected from the ionosphere, an electrically charged layer in the Earth’s upper atmosphere

Dangers: No real danger, although too

much TV can make you ‘square eyed’

Microwaves:

Frequency (Hz): 109 - 3x1011

Wavelength (m): 0.001 - 0.3Uses:

Satellite communication, as they pass easily through the Earth’s atmosphere

Cooking, because microwaves are absorbed by water molecules, causing them to heat up

Dangers: Absorbed by water in

cells where heat is released my DAMAGE or KILL CELLS

Infrared:

Frequency (Hz): 3x1011 - 3.9x1014

Wavelength (m): 7.6x10-7 - 0.001Uses:

Grills, toasters and heaters Remote control for TV and

VCR’s Optical Fibre

communicationDangers:

Absorbed by skin and FELT as HEAT. Excessive amounts can cause BURNS

Visible:

Frequency (Hz): 3.9x1014 - 7.9x1014

Wavelength (m): 3.8x10-7 - 7.6x10-7

Scale size: BacteriaUses:

Seeing Optical fibre communication

Dangers: Excessive amounts can

damage the retina

Ultraviolet waves:

Frequency (Hz): 7.9x1014 - 3.4x1016 Wavelength (m): 8x10-9 - 3.8x10-7

Scale Size: VirusesUses:

Fluorescent lamp and security coding, where surfaces coated with special paint absorb UV and emit LIGHT

Dangers: Passes through skin to the TISSUES below. Darker

skin allows less penetration and provides more protection

HIGH DOES can KILL NORMAL CELLS and LOW DOSES can cause CANCER

X-rays:

Frequency (Hz): 3.4x1016 - 5x1019

Wavelength (m): 6x10-12 - 8x10-9

Scale size: AtomsUses:

Produce shadow pictures of BONES and METALS, materials X-rays do not easily pass through

Dangers: Pass through SOFT TISSUES,

although SOME is ABSORBED

Gamma rays:

Frequency (Hz): > 5x1019

Wavelength (m): < 6x10-12

Scale size: NucleiUses:

Killing cancer cells Killing bacteria on food and

surgical instrumentsDangers:

Pass through SOFT TISSUES, although SOME is ABSORBED

6th Form Physics Notes (GSM)

Pressure

P = FA

Units: N

m2or Pascals

Kinetic Theory of Matter SOLID LIQUID GAS

P

F

AS

AS

hS

Density of fluid = d

(d = mv

)

AS

P = FA

= WeightA

= m x gA

= d x V x gA

= d x A xh x gA

P = d x h x g

melt

break bonds

evaporation

or boiling

tight packvibrate fixed positionEk relatively low

further apartattraction lessEk highermove around

much further apartEk much higherforces are negligiblemove around at high speed

If the atoms are [totally] stationary, this is equivalent to Ek = 0, and a temperature of 0K, or -273oC is reached.

There is a range of kinetic energies in the atoms of a liquid. During evaporation, some of the fastest molecules leave the surface of the liquid.

At the boiling point, all of the particles are effectively evaporating, but at the same temperature.

Gas PressureThe atoms in a gas have a range of speeds and kinetic energies. All of the atoms are moving in random directions. When they hit each other and the walls of the container, they will exert a force, and change direction. The forces due to the individual atoms will be spread over an

area, and since P = FA

, the atoms of the gas will exert a pressure.

The pressure due to a gas is used in both external and internal combustion engines. The energy of the moving atoms is used to drive a piston which in turn can be used to turn the wheels of a steam engine, car, etc.

There are three macroscopic properties for a gas – pressure, volume, and temperature. In this experiment, the pressure and temperature variations are investigated, while the volume is kept constant. The graph is extrapolated to find the temperature at which the pressure would be zero, i.e. the molecules have stopped moving. Kelvin moved the pressure axis to -273oC, and redefined this as 0 Kelvin. The graph is now a straight line passing through the origin, and therefore:

P absolute temp PT

= constant

Boyle’s Law Experiment

Ruler to measure ‘volume’ of air

Oil

Fixed mass of dry air at constant temperature

To

compression pump

Bourdon Pressure Gauge to measure pressure of air

P x V = constant

P x 1V

= straight line through (0,0)

P 1V

P = constant x 1V

(0,0)

y = m x + c

Pressure inversely proportional to volume

P T (v constant); PT

= constant

P x V = constant (temp. constant)a third law states that

V T (P constant) VT

= constant

PV = constant; PT

= constant; VT

= constant

P x VT

= constant

Radioactivity and Nuclear PhysicsAtomic nuclei are made up of protons and neutrons. These comprise nearly all of the mass of the atom. The orbiting electrons have negligible mass by comparison. Nuclei are represented

by the X notation (A = neutrons + protons; Z = protons). Nuclei which contain the same

number of protons but different numbers of neutrons are called isotopes. Some of the isotopes of a given element will be unstable. To become more stable, they emit radioactive radiation –

The results show that within experimental error, P x V = constant, and the graph shows that

pressure 1

volume(This can also be stated pressure inversely proportional to volume)This is Boyle’s Law.

