PHYSICS UNIT ONE
Feb 25, 2016
PHYSICS UNIT ONE
What is in Unit One?
Nuclear Physics and Radioactivity Electricity
One of … Astrophysics Astronomy Energy from the Nucleus…
What is required?
Pass the SACs in the three areas of study
Turn up to <90% of classes Complete all weekly homeworks
Optional … 40+ club homework
What do I need?
Yr 11 Physics Textbook Scientific Calculator (CAS not allowed
in exam) Workbook
NUCLEAR PHYSICS AND RADIATION
What is an atom
Three key historic figures in discovery of what an atom is:1) Democritus – concept of atomos2) J. J. Thompson – the plum pudding
model3) Lord Rutherford – the solar system
model
Rutherford’s Gold foil experiment
Most radiation particles pass through undeflected
A small amount of the radiation is deflected by very large angles
Fired radiation at a thin sheet of gold (only 200 atoms thick)
Rutherford’s gold foil experiment He concluded two things: 1) The atom is mainly empty space 2) It has a small, but dense and
positive nucleus.
Activity – work in groups
Your teacher wants to make a scale model of an atom for the classroom. We want to use a tennis ball for the nucleus and marbles will be the orbiting electrons. If the size of a nucleus is r = 1x10-14m, and the electrons orbit at a distance of 1 x 10-10m; how far away do my electron marbles have to orbit the tennis ball?
Activity: Extension
How small will the scale model nucleus have to be such that the entire scale model atom will fit in the classroom?
The atom
Nucleus: Made of neutrons and protons
Electrons orbit the nucleus
The atom
Name Charge Mass (atomic units: a.u.)
Proton +1 1Neutron 0 1Electron -1 0.0005
Particles that live in the nucleus are called nucleons. Protons and neutrons are nucleons.
Electromagnetic Force and Nuclear Force Protons have the same charge, and just like
having two magnets with similar poles, protons repel each other
How does the nucleus stick together? There is a force called the strong nuclear
force. It only works at very small distances (when nucleons are almost touching).
Adding neutrons to a nucleus increases the attractive strong nuclear force without increasing the repulsive electromagnetic force
Electromagnetic Force and Nuclear Force This is similar to adding a piece of
metal between “like” poles of two magnets. It helps them stick!
Isotopes
Isotopes are atoms of the same element that differ in the number of NUETRONS they have.
Isotopes have similar chemical properties but different physical properties.
Example: Carbon has 3 isotopes: Carbon -12, 13, and
14. All three isotopes have 6 protons. C-12 has 6 neutrons. C-13 has 7 neutrons. C-14 has 8 neutrons.
Describing an atom
EAZ
This is the isotopic symbol for an atom
Symbol of element
Mass Number: Protons + Neutrons
Atomic Number: Protons
Isotopic Symbol
Why are there mass numbers with decimal places on the periodic table? eg
How can you half a neutron? On the periodic table, the mass
numbers are called the relative atomic mass numbers.
They are an average of all isotopes of that element found on earth.
Questions: Complete the table
Element No. of protons
No. of neutrons
Name
143 Uranium
53 72
Radioisotopes Sometimes the nucleus of a
particular isotope is unstable, and it may undergo a radioactive decay or transmutation
This is known as a radioisotope Radiation is emitted Every element heavier than Bismuth
(A = 83) is radioactive. Every element heavier than Uranium
(A = 92) has to be produced artificially.
Nuclear Radiation
Three types of radiation can be emitted in nuclear decay
Alpha (α) Beta (β) Gamma (γ)
RadiationSymbol α
β γ
Name alpha
beta Gamma
Made of
Charge
Mass
Typical Energy
Two protons, two neutrons (helium
nucleus)
electron energy
+2 -1 0
NoneLight (0.0005 a.u.)
Heavy (4 a.u.)
