Cambridge IGCSE0625

Electromagnetic effects

Physics 0625 Chapter Notes

What this chapter covers

Electromagnetic effects - Electromagnetic inductionElectromagnetic effects - The a.c. generatorElectromagnetic effects - Magnetic effect of a currentElectromagnetic effects - Force on a current-carrying conductorElectromagnetic effects - The d.c. motorElectromagnetic effects - The transformer

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Electromagnetic effects notes

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1. Inducing an e.m.f.

An e.m.f. is induced in a conductor when the conductor moves across a magnetic field. An e.m.f. is also induced when a changing magnetic field links with the conductor. This is electromagnetic induction.

What matters is relative movement between the magnet and the conductor. The e.m.f. is induced when the magnet moves towards the coil and when it moves away. A stationary magnet induces no e.m.f. No battery or power supply is needed.

Examiner insight

“The majority of candidates showed little understanding of how to induce an e.m.f. in a copper wire in a magnetic field and thought that a power supply was required, resulting in option A being chosen.” “Thus no relative movement, whether it be a strong or weak magnet means that there is no induced e.m.f. and, also that an e.m.f. will be induced if the relative movement is either away or towards each other.”

Worked example 11 mark

A solenoid is connected to a very sensitive ammeter. A rod is inserted into one end of the solenoid.

The ammeter shows that there is a small electric current in the solenoid while the rod is moving.

Which rod is being inserted?

A a heated copper rod · B a magnetised steel rod · C an uncharged nylon rod · D a radioactive uranium rod

1
Only a magnet gives a changing magnetic field linking with the solenoid as it moves.
2
B: a magnetised steel rod. [1]

2. An experiment to show induction

Connect a coil of wire to a sensitive ammeter. Move a bar magnet into the coil. The pointer deflects while the magnet is moving.

Pull the magnet out and the pointer deflects the other way. Hold the magnet still and there is no deflection. Do not add a power supply and do not connect wires to the magnet.

Examiner insight

“Candidates needed to make it clear in their responses that there was relative movement between a magnetic field and a conductor. Vague statements such as “put the magnet near the coil” were insufficient.” “Very few stated that the wire would cut the magnetic field lines of the magnet when it moved.”

3. Factors affecting the induced e.m.f.

The induced e.m.f. is larger when the magnet moves faster: increase the speed of the magnet.

It is also larger with a stronger magnet, and with more turns on the coil: increase the number of turns.

Give the change in full. Say “increase”, not just “change”.

Examiner insight

“A common error was to be too vague, e.g. “change the number of coils” or “change the magnet”, when they needed to state “increase the number of coils” or “use a stronger magnet”.” “a common misconception of both stronger and weaker candidates was that the speed of the magnet did not affect the magnitude of the induced e.m.f.”

Worked example 14 marks

The student uses a bar magnet to demonstrate electromagnetic induction as shown in Figure 9.3.

When the S pole of the bar magnet is moved into the coil of wire, the pointer on the sensitive ammeter deflects to the left.

(i) Describe two ways by which the student can increase the deflection to the left.

(ii) Describe two ways by which the student can make the pointer deflect to the right.

1
(i) increase the speed of the magnet [1]
2
(i) use a stronger magnet (or increase the number of turns on the coil) [1]
3
(ii) move the S pole out from the coil [1]
4
(ii) move the N pole into the coil (or reverse the connections on the ammeter) [1]

4. The direction of the induced e.m.f.

The direction of an induced e.m.f. opposes the change causing it.

When a magnet moves towards a coil, the induced current gives the coil a magnetic field that opposes the movement. The end of the coil nearest the approaching magnet behaves like a magnet that repels it.

When the magnet moves away, the coil end nearest it attracts it. This again opposes the change.

Examiner insight

“This question tested the concept that an induced e.m.f. is always in the opposite direction to the change causing it.” “To achieve the second mark candidates needed to recognise that the coil behaved like a magnet that repelled the magnet moving in, or that the direction of the e.m.f. opposes the change producing it.”

Worked example 13 marks

The bar magnet is moved into a coil of wire. Complete the sentences to explain what happens.

