P2 Part B

Question 1

What is the average voltage of UK mains supply?

Answer 1

230 volts

Question 2

What does AC and DC stand for?

Answer 2

AC: Alternating current, it is constantly changing direction DC: Direct current, always flowing in the same direction

Question 3

What is the main difference between mains supply and battery power?

Answer 3

Mains supply is AC, whereas battery power is DC

Question 4

Give the frequency of AC mains supply

Answer 4

50 Hz (50 cycles per second)

Question 5

What is a CRO?

Answer 5

A cathode ray oscilloscope (type of voltmeter)

Question 6

When you plug an AC current in to a CRO what will be shown in the ‘trace’?

How can potential difference be calculated from this?

Answer 6

The trace shows how the voltage of the supply changes with time. This ‘trace’ goes up and down in a regular pattern - some of the time it’s positive, some of the time it’s negative.

The vertical height of the AC trace at any point will show the input voltage at that point. By measuring the height of the trace, you can find out the potential difference of the AC supply.

Question 7

When you plug a DC current in to a CRO what will the trace look like? How can you work out the potential difference of the DC suppy?

Answer 7

The trace will be a straight line. The potential difference will be the distance from the straight line trace to the centre line.

Question 8

How can you calculate the frequency of an AC supply from the trace on a CRO reading?

Answer 8

Use the formula:

Frequency (Hz) = 1 ÷ Time period (seconds)

You must work out the time period by measuring the distance between two peaks on the trace. Times the number of squares/divisions by the timebase.

Question 9

List 9 examples of electrical hazards in the home

Answer 9

1. Long cables
2. Frayed cables
3. Cables in contact with something hot or wet
4. Water near sockets
5. Things being shoved into sockets
6. Damanged plugs
7. Too many plugs in one socket
8. Appliances without their covers on

Question 10

Outline the purpose of each of the three wires inside a 3 core cable

Answer 10

1. The brown live wire in a mains supply alternates between a high positive and negative voltage.
2. The blue neutral wire is always at 0V. Electricity normally flows in and out through the live and neutral wires only.
3. The green and yellow earth wire is for protecting the wiring - it works together with a fuse to prevent fire and shocks. It is attached to the metal casing of the plug and carries the electricity to the earth if somethings goes wrong and the live/neutral wire touches the metal case.

Question 11

Draw a labelled diagram of the wiring of a three pin plug

Answer 11

Question 12

Why are different parts of a three pin plug made of different materials?

Answer 12

• The metal parts are made of copper/brass because they are good conductors
• The case, cable grip and cable insulation are made of rubber or plastic because they’re good insulators and flexible.

Question 13

How does an earth wire and fuse work together to prevent electrical overloads? (3 main stages)

Answer 13

1. If a fault develops when the live wire touches the metal case, a great current travels through the live wire, through the case and out down the earth wire
2. This surge in current melts the fuse (or trips the circuit breaker in the live wire) when the amount of current is greater than the fuse rating. This cuts off the live supply and breaks the circuit.
3. The whole appliance becomes isolated, meaning it is impossible to get an electric shock from the case. It also prevents risk of fire caused by the heating effect of the large current.

Question 14

Why does fuse rating usually increase with cable thickness?

Answer 14

The larger the current, the thicker the cable needs to be. Fuses should be rated as near as possible but just higher than normal operating current.

Question 15

What makes an appliance ‘double insulated’?

Answer 15

If an appliance has a plastic casing and no metal parts, it’s said to be double insulated.

Question 16

Give three advantages and one disadvantage of using a circuit breaker instead of a fuse

Answer 16

Advantages:

1. A circuit breaker can be easily reset by flicking a switch, whereas fuses must be replaced
2. They operate faster than fuses, making them safer.
3. They work for small current changes that might not be large enough to melt a fuse and are therefore more effective at protecting against electrocution.

Disadvantages:

1. They are more expensive than fuses

Question 17

What is a RCCB?

Answer 17

A Residual Current Circuit Breaker - a circuit breaker commonly used instead of a fuse and an earth wire

Question 18

How do filament bulbs work?

Answer 18

By passing a current through a very thin wire, heating it up so much that it glows (and wasting a lot of heat energy in the process)

Question 19

What is the formula for calculating energy transferred by an appliance?

Answer 19

Energy transferred = Power rating (watts) x time (seconds)

Question 20

A 2.5 kW kettle is on for 5 minutes, calculate the energy transferred by the kettle in this time.