0.1MeV1MeV5MeV
Radiation
α β γ
Range in air Shielded by
Ionizing ability Speed
Isotopic Symbol
Many metres1 or 2 metresFew cm
Many cm of Pbcm of AlSheet of paper
PoorReasonableHigh
c0.9c0.1c
𝛾00𝛽−10𝛼24
c = the speed of light
What is an electron volt? (eV) We normally measure energy in the
unit Joules (J) Atoms and electrons often contain
very small amounts of energy if measured in Joules (~10-19J)
1eV = 1.6x10-19J It is the energy 1 electron gains after
being accelerated by 1V
Questions
1) Convert into Joulesa) 5eV b) 1keV c) 3MeV d)0.2keV
2) Convert into eVa) 3.2x10-19J b) 6.4x10-14J c) 10-15J
Prac: Detecting radiation
Cloud chamber and glow-in-the-dark and geiger
How radiation is detected
1) Geiger counter
Incoming radiation strikes an atom in the Geiger counter
This collisions causes an electron to be ejected. It is attracted to the positive anode
When it touches the anode, it becomes an electrical pulse. The counter counts these electrical pulses
How radiation is detected
2) Cloud Chamber
Super saturated alcohol vapour
Incoming radiation
Clouds form around ionized alcohol atoms
The radiation ionizes some of the alcohol atoms
How radiation is detected
3) Fluorescent Materials – Some materials (like glow in the dark stickers, or phosphorus screens) will emit light when bombarded with nuclear radiation.
Decay equations
Polonium-210 decays via alpha emission. What product is formed?
The mass numbers must balance on both sides of the equation
The atomic numbers must balance on both sides of the equation
x= 206y= 82 z= Pb
Decay equations
Carbon-14 decays via beta emission.
Note about beta decay: An antineutrino ( a particle of very small mass, but no charge) is also emitted. Symbol: ῡ
x= 14 y= 7 z= N
Decay equations
The product nucleus is called the daughter nucleus
Any further decay will lead to a grandaughter nucleus
Questions: Write equations for the following: 1) Uranium-238 decays via alpha
emmision 2) Francium-222 decays via beta
emission 3) Polonium-214 decays via alpha
emission 4) Carbon-12 emits gamma radiation 5) A new isotope is formed by adding
a neutron to U-238
Decay equations prac
Half-life Prac
Half life
It is random when a radio-isotope might decay.
However, with a large number of atoms, they obey an exponential decay.
Half the atoms have decayed in one half life.
Half the remaining atoms have decayed in a second half life.
Half Life
Activity is a measure of atomic decays. Units are Becquerels 1 Bq = 1 decay per second
Half Life Questions
From the following graph, estimate the half life of the sample.
Using this half-life, estimate at what time the sample will have an activity of 100 kBq
Half Life Questions
A hospital keeps a sample of Iodine-131 (I-131) for use in radiation therapy. It is noted that the activity of the sample has reduced by 75% in 17 days. What is the half life of I-131?
Half-Life Question
Carbon-14 is a radioisotope that is useful in obtaining the age of carbon containing materials (such as wood) up to about 50’000 years. Carbon-14 is a naturally occurring radioisotope that plants will absorb from the atmosphere, along with the non-radioactive isotopes carbon-13 and carbon-12. After the plant dies, it stops absorbing carbon, and the carbon-14 within the plant decays away. Carbon-14 decays through beta decay with a half-life of 5730 years.
Half-Life Questions
A team of archaeologists have discovered the remains of what looks to be a fire-pit used by primitive homo sapiens. They measure the amount of carbon-14 in samples taken from the fire-pit and find the measured ratio of carbon-14 to be approximately 6.25% of the ratio of atmospheric carbon-14.
Approximately how many half-lives have elapsed since this sample was alive?
Approximately how old is the sample?
Half-Life Questions
Radiation
Isn’t always a bad thing! Has many uses Medical Imaging and treatment Carbon Dating Energy production (nuclear fission) Smoke alarms But in large quantities it can be
harmful…
Which radiation is the most harmful? Alpha radiation has the highest ionizing
ability. Ionizing: The radiation removes electrons
from atoms in our cells/DNA. This causes the cells/DNA to mutate/die.
However, alpha radiation is also the easiest to shield
Our skin will effectively shield it. However, if we breath it in, or eat
contaminated food = GAME OVER!
Which radiation is the most harmful?What radiation are these Fukishima workers being protected from? How do you know?
Absorbed Dose
The absorbed dose is the amount of radiation energy that has been absorbed per kilogram.
Units: Gray (Gy). 1 Jkg-1 = 1 Gy
Dose Equivalent
Different forms of radiation have different ionising abilities, and so cause varying amounts of damage to humans.
As each type of radiation can affect tissue differently, the Dose Equivalent is used compare accurately radiation effects.