As the magnet moves into the coil, the coil experiences a changing magnetic field. An e.m.f. is ______ in the coil. This causes a ______ in the coil which produces a magnetic field around the coil. The magnetic field opposes ______.

1
e.m.f. is induced in the coil [1]
2
this causes a current in the coil [1]
3
the field opposes the change causing it (the movement of the magnet) [1]

5. Force, field and induced current

The force (the movement of the conductor), the magnetic field and the induced current are perpendicular to each other.

The magnetic field points from the N pole to the S pole. Use the right-hand rule for induced current. Do not use the left-hand rule. Electrons flow the opposite way to the current.

Examiner insight

“Candidates should have realised that, as the conductor had to move in a perpendicular direction to both the flow of electrons and the direction of the magnetic field, only one of the four options was possible.” “Many stronger candidates determined the direction correctly but a significant number chose option B, suggesting that they used the left-hand rule instead of the right-hand rule.”

Worked example 11 mark

The diagram shows a conductor WZ.

Part of the conductor lies between the magnetic poles N and S.

When the conductor is moved, electrons flow along the conductor from Z to W.

In which direction is the conductor moved?

A in the direction of arrow P · B in the direction of arrow Q · C in the direction of arrow R · D out of the paper towards the viewer

1
The movement is perpendicular to the field (N to S, down the page) and to the flow of electrons (along WZ).
2
D: out of the paper towards the viewer. [1]

Recap

An e.m.f. is induced when a conductor moves across a magnetic field, or when a changing magnetic field links with it.
No relative movement means no induced e.m.f. No battery is needed.
Experiment: coil, sensitive ammeter, bar magnet moved in and out of the coil.
A larger e.m.f. needs a faster magnet, a stronger magnet or more turns on the coil.
Extended: the direction of the induced e.m.f. opposes the change causing it.
Extended: movement, field and induced current are perpendicular. Use the right-hand rule.

6. A simple a.c. generator

An a.c. generator has a coil of wire and a magnet. Either the coil turns in the magnetic field of the magnet, or the magnet turns near the coil.

As it turns, the coil cuts the magnetic field and an e.m.f. is induced in it.

Examiner insight

“Stronger answers to this question referred to the need for the coil to cut the magnetic field provided by the permanent magnets.”

7. Slip rings and brushes

Slip rings and brushes provide continuous contact between the turning coil and the circuit. They transfer the induced current from the coil to the circuit and prevent the wires tangling. They are needed only where the coil turns.

A generator has slip rings. A split-ring commutator belongs to a d.c. motor.

Worked example 11 mark

Which feature is not part of a d.c. motor?

A a coil of wire · B a pair of slip rings · C a magnetic field · D a split-ring commutator

1
B: a pair of slip rings. A d.c. motor has a split-ring commutator. [1]

8. Graph of e.m.f. against time

One full turn of the coil gives one full cycle on the graph of e.m.f. against time. The current changes direction every half turn (180°), as the coil passes the vertical position. At these moments the e.m.f. is zero.

A quarter turn from each zero, the e.m.f. is at a peak or a trough. The e.m.f. is largest when the magnetic field is changing most rapidly.

Examiner insight

“Instead of recalling that the maximum induced e.m.f. occurs when the magnetic field at position Q is changing most rapidly, most stronger candidates incorrectly thought that the induced e.m.f. is zero when the magnetic field at position Q is zero (option B).”

9. Changing the output

A larger e.m.f. comes from more turns on the coil or faster rotation of the coil.

The frequency of the output rises only if the speed of rotation increases. More turns do not change the frequency.

More turns give a taller wave with the same spacing. Faster rotation gives a taller wave with the peaks closer together.

Examiner insight

“Few candidates recalled that the frequency of the output potential difference from an a.c. generator can only be increased if the speed of rotation is increased.” “Many candidates had the misconception that by increasing the number of coils on the a.c. generator, both the magnitude and the frequency of the output potential difference will increase.”

Worked example 11 mark

Graph 1 shows the output from an a.c. generator. Which changes can be made so that the generator produces graph 2?