Answer 20

2500 x 300 = 750,000joules

= 750 kJ

Question 21

What is the formula for electrical power?

Answer 21

Power (watts) = current (amps) x potential difference (volts)

in symbol form: P = I x V

Question 22

What is potential difference in a circuit?

Give the equation for energy transformed

Answer 22

It is the energy transferred per charge passed. When an electrical charge (Q) goes through a change in potential difference (V) then energy (E) is transferred.

Energy transformed = charge x potential difference

E = Q x V

Question 23

What was the “plum pudding” theory?

Answer 23

A few years into the 1900s, JJ Thomson discovered that electrons could be removed from atoms - disproving John Dalton’s theory (1804) that atoms couldn’t be broken up. Thomson suggested atoms were spheres of positive charge with tiny negative elctrons stuck in them (like plums in a plum pudding).

Question 24

How was the “plum pudding theory” disproved?

Answer 24

In 1909, Rutherford and Marsden tried firing a beam of alpha particles at thin gold foil. They expected that the positively charged alpha particles would be slightly deflected by the electrons in the plum pudding model. However, most of the alpha particles went straight through, but the odd one came back at them. Rutherford and Marsden realised that this meant most of the mass of the atom was concentrated at the centre in a tiny nucleus and realised this must have a positive charge (as it repelled positive alpha particles). It also showed that most of an atom is empty space.

Question 25

What is the mass and charge of:

a) a proton
b) a neutron
c) an electron

Answer 25

a) Proton - Mass: 1, Charge: +1
b) Neutron: Mass 1, Charge: 0
c) Electron: Mass: 1/2000, Charge: -1

Question 26

Why do atoms have no charge?

Answer 26

The number of protons will be the same as the number of electrons.

(When electrons are added/taken away, the atom will become an ion)

Question 27

What are isotopes and how do they relate to radioactivity?

Answer 27

Isotopes are atoms with the same number of protons but a different number of neutrons. Most elements have different isotopes but there’s usually only one or two stable ones - the others tend to be radioactive.

Question 28

If an isotope is radioactive, what does it mean?

Answer 28

The isotope will decay into other elements and emit radiation from its nucleus.

Question 29

How is radioactivity a totally random process?

Answer 29

If you have 1000 unstable nuclei, you can never tell when any one is going to decay and nothing can be done to make decay happen (completely unaffected by temperature/chemical bonding).

Question 30

Where does background radiation come from?

Answer 30

1. The radioactivity of naturally occurring unstable isotopes which are all around us - in the air, food, building materials and rocks.
2. From space (cosmic rays) eg. from the Sun
3. Radiation due to man-made sources eg. a fallout from nuclear weapons test, nuclear accidents, dumped nuclear waste

Question 31

Give the percentage of the relative proportons of background radiation from: radion gas, food, medical x-rays, the nuclear industry, rocks and building materials, cosmic rays

Answer 31

Radon gas: 51%

Food: 12%

Medical X-rays: 12%

Nuclear industry: 1%

Rocks/building materials: 14%

Cosmic rays: 10%

Question 32

What are the three forms of radiation that can be emitted?

Answer 32

Alpha (α)

Beta (β)

Gamma (Y)

Question 33

What are alpha particles? Describe their properties

Answer 33

An alpha particle is two neutrons and two protons - the same as a helium nucleus

They are relatively big and heavy and slow moving.

They don’t penetrate very far into materials and are stopped quickly, even when travelling through air.

They are strongly ionising because bash into a lot of atoms and knock electrons off them.

Question 34

What are beta particles? Describe their properties

Answer 34

A Beta particle is an electron with virtually no mass and a charge of -1.

They move quite fast and are small

They penetrate moderately into materials before colliding, have a long range in air, and are moderately ionising too.

For every beta particle emitted, a neutron turns to a proton in the nucleus.

Question 35

What are Gamma rays?

Answer 35

Gamma rays are very short wavelength EM waves with no mass and no charge.

They penetrate far into materials without being stopped and pass straight through air.

This means they are weakly ionising because they tend to pass through rather than collide with atoms. Eventually they hit something and do damage.

Question 36

Why are alpha and beta particles deflected by electric and magnetic fields?

Why are alpha particles deflected less than beta?

Answer 36

Alpha and beta particles are deflected due to their positive/negative charge. They are deflected in opposite directions because alpha particles have a positive charge but beta particles have a negative charge.