Dose Equivalent
Units: Sieverts (Sv)Radiation Quality Factor
Alpha 20Beta 1
Gamma 1Neutron 10
Effective Dose
Different organs in the body have different sensitivities to radiation doses.
eg. A person’s lungs would be more likely to develop cancer than the liver if they were both given the same amount of radiation.
W = Weighting Σ = “the sum of”
Effective Dose
Body Part Weighting (W)Ovaries/testes 0.2Bone Marrow 0.12Colon 0.12Lung 0.12Stomach 0.12Bladder 0.05Breast 0.05Liver 0.05Oesophagus 0.05Thyroid 0.05Rest of body 0.07Total 1.00
Effects – Short Term
Called Somatic effects Radiation ionizes molecules, causing
cells to mutate or die Immediate Sickness/Death
Effects – Long term
Genetic Effects (mutates DNA in testes/ovaries)
Radiation causes genetic defects that show up in offspring
Questions
1. An 80kg tourist absorbs a gamma radiation dose of 200μGy during a return flight to England. Calculate the amount of radiation energy absorbed
2. Calculate the dose equivalent that has been received
3. During a medical procedure a patient receives a dose of gamma radiation. The organs which are affected include a bladder (5000Sv) and the ovaries (3000 Sv). Calculate the effective dose of radiation to which this woman has been exposed
Area of Study Two: Electricity!
Electrical Circuits
A circuit must be closed Must have a voltage source (EMF) –
otherwise no power! Must have a resistive load (bulb,
resistor, bell etc) – otherwise you get a short circuit!
Electrical SymbolsDevice Symbol Device SymbolWires crossed (not joined)
Cell
Wires joined Battery of cells
Resistor or other load
AC supply
Resistor Ammeter
Filament Lamp Voltmeter
Diode DC Supply
Earth or ground
Switch
CVCV
A
V
Circuit Diagrams
Draw:A) Two 1.5V cells connected to a lightbulb, and a switchB) A heating element (a resistor) connected to an AC power supply with a switch
Circuit Diagrams
Are the following circuits different?
No! They are the same. One bulb, and a DC power supply, connected to each other.
Circuit Diagrams
Just because there are corners in a circuit diagram, doesn’t mean there has to be corners in the real circuit.
This brings some confusion. Circuits that may appear different as diagrams, may actually be identical.
These are called equivalent circuits.
Circuit Diagrams
Which of the following are equivalent circuits? Why?
Prac
Simple Circuit. Part B. Using volt and ammeter Graph V/I
Ohm’s Law
Ohm’s Law relates the voltage across a load to the current it draws
V = IR V = Voltage Units: Volts (V) I = Current Units: Amps (A) R = Resistance Units: Ohms
(Ω)
Ohm’s Law
Ohmic Conductors obey Ohm’s Law They have linear relation between
the voltage and current (straight line graph)
Ohm’s Law
Some devices don’t obey Ohm’s Law. Their resistance is not constant
Called non-ohmic devices
Ohm’s Law
Examples of Ohmic Conductors: Resistors, most bulbs
Example of non-Ohmic Conductors: Diodes
Questions
A resistor draws a current of 1mA and has a voltage across it of 8V. Find the resisitance
What current does a 1000Ω light bulb draw from a 24V power supply
You have an Ohmic resistor, which draws 75mA when connected to 12V power supply. You decrease the voltage to 5V, what current do you expect it to draw?
Resistance
Which has more resistance?a. Fuse wire or power lines?b. Copper wire or Iron wire?
Resistance
The more resistance something has, the less current can go through it Resistance in a wire depends on: 1) The length of the wire 2) The cross sectional area of the
wire 3) What the wire is made of
Resistance
ρ = resistivity (depends on the material) Unit: Ωm L = Length Unit: m A = Cross sectional area. Unit: m2
Resistance
Example Normal household wire has a
diameter of 1.8mm, and is made from copper (ρ =1.7x10-8)
What is the resistance of a 10m long section?
Resistance
1) A 100m long section of telegraph wiring has a diameter of 3.6mm and a resistivity of 2.8x10-8. What is the resistance?
2) You have to design a household’s wiring system. Name three ways you could decrease the resistance as much as possible.
3) Why don’t all electricians try and use really thick wire all the time?
Electrical Power
Power is how much energy is used per second.