A decreasing the magnetic field strength and decreasing the speed of rotation only · B increasing the magnetic field strength and decreasing the number of coils only · C increasing the number of coils only · D increasing the speed of rotation only

1
D: increasing the speed of rotation only. [1]
2
Graph 2 has two cycles in the time of one cycle of graph 1, so its frequency is higher. Only faster rotation raises the frequency.

Recap

Extended: an a.c. generator has a coil and a magnet. The coil turns in the field, or the magnet turns near the coil.
Extended: the coil cuts the magnetic field, so an e.m.f. is induced.
Extended: slip rings and brushes keep the turning coil in contact with the circuit. A split-ring commutator is for a d.c. motor.
Extended: one turn of the coil gives one cycle of e.m.f. against time.
Extended: the current changes direction every half turn (180°), where the e.m.f. is zero.
Extended: more turns or faster rotation gives a larger e.m.f. Only faster rotation raises the frequency.

10. Field around a straight wire

A current in a straight wire has a magnetic field around it. The field lines are concentric circles, centred on the wire.

The direction of the field depends on the direction of the current. Grip the wire in your right hand with the thumb along the conventional current. The fingers show the direction of the field. For a current into the page, the field is clockwise. Draw one arrow on each circle.

Examiner insight

“A significant number of candidates recognised the correct shape of the magnetic field of a current in a straight conductor but did not know the field direction.” “Some candidates who drew multiple arrows on the field lines contradicted themselves. It is better to place one, correct arrow on each field line.”

11. Field around a solenoid

A solenoid is a coil of wire. A current in it gives a magnetic field. The field lines come out of one end of the coil, curve round the outside in loops and go back in at the other end.

Plotting compasses show the direction. At the two ends, the needles point the same way, along the solenoid. Above and below the solenoid, the needles point the opposite way to these.

Examiner insight

“Most candidates thought that the compass needles above and below the solenoid would point directly towards the centre of the solenoid.”

12. Investigating the field pattern

Place plotting compasses at points around the wire or solenoid while it carries a current. Each needle shows the direction of the field at that point.

Iron filings show only the shape of the field. They cannot show its direction, so a compass is still needed.

Examiner insight

“Some candidates identified the device used to determine the direction of the magnetic field as iron filings. Iron filings can only give the shape of the magnetic field.”

Worked example 11 mark

State the name of the device that is used to determine the direction of the magnetic field.

1
(plotting) compass [1]

13. Relays

A relay is a type of switch, turned on and off using an electromagnet. A current in the coil makes a magnetic field. This attracts a soft-iron armature. Contacts are pushed together, completing the circuit.

The two circuits are separate: no current passes from the coil circuit through the armature. A relay lets a low-voltage circuit, such as the output of a logic gate, switch a higher-voltage circuit. Soft iron is magnetised and demagnetised easily. A steel armature would remain magnetised, so the second circuit would continue working.

Examiner insight

“Some candidates misunderstood how the current flowed in circuit B and stated that the current flowed from circuit A via the iron armature to circuit B.” “Stronger candidates correctly identified that the motor or circuit B would continue working if the soft iron armature was replaced with a steel one as the steel would remain magnetised.”

Worked example 11 mark

A relay consists of a coil and it is a type of switch. It is turned on and off using an electromagnet.

Four statements explaining how the relay works are listed.

1 An electromagnetic field forms around the coil. 2 Contacts are pushed together, completing the circuit. 3 There is a current in the coil. 4 This attracts a soft-iron armature.

What is the correct order for the statements that explains how the relay works?

A 1234 · B 1432 · C 3124 · D 3142

1
D: 3 1 4 2. [1]

14. Loudspeakers

A loudspeaker uses the magnetic effect of a current. An alternating current in the coil makes a changing magnetic field. This interacts with the field of a permanent magnet, so the cone vibrates.

The cone makes the air vibrate, and energy is transferred as sound.

Examiner insight

“Many candidates gave answers in terms of the loudspeaker receiving sound and did not refer to the loudspeakers being a vibrating source.”

Worked example 13 marks

Complete the sentences to describe and explain how a loudspeaker produces sound.

A changing magnetic field is produced by the ______ in the coil.

This magnetic field interacts with the magnetic field due to the ______.

This causes the cone to vibrate. The vibration of the ______ transfers energy to a student as sound.

