Physics Unit one

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 1

Neutron 0 1

Electron -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−1

0𝛼24

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 20

Beta 1

Gamma 1

Neutron 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 0.2

Bone Marrow 0.12

Colon 0.12

Lung 0.12

Stomach 0.12

Bladder 0.05

Breast 0.05

Liver 0.05

Oesophagus 0.05

Thyroid 0.05

Rest of body 0.07

Total 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 Symbol

Wires 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

3Ω

6Ω

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)

6Ω

3Ω

Questions

Find the equivalent resistance of the following circuits:

3Ω

4Ω

1Ω 2Ω 3Ω1)

2)

Total Resistance of Hard Circuits! Rules: Work out one section at a time : Work from the inside out

6Ω

12Ω1Ω

Questions

6Ω

3Ω4Ω

5Ω

5Ω

3Ω

12Ω

3)

4)

Questions

6Ω

3Ω10Ω

3Ω

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