Power = VI Measured in Watts (W) 1W = 1Js-1
Questions
1) A bulb is connected to a 12V supply and draws 0.875A, what power is it using?
2) A toaster connected to the 240V mains supply uses 1kW of power. What current does it draw?
3) Which of the three identical bulbs in the picture shown is using the most power?
4) Which of these two bulbs (connected to different power supplies) is the brightest? Bulb 1 connected to 10V supply draws 0.8A of current. Bulb 2 connected to a 15V supply draws 0.4A.
5) A 50Ω heating unit is connected to a 20V power supply. What power does it use?
Questions
Electrical Energy
If power is “energy per second”, then energy is power multiplied by the time. Measured in Joules (J) kWh is another unit of energy (Note
it isn’t power!). 1kWh = 3.6MJ
Activity
The price for electricity supplied by the grid is about 20c per kWh. Find: The cost of heating something in the microwave for
5minutes How much a 1.5V battery should cost How much does it cost to boil a kettle (for 2 minutes) How much money are the solar panels on the science
block roof saving us per day (with approx 8 hours sun)
A iphone charger left plugged in to the wall will still use 1W. How much does this cost, if it is left plugged in all year?
What is electricity?
Electricity is moving charges carrying energy
An example of a charge is an electron
Charge is measured in Coulombs (C) 1e- = 1.6x10-19C How many electrons in 1 C?
Current and Voltage
What is current? Current is how much charge goes
passed a given point in 1 second
What is voltage? Voltage is how much energy each
Coulomb of charge carries
Questions
1. How many electrons make up a charge of 1μC?
2. A) A hairdryer draws a current of 1.6A. What charge flows through the hair dryer every second?
B) How many electrons flow through the hairdryer in 1 minute?3. Use the formulae and , and
substitute into P=VI, and simplify
Electrostatics
Charges don’t always have to move They may just accumulate
somewhere Law of conservation of charges:
Charges aren’t just created! To make some negative charge you have to start with neutral atoms and split into an equal negative charge (electrons) and positive charge (remaining ion)
Electrostatics
Like charges repel Unlike charges attract (like a
magnet) An excess of electrons on something
gives it a negative charge Deficit of electrons on something
gives it a positive charge
Prac: Charged Rods
Demo: Van de Graff generator
Van de Graaff Generator
How does it work?Charge jumps!
Negative charge is taken to bottom of the machine.
Metal comb touches belt, causing friction and separating positive and negative charges
Positive charge left on dome
Conductors and Insulators
Conductors: Allow the flow of electrons
Insulators: Inhibit the flow of electrons
Coulomb's Law
What caused the Al can to roll? Charges exert a force on one another. k = 9x109 Nm2C-2
Positive force = repulsion ( a push ) Negative force = attraction ( a pull )
A force!
Coulomb’s Law
Example: Two charges, with a charge equal to 1μC, are held 2m apart from each other. What is the force each charge exerts on each other?
Coulombs law
Examiners like to ask proportionality questions with Coulomb’ law
Eg. Two charges, each with a charge of q, are positioned a set distance apart from each other. If the distance is doubled, how does the force change?
Questions
1) A - 1μC and 6 μC are separated by 20cm. Calculate the force.
2) Two 1μC are separated by an unknown distance and experience a force of 3N. Calculate the distance separating the two charges.
3) A charge of q, and another of 2q are separated by a distance x. If the charge on the second is doubled to 4q, and the distance between them halves to x/2, how does the force change?
Electric Fields
The electrostatic force is invisible. However, as scientists, we have added lines around charges to help visualise this force
Called “The Electric Field” Shows where a positive charge would fly off to
if we dropped it into the field A negative charge would head in the opposite
direction. Where lines are close together, the field is
strong Where lines are far apart the field is weak
Electric Fields
If we had a positive charge that couldn’t move
Electric Fields
And we dropped another positive charge just north of it, which direction would the 2nd charge fly off to?
Electric Fields
So we put an arrow there
Electric Fields
If we were to drop another positive charge to the right of our main positive charge…
Electric Fields
If we were to drop another positive charge to the left of our main positive charge…
Electric Fields
Continue putting field lines around our charge…
Electric Fields
Question: Where is the electric field the strongest?
Answer: Near the charge!
Electric Fields
Question: What would the field lines created by a negative charge look like? Discuss with your neighbour.