1
(alternating) current / a.c. [1]
2
(permanent) magnet [1]
3
air (particles) [1]

15. Strength and direction of the field

The field is strongest close to the wire. It gets weaker as the distance from the wire increases, so the field lines become further apart. A larger current gives a stronger field.

Reversing the current reverses the direction of the field. The same changes apply to a solenoid: increasing the current makes the field strength increase, and reversing it reverses the field.

Examiner insight

“A number of candidates did not recognise that the magnetic field decreases as you move further away from the current carrying conductor and consequently the field lines become further apart.” “Many candidates stated that the magnetic field strength increases and the direction of the field reverses. Weaker candidates sometimes stated only that the magnetic field direction changes, which was insufficient to gain credit.”

Recap

Straight wire: the field lines are concentric circles centred on the wire. Draw one arrow on each circle.
Right hand: thumb along the conventional current, fingers show the direction of the field.
Solenoid: needles at the two ends point the same way. Needles above and below point the opposite way, not towards the centre.
Plotting compasses show the direction of the field. Iron filings show only its shape.
Relay: current in the coil, magnetic field, soft-iron armature attracted, contacts pushed together.
Loudspeaker: alternating current in the coil, its field interacts with the field of a permanent magnet, the cone vibrates.
Extended: the field is weaker further from the wire and stronger with a larger current.
Extended: reversing the current reverses the direction of the field.

16. Showing the force on a wire

Rest a thin copper wire on two copper rails connected to a d.c. power supply. Place the wire in the magnetic field between a N pole and a S pole. Switch on. The wire experiences a force, so it moves along the rails.

When the current direction in the wire is reversed, the force reverses. When the magnetic field is reversed, by swapping the poles, the force reverses. Do both together and the force returns to its original direction.

Examiner insight

“Answers such as “change the poles” or “change the current” could not be credited. Candidates should be encouraged to state that the current direction in the wire is reversed, or the magnetic field is reversed.” “However stronger candidates recognised that swapping the poles reverses the direction of the force on the wire and changing the current direction also reverses the direction of the force, hence returning it to the original direction.”

17. Changing the size of the force

To make the force on the wire bigger, increase the current or increase the magnetic field strength.

Say a stronger magnet or a more powerful magnet. A weaker field between the poles gives a smaller force.

Examiner insight

“A common error was to state “bigger” magnets instead of “stronger” or “more powerful” magnets.”

Worked example 12 marks

A copper cylinder is placed on two metal rods in the magnetic field between the poles of a magnet. The switch is closed. Suggest two changes that each increase the force on the copper cylinder.

1
increase the current [1]
2
increase the magnetic field strength [1]

18. Directions of force, field and current

The force on the wire, the magnetic field and the current are mutually perpendicular.

To find the direction of the force, use the left-hand rule. The right-hand rule gives an incorrect answer. Name both the direction of the field and the direction of the current when you explain it.

Examiner insight

“Some candidates mentioned that magnetic field and current were perpendicular without any reference to force direction and this was insufficient.” “Weaker answers only made reference to the left-hand rule or that current, field and motion were perpendicular to each other.”

19. Beams of charged particles

A magnetic field exerts no force on a stationary charge. It exerts a force on a moving charge, because a moving charge has a magnetic field.

A beam of charged particles is deflected like a current-carrying wire. The deflection is at right angles to both the magnetic field and the velocity of the particles. It is not towards a magnetic pole.

Electrons and beta particles are negatively charged. The current points opposite to the way they move, so use that direction in the left-hand rule. On a diagram, crosses show a field directed into the page.

Examiner insight

“The majority of candidates incorrectly thought that the β-particles are deflected towards one or other of the two magnetic poles.” “Candidates could have, if they wished, confirmed that the diagram was correct by applying the left-hand motor rule, remembering that the second finger (current) needed to point to the left, the moving particles being negatively charged.”

Worked example 11 mark

The diagram shows a beam of beta particles entering a magnetic field. The direction of the magnetic field is into the page. In which direction is the beam of beta particles deflected?