IDEAL GAS EQUATION

AZ

42

1

0

1

1

0

-1

P / x105 Pa V / cm3 1V

/ x10-2PxV

2.5 16 6.3 40.02.3 17 5.9 39.12.1 19 5.3 39.92.0 20.5 4.9 41.01.85 22 4.5 40.71.68 25 4.0 42.01.5 28 3.6 42.01.4 30 3.3 42.01.3 33 3.0 42.91.0 42 2.4 42.0

these include α, β, and γ radiation. α is a helium nucleus, He. β is an electron emitted

when a neutron turns into a proton and an electron. n p + eGamma radiation is a very high frequency electromagnetic wave. The emission of radioactive radiation always takes place in an attempt to improve the stability of the remaining nucleus.

Background RadiationThis is a totally random process and is present everywhere and comes from all directions. Some of the sources of background radiation are building materials (especially granite), cosmic radiation (from the Big Bang), the Sun, etc. Background radiation is greater in mountainous areas because of all the granite rocks. In radioactive experiments, the background count in Becquerels (Bq) should always be subtracted from the actual count to reduce the corrected count.

Half-lifeThe half-life of a radioactive sample is the time taken for half of the radioactive nuclei to decay.

This decay is totally random and the initial number of radioactive nuclei is immaterial. Its value can vary for different isotopes between fractions of a second and millions of years. The curve is called an exponential decay curve.

Rutherford Alpha Particle ScatteringPositively charged alpha particles were directed at a thin gold foil. It was expected that the alpha particles would pass straight through the foil. However, about 1 in 2000 bounced back in the original direction. The conclusion was that the atom was not solid as in the kinetic theory model, but was made up of a tiny positive nucleus (the electron shells are a very long way from this nucleus).

The nucleus is approximately 10-15m in size, the atom is about 10-10m, i.e. about 105 or 100,000 times bigger.

Geiger, Marsden, Rutherford Expt. (1909)

vacuum

gold foil

beam of

α particles

most pass straight through

zinc sulphidescreen

Nuclear EnergyNatural radioactive decay energy, but at a slow rate. In a nuclear reactor, the rate of decay is accelerated by bombarding nuclei with other particles, typically neutrons which are not charged. E.g.:

U + n U Ba + Kr + 2 n + energy

Spontaneous fissions starts reaction Process called a chain reaction

NB: U must be above a certain critical mass else too many neutrons escape

without causing further fission

NB: natural uranium consists mainly of two isotopes, U (over 99%) and U (less than 1%)

zinc sulphidescreen

23592

10

23692

very unstable

144 56

90 36

10

neutrons available for other fission reactions

235

238 235

Basic layout of Advanced Gas Cooled Reactor (AGR)A controlled chain reaction takes place and thermal energy is released at a steady rate. The energy from the chain reaction is used to boil water. The resulting steam is used to turn turbines which drive generators to produce electricity.

Nuclear ReactorNUCLEAR FUEL ELEMENTS – Uranium dioxide, with natural uranium enriched with extra uranium-235.

GRAPHITE CORE – Slow neutrons are more effective at causing fission. Graphite blocks are used to slow down the neutrons – the graphite acts as a MODERATOR.

CONTROL RODS – Rate of fission process controlled by raising or lowering boron-steel control rods. Boron absorbs neutrons. When the rods are raised, more neutrons are available to cause fission and core temperature rises. The reactor can be shut down by keeping the rods lowered.

COOLANT – Heat from the fission reaction is carried away by carbon dioxide at high pressure. This heat is used to make steam to drive turbines and hence dynamos to generate electricity.

WASTE PRODUCTS – Spent fuel rods are removed from the core and sent to a reprocessing plant. Here, unused uranium is separated from the radioactive waste products together with small quantities of Plutonium-239. This is used as the fuel in fast breeder reactors and in the production of nuclear weapons – it is the most hazardous substance known.