Electric Fields
Some more complicated fields…
Electric Fields
E = Electric Field. Units: NC-1
Electric Fields - Questions Some small charged spheres are to
be placed in an electric field which points downwards and has a strength of 5000NC-1. What force would be experienced by a charge of +5 μC and -0.6 μC ?
Draw the electric field you would expect around the three charges below-q -q+
q
Electric Fields Questions
Draw the electric field you would expect around the two charges below
-2q
+q
Back to Electric Circuits
Draw two different ways you could place two bulbs in a circuit with a battery
One is called series One is called parallel
Kirchhoff’s Laws
Law 1: The current law Law: “In any circuit the sum of all currents
flowing into a point equals the sum of all currents flowing out of that point
Analogy: River: The amount of water flowing into a junction equals the sum of water flowing out. [Draw this]
What this means. Current is same at all places on a series circuit. It splits up in a parallel circuit.
Kirchhoff’s Laws
Law 2: The voltage law Law: “The total potential drop around a
closed circuit must be equal to the total EMF in the circuit
Analogy: River. The pump (EMF) pumps water to the top. It drops down as it goes through devices [Draw this]
What this means: Voltage is supplied by battery, and used in devices. Voltage is the same in parallel circuit.
Activity
Smarties run. Questions?
Prac: Series and Parallel Circuits
Resistors in Series
Eg Total resistance in the circuit?6Ω 2Ω 3Ω
Resistors in Parallel
3Ω6Ω
Resistors in Parallel
The total resistance in a parallel circuit is always less than the smallest resistor.
Sometimes examiners ask for the equivalent resistance (it’s the same thing as total resistance)
6Ω3Ω
Questions
Find the equivalent resistance of the following circuits:
3Ω4Ω
1Ω 2Ω 3Ω1)
2)
Total Resistance of Hard Circuits! Rules: Work out one section at a time : Work from the inside out
6Ω12Ω 1Ω
Questions
6Ω3Ω 4Ω
5Ω5Ω
3Ω12Ω
3)
4)
Questions
6Ω3Ω 10Ω3Ω
6Ω
2Ω
3Ω3Ω
5)
6)
Questions
7) Draw a circuit with three resistors that has a total resistance of 5Ω.
8) Draw the above circuit differently.9) Using a 1Ω, 2Ω, 3Ω, 4Ω and 5Ω
resistor draw a circuit with total resistance = 6Ω
Final note about Voltages and Current in circuits The total voltage in a circuit is set by the
battery/powerpack. However, the current drawn from this battery
depends ON WHAT IS IN THE CIRCUIT Eg…POE Draw two circuit diagram with two bulbs (series
and parallel). Guess which will draw the most current Measure total voltage in both, and total current. Which draws most power? Explain why one draws more current than the other
CUPS 9 and 10.
Prac: Which bulb is the brightest Report required for this prac.
Demo: Internal Resistance
Resistance within the powerpack/battery. Uses some of the voltage up!
A
V
Internal Resistance
Voltage without bulb = Voltage measured with bulb = Current = Voltage used in internal resistance Internal Resistance =
Other sources of EMF
EMF = electro-motive force (what drives the circuit)
Solar 20% efficient 2kW per square meter of sunlight
energy Don’t work at night! Less power in winter. Why?
Household Electricity
We have only been dealing with DC electricity (Direct Current)
Household electricity is AC (Alternating Current)
Why? We use AC because its relatively
easily to change between voltages
Household Electricity
Power lines have a low resistance, but because electricity has to travel a large distance, this becomes an issue, and much power could be lost
A high voltage has a lower current, and a lower current loses less power!
Transformers change between high voltage (14000V) and low voltage (240V)
Household Electricity
Current wants to flow from Active to Neutral just like it flows from +ve to -ve in a battery.
Active: 240V
Neutral: 0V
Earth/Ground
Household Electricity
Why do we have an earth wire? Imagine a fault in your toaster that
touched the active wire to the casing. Then you touched the casing!
The casing is safely connected to earth, so if a fault did occur, high current would flow to earth, and a fuse would blow.
Electric Shocks!
Our muscles move when they get small electrical shocks sent from our brain.
Above 20mA of electric current, and we cant let go.
A shock across the heart of 50mA for 5s = possible death
Prac: Build a buzzer game with a bell