A downwards towards the bottom of the page · B into the page · C out of the page · D upwards towards the top of the page

1
beta particles are negative, so the current is opposite to their motion: to the left
2
the force is at right angles to the field (into the page) and the current
3
left-hand rule: A, downwards towards the bottom of the page [1]

Recap

A current-carrying wire in a magnetic field experiences a force.
Reversing the current reverses the force. Reversing the magnetic field reverses the force.
Reverse both and the force is in its original direction.
A larger current or a stronger magnet gives a larger force.
Extended: force, field and current are mutually perpendicular. Use the left-hand rule.
Extended: a magnetic field acts on moving charges, not stationary ones.
Extended: electrons and beta particles are negative, so the current is opposite to their motion.

20. The turning effect on a coil

A current-carrying coil in a magnetic field may experience a turning effect.

The turning effect is increased by increasing the number of turns on the coil, the current, or the strength of the magnetic field.

Say that each one is increased. Naming the quantity alone is not enough. For the magnetic field, write stronger magnets, not bigger magnets.

The turning effect is not the rate at which the coil turns or spins. The measure of a turning effect is called the moment.

Examiner insight

“Many candidates answered in terms of what the effect would be on the coil and said it would be faster which was insufficient.” “Turning effect does not refer to the rate at which an object turns or spins.”

21. The split-ring commutator

In a d.c. motor, a current-carrying coil lies in a magnetic field. The turning effect on the coil is what makes it rotate.

A split-ring commutator changes the direction of the current in the coil every half turn. This happens when the coil is vertical, at right angles to the magnetic field.

This keeps the turning effect in the same direction, so the coil keeps rotating one way.

Without it, the coil would stop at the vertical position and not carry on turning in the same direction.

If you write that the coil turns or moves, make it clear that you mean continuously in one direction, not oscillating.

Examiner insight

“The most common acceptable answers were either that the split-ring commutator is needed to change the direction of the current in the coil every half turn, or that without the commutator the coil will not continue to rotate in the same direction.” “In this case, if viewed from a stationary frame of reference there can be no doubt that the aim is to keep the turning effect in the same direction at all times.”

Worked example 12 marks

Explain how the split-ring commutator allows the coil to rotate continuously.

1
allows the current in the coil to change direction [1]
2
every half turn (180°), or when the coil is vertical [1]

22. The brushes

The brushes transfer current to the coil from the circuit. They give continuous contact between the split-ring commutator and the circuit while the coil rotates. The brushes also prevent the wires tangling. That is a purpose of the brushes, not of the commutator.

Examiner insight

“Some weaker answers suggested that the split ring commutator was transferring the current from the coil to the circuit, again confusing motors and generators.”

Worked example 11 mark

Explain the purpose of the brushes.

1
any one: transfer current to the coil (from the circuit) [1]
2
or: provide continuous contact between the split-ring commutator and the circuit
3
or: prevent the wires tangling

23. Which parts belong to a d.c. motor

A d.c. motor has a coil of wire, a magnetic field, a split-ring commutator and brushes.

It uses the split-ring commutator to produce continuous rotation of the coil. A pair of slip rings is not part of a d.c. motor.

Examiner insight

“Many candidates answered this correctly, but weaker candidates thought that neither the motor nor the generator contained a magnet.” “The name and purpose of slip rings in an AC generator was not well known and many candidates confused them with a split ring commutator.”

Worked example 11 mark

Which feature is not part of a d.c. motor?

A a coil of wire; B a pair of slip rings; C a magnetic field; D a split-ring commutator

1
B: a pair of slip rings [1]

Recap

A current-carrying coil in a magnetic field may experience a turning effect.
The turning effect is increased by more turns, more current or a stronger magnetic field.
Turning effect is not the rate at which the coil turns or spins.
Extended: the split-ring commutator changes the direction of the current in the coil every half turn, so the coil keeps rotating in one direction.
Extended: the brushes transfer current to the coil, keep continuous contact between the commutator and the circuit, and prevent the wires tangling.
Extended: a d.c. motor has a coil, a magnetic field, a split-ring commutator and brushes. Slip rings are not part of it.

24. Construction and terms

A simple transformer has two coils of wire wound on a soft-iron core. The input coil is the primary coil. The output coil is the secondary coil.