Alpha particles have a larger charge than beta particles and feel a greater force in magnetic and electric fields. But they’re deflected less because they have a much greater mass.

Question 37

Why isn’t gamma radiation deflected by magnetic/electric fields?

Answer 37

Gamma rays are electromagnetic so they have no charge, meaning they aren’t deflected

Question 38

Give some examples of how location and occupation can effect the amount of radiation a person is exposed to? (5 in total)

Answer 38

1. Certain underground rocks (eg. granite) can cause higher levels at the surface, especially if they release radon gas.
2. Nuclear industry workers are uranium miners are ttpically exposed to 10x the normal amount of radiation (must wear protective clothing).
3. Radiographers work in hopitals using ionising radiation
4. At high altitudes, the background radiation increases because of more exposure to cosmic rays
5. Underground (eg. in mines) it increases because of the rocks all around, posing a risk to miners.

Question 39

The radioactivity of a sample always decreases over time. True or false?

Answer 39

True

Question 40

Fill in the gaps:

Everytime a ……. happens and an alpha, beta or ….. is given out, it means one more radioactive ….. has dissappeared. As the …… nuclei dissappear, the activity (number of nuclei that decay per second) will decrease. This means the older a sample becomes, the less radiation it will admit.

Answer 40

Everytime a decay happens and an alpha, beta or gamma is given out, it means one more radioactive nucleus has disappeared. As the unstable nuclei disappear, the activity (number of nuclei that decay per second) will decrease. This means the older a sample becomes, the less radiation it will admit.

Question 41

What is a half-life?

Answer 41

A half-life is the average time it takes for the number of nuclei in a radioactive isotope sample to halve.

Question 42

What does it mean if a radioactive sample has

a) a short half-life?
b) a long half-life?

Answer 42

a) A shory half-life means the acticity falls quickly because the nuclei decay quickly.
b) A long half-life means the activity falls more slowly because most of the nuclei don’t decay for a long time - they just remain unstable.

Question 43

The activity of a radioisotope is 640cpm (counts per minute). Two hours later, it has fallen to 80cpm. Find the half life of the sample.

Answer 43

1. Initial count: 640
2. After one half life: 640 ÷ 2 = 320
3. After two half lives: 320 ÷ 2 = 160
4. After three half lives: 160 ÷ 2 = 80
5. Three half lives gets us to 80cpm, so two hours represents 3 half lives. This means the half life of the sample is 120minutes ÷ 3 = 40 minutes

Question 44

How is a half life of a sample found by using a graph?

Answer 44

The half life is found from the graph by finding the time interval on the bottom axis, corresponding to a halving of the activity on the vertical axis.

Question 45

How is radiation used in smoke detectors?

Answer 45

A weak source of alpha radaiation is placed close to two electrodes. The source causes ionisation and a current flows between the electrodes. If there is a fire then smoke will absorb the radiation - so the current stops and the alarm sounds.

Question 46

How is radiation used in tracers in medicine?

Answer 46

Certain radioactive isotopes (gamma or beta, with a short half life) can be swallowed/injected and their progress around the body can be followed using an external detector. A well known example is the use of iodine-131, which is absorbed by the thyroid gland but it gives out radiation that can be detected to indicate whether the gland is taking in iodine as it should.

Question 47

How is radiation used in the treatment of cancer?

Answer 47

Gamma rays can be directed carefully and at the right dosage so as to kill the cancer cells, without damagine too many normal cells. However, a fair bit of damage can still be done so that the patient feels extremely ill.

Question 48

What is irridation? Outline 3 advantages of this method of sterilisation

Answer 48

Food and surgical instruments are exposed to gamma rays, which will kill all microbes.

Advantages: no high temperatures, so things like fresh apples and plastic instruments can be sterilised without them being damaged. Food is not radioactive afterwards. Isotope used must be a strong emitter of gamma rays with a long half-life, so it doesn’t need replacing too often.

Question 49

How does radiation harm living cells?

Answer 49

Alpha, beta and gamma radiation will enter living cells and collide with molecules. These collisions cause ionisation, which damages or destroys the molecules. Lower does can cause minor damage without killing the cell, sometimes giving rise to mutant cells which divide (and cause cancer).

Question 50

Why are beta and gamma sources most dangerous outside the body?

Answer 50

They can get inside the delicate organs, whereas alpha is less dangerous because it can’t penetrate the skin.

Question 51

Why are alpha sources most dangerous inside the body?