Investigation of Factors Affecting the Strength of an Electromagnet

Strength of electromagnet depends on:

1. Current2. Number of turns / length3. Nature of core (best with soft iron)

Domain theory of Magnetism

A

Paperclips

demagnetisedS SN

Fleming’s Left hand ruleThis is used to predict the direction of motion of a conductor which is carrying a current through a magnetic field. The forefinger and second finger of the left hand are set at right-angles to each other and to the thumb.

Forefinger Second finger Thumbi u re r ul r sd e t(NS) n t

(+-)

The interaction of the magnetic field due to a current and a second magnetic field is used in numerous devices, e.g. electric motor, loudspeaker, analogue television, etc.

Loudspeakers

ElectromagnetismMaxwell, Faraday, Fleming.

partially magnetised

fully magnetised

S

S S

S

S

S

S

S

S

S

S

N

N

N

N

N

N

N

N

N

N

N

N

An electric current is fed into the coil. There is an interaction between this electromagnetic field and the permanent magnetic field due to the pole pieces. Using Fleming’s left hand rule, the coil will move in and out as shown on the diagram. Music is made up of of a series of alternating patterns, and these cause the cone to move backwards and forwards. This movement causes the surrounding air molecules to move backwards and forwards, so that a sound wave is produced.

-CURRENT

Maxwell’s Right Hand Corkscrew RuleRight thumb in direction of conventional current (+-); fingers of right hand in the direction of magnetic field.

The Electric MotorThis uses the interaction of an electromagnetic field in the coil with the permanent magnetic field from the two permanent magnets. The current is reversed every half revolution by the split ring commutator. This ensures continual rotation in one direction because it makes sure that the current always flows either clockwise or anticlockwise in the coil. The rate of turning and the power of the motor can be changed by using stronger magnets, larger currents, and a soft iron core.

In a practical motor, there will be more than one coil, so that the motion is made smoother.

+

current going away current towards

coil facecurrent anti-clockwise

carbon brushes

split ring commutator

armature

permanent magnet

Electromagnetic inductionWhen there is relative motion between a conductor and a magnetic field, a voltage (or electromotive force, or emf) is induced and a current will flow. The size of the induced emf and current depends on: –

1. The rate at which the magnetic field is cut2. The strength of the magnetic field3. The length of wire affected by the field

Michael Faraday discovered this around 1830.

When the magnet is brought towards the coil, the induced current flows in such a direction that it opposes the motion (i.e. a north pole is induced in the coil). The galvanometer deflects to the right. When the magnet is withdrawn from the coil, a south pole is induced, and the galvanometer deflects to the left. The induced current flows in such a direction as to oppose the motion of the magnet. This is called Lenz’s Law, and it ensures that the Principle of Conservation of Energy is obeyed. Fleming’s Right Hand Rule can be used to predict the direction of the current.

S N

galvanometer(sensitive ammeter)

Electromagnetic braking

When the magnet is moving the aluminium tube is a conductor in a moving magnetic field. As a result, an emf and current are induced in the aluminium tube. These are in a direction to oppose the motion. The resultant force (weight-induced force) is much smaller than the weight alone, and so the magnet falls at a slow, steady speed.

This type of electromagnetic braking has various uses, e.g. electric trains, some theme park rides, and protection systems in lifts. Another use is in speedometers.

strong magnet

aluminium tube

weight

induced magnetic force

AC GeneratorThis is very similar to the motor, except that it does not involve a battery. The coil is turned by some external force, e.g. steam, in a gas-fired power station, the turning blades of a windmill, etc. When the coil is in the position shown, it is cutting the magnetic field at right-angles, and so the maximum emf and current are induced. When the coil has moved through 90o, it is then moving parallel to the magnetic field, i.e. no longer cutting the field, and the induced emf and current will be zero. It can be shown that the emf and current are sinusoidal or cosinusoidal [sin/cos waves].

N

S

slip rings

brushes

V- +

U

V

Transformer theoryCurrent and voltage are only induced in the secondary coil when the magnetic field is changing in the soft iron core. This requires AC in the primary coil.

It can be shown that: –

secondary voltageprimary voltage

= secondary turns

primary turns or

VsVp

= nsnp

Also, by conservation of energy: –

input power = output power

Ip x Vp = Is x Vs

Transformer equation: IpIs

= VsVp

(= nsnp

)