A step-up transformer has more turns on the secondary coil than on the primary coil. A step-down transformer has fewer.

Examiner insight

“Weaker candidates often omitted the soft-iron core, did not label it, or drew two cores.” “Many had the misconception that a step-up transformer had more turns on the primary coil than on the secondary coil, and vice versa.”

25. The transformer equation

The voltages across the coils are in the same ratio as the numbers of turns. The equation can be used in any form.

Write the equation in symbols, then substitute the values.

V_p/V_s = N_p/N_s

Examiner insight

“Weaker candidates did not correctly recall the equation Vs ÷ Vp = Ns ÷ Np.” “Candidates should be advised to always give the equation they are using in symbols first before substituting numerical values.”

Worked example 13 marks

The charger includes a transformer. The voltage to the primary coil of the transformer Vp is 240 V. The secondary voltage Vs for the transformer is 16 V. The number of turns on the primary coil Np is 900. Calculate the number of turns on the secondary coil Ns for the transformer.

1
V_s/V_p = N_s/N_p in any form [1]
2
16/240 = N_s/900 [1]
3
N_s = (16 × 900)/240 = 60 [1]

26. High-voltage transmission

A transformer at the power station steps up the voltage before the electricity is transmitted. A step-down transformer steps the high voltage down again.

For the same power, a high voltage means a low current. That gives less power loss in the cables, and thinner cables can be used.

Examiner insight

“Many candidates had the misconception that current is transmitted by a transformer instead of power.” “However, many candidates thought that high voltage would transmit electricity faster.”

Worked example 11 mark

Which statement is correct?

A A transformer is used to step up the voltage so that current is transmitted more efficiently · B A transformer is used to step up the current so that power is transmitted more efficiently · C A transformer is used to step up the voltage so that power is transmitted more efficiently · D A transformer is used to step up the current so that voltage is transmitted more efficiently

1
C: step up the voltage so that power is transmitted more efficiently. [1]

27. How a transformer works

A transformer works only with alternating current. The a.c. in the primary coil produces a changing magnetic field.

The soft-iron core carries this field to the secondary coil. The field cuts the secondary coil, which induces an e.m.f. across it by electromagnetic induction.

Examiner insight

“Where full credit was not gained, it was often for a lack of precision, for example, mentioning current in the primary coil without specifying that it must be a.c.” “A significant number of weaker candidates had the misconception that the iron core was used to conduct current from the primary coil to the secondary coil.”

Worked example 12 marks

A student connects the primary coil of the transformer to a d.c. supply. Explain why there is no voltage across the secondary coil.

1
there is no changing magnetic field [1]
2
d.c. does not produce the changing magnetic field needed for electromagnetic induction [1]

28. Power in equals power out

For a transformer that is 100% efficient, the power supplied to the primary coil equals the power delivered by the secondary coil.

Convert kilowatts to watts before you substitute.

I_pV_p = I_sV_s

Examiner insight

“Candidates needed to realise that it is the fact that the transformer is 100% efficient that allows the use of the formula IpVp = IsVs.” “Some candidates attempted to use P = IV with the secondary voltage to calculate the primary current.”

Worked example 15 marks

A transformer steps down high-voltage electricity. The transformer has 6200 turns on the primary coil and 43 turns on the secondary coil. The output voltage is 230 V.

(i) Calculate the input voltage in the step-down transformer.

(ii) A 2.2 kW kettle is connected to the secondary coil of the transformer. Calculate the current in the primary coil of the step-down transformer. Assume the transformer is 100% efficient.

1
(i) V_p = N_pV_s/N_s = (6200 × 230)/43 [1]
2
V_p = 33 000 V [1]
3
(ii) convert to watts: 2.2 kW = 2200 W [1]
4
I_p = I_sV_s/V_p = 2200/(33 000) [1]
5
I_p = 0.067 A [1]

29. Why high voltage cuts power loss

The power lost in a cable is given by P = I²R. A higher voltage means a lower current for the same power, so high voltage reduces current. The resistance of the cables does not change.

The current is squared, so halving the current cuts the power loss to a quarter. A low current gives less heating effect in the cables.