Answer 51

Alpha sources do damage in very localised areas, whereas beta and gamme sources mostly pass straight out without doing much damage.

Question 52

What four safety precautions can be made when protecting against radioactive material?

Answer 52

1. Radioactive sources must be used for a short time
2. Never allow skin contact (use tongs)
3. Hold the source at arm’s length as this will decrease the amount of radiation that hits you.
4. Keep the source pointing away from you and don’t look directly at it.

Question 53

Why do people who work with radiactive material wear lead aprons/stand behind lead screens?

Answer 53

Lead absorbs all three types of radiation (although a lot is needed to stop gamma radiation completely)

Question 54

What is nuclear fission?

Answer 54

The splitting up of big atomic nuclei

Question 55

Explain what happens in a nuclear power station

Answer 55

In a nuclear reactor, a controlled chain reaction takes place in which atomic nuclei split up steam, which is used to drive a steam turbine connected to an electricity generator.

The fuel that is usually split is uranium-235, though sometimes its plutonium-239.

Question 56

Outline the chain reactions involved in nuclear fission (3 steps)

Answer 56

1. A slow moving neutron must be absorbed into a fuel nucleus. This makes the nucleus unstable and causes it to split.
2. Each time a uranium or plutonium nucleus splits, it spits out two or three neutrons, one of which might hit another nucleus (causing it to split also) and thus keeping the chain reaction going.
3. When a large atom splits in two it will form two new smaller nuclei, these are radioactive because they have the ‘wrong’ number of neutrons in them.

Question 57

Why is nuclear fission so useful? (2 main points)

Answer 57

1. It gives out much more energy than a chemical reaction
2. Nuclear fuel is cheap

Question 58

What are the risks involved in nuclear fission? (3)

Answer 58

1. The products left over are difficult to dispose of because they are highly radioactive
2. Dismantling a nuclear plant takes decades
3. Nuclear power carries the risk of leaks or a major catastrophe (eg. Chernobyl)

Question 59

What is nuclear fusion?

Answer 59

Nuclear fusion is the joining of two light nuclei (eg. hydrogen) to create a larger nucleus.

Question 60

What would be the advantages of nuclear fusion over fission, if scientists were able to sustain it? (3)

Answer 60

1. Releases much more energy than fission
2. Doesn’t leave behind nuclear waste
3. Plenty of hydrogen around, to act as a fuel

Question 61

What are the disadvantages attached to nuclear fusion? (3)

Answer 61

1. Can only happen at high temperatures - around 10,000,000°C
2. You can’t hold hydrogen at the high temperatures and pressures required for fusion in an ordinary container (need an extremely strong magnetic field)
3. No fusion reactors are producing electricity yet (takes more energy to get up to the temperature than they are able to produce)

Question 62

Draw a flowchart to outline the life cycle of stars

Answer 62

Question 63

How do stars initially form?

Answer 63

The force of gravity makes clouds of gas and dust spiral in together to form a protostar.

Question 64

What happens after a protostar is formed?

Answer 64

Gravitational energy is convertes into heat energy so the temperature rises. When temperature is high enoough, hydrogen nuclei undergo nuclear fusion and give out massive amounts of heat and light. Smaller masses of gas and dust may also pull together to make planets that orbit this newly born star.

Question 65

What happens to the star after it begins emitting heat and light?

Answer 65

It enters a long stable period, where the heat created by nuclear fusion provides an outwards pressure to balance the force of gravity pulling everything inwards. In this stable period it’s called a main sequence star.

Question 66

What happens towards the end of a main sequence star’s life?

Answer 66

When the hydrogen begins to run out, heavier elements such as iron are made by the nuclear fusion of helium. The star then swells into a Red Giant (if it is about the same size as the sun) or a Red Super Giant (if it is much bigger than the sun). The surface of the star becomes red as it cools.

Question 67

How does the life of a Red Giant end?

Answer 67

It becomes unstable and ejects its outer layer of gas and dust as a planetary nebula. This leaves behind a hot, dense solid core - a white dwarf, which cools to become a black dwarf and disappears.

Question 68

What happens to a Red Super Giant towards the end of its life?

Answer 68

They begin to glow brightly again as they undergo more nuclear fusion and expand/contract several times forming elements as heavy as iron and eventually exploding into a supernova. The elements are then ejected into the universe to form new planets and stars. The exploding supernova throws the outer layer of dust and gas into space, leaving a very dense core called a neutron star. If the star is big enough, it will become a black hole.