P = I²R

Examiner insight

“Some candidates were confused about the difference between power transmission and power loss during transmission.” “some candidates believed that stepping up the transmission voltage reduced the resistance of the cables.”

Worked example 12 marks

A power station produces 1.3 GW of electricity. A transformer at the power station steps up the voltage before the electricity is transmitted. This reduces the power loss during transmission. Explain how the step-up transformer reduces power losses during electricity transmission.

1
high voltage means low current (for the same power) [1]
2
low current gives less heating effect [1]

Recap

A transformer has primary and secondary coils on a soft-iron core.
Step-up: more turns on the secondary. Step-down: fewer.
V_p / V_s = N_p / N_s.
High-voltage transmission: low current, less power loss, thinner cables.
Extended: a.c. in the primary gives a changing magnetic field, which induces an e.m.f. in the secondary.
Extended: for 100% efficiency, I_pV_p = I_sV_s.
Extended: P = I²R, so a lower current means a smaller power loss.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

A coil is connected to a sensitive ammeter. A strong bar magnet is held still inside the coil. State the reading on the ammeter.1 mark
A bar magnet is pulled out of a coil connected to a sensitive ammeter. State whether an e.m.f. is induced and give a reason.2 marks
Describe an experiment to show electromagnetic induction using a coil and a bar magnet.3 marks
State two changes that give a larger induced e.m.f. when a bar magnet is pushed into a coil.2 marks
Student A pushes a magnet into a coil in 2.0 s. Student B pushes the same magnet into the same coil in 0.5 s. State which student induces the larger e.m.f. and give a reason.2 marks
A small magnet is fixed to the rim of a bicycle wheel. A coil on the frame is connected to a sensitive ammeter. The ammeter deflects each time the magnet passes the coil. (a) Explain why an e.m.f. is induced in the coil. (b) The wheel turns at the same speed. State two changes that increase the e.m.f.4 marks
State the direction of an induced e.m.f. compared with the change that causes it.1 mark
The N pole of a bar magnet is pushed towards a coil. Then it is pulled away from the coil. State the pole induced at the end of the coil nearest the magnet each time, and explain each.4 marks
A train travels due north on a level track. A metal axle joins two of its wheels and lies along an east–west line. The magnetic field at this place points vertically downwards. (a) State the direction of the induced current in the axle. (b) State the direction in which electrons flow in the axle. (c) The train later travels due south at twice the speed. State two ways in which the induced e.m.f. in the axle changes.5 marks
A coil is held with its axis vertical and is connected to a sensitive ammeter. A bar magnet is dropped through the coil, N pole first. The two ends of a coil that carries a current are opposite poles. As the N pole enters the top of the coil, the pointer deflects to the right. (a) State the pole induced at the top of the coil as the magnet enters. Explain your answer. (b) The S pole of the magnet is the last part to leave the bottom of the coil. State the pole induced at the bottom of the coil as the magnet leaves. Explain your answer. (c) The magnet is moving faster when it leaves the coil than when it enters. Describe and explain the deflection of the pointer as the magnet leaves.6 marks
Name the two parts that keep the coil of an a.c. generator connected to the external circuit as it turns.2 marks
A coil turns in a magnetic field and an e.m.f. is induced. State what the coil does to the magnetic field.1 mark
State the two main parts, other than slip rings and brushes, in a simple a.c. generator with a rotating coil.2 marks
Describe how an a.c. generator can work with a rotating magnet instead of a rotating coil.2 marks
Through what angle does the coil of an a.c. generator turn between one change in the direction of the current and the next?1 mark
The graph shows the e.m.f. of an a.c. generator during one turn of the coil. At which two labelled points does the current in the circuit change direction?2 marks
A student adds more turns to the coil of a generator. The speed of rotation stays the same. State what happens to (a) the frequency and (b) the size of the e.m.f.2 marks
The graph shows the e.m.f. from the generator of a small wind turbine, with the blades turning steadily. The wind gets stronger and the generator turns faster. State two ways the graph changes.2 marks
The graph shows how the e.m.f. of the generator in a hand-cranked torch changes with time. The e.m.f. is zero at 0.05 s and at 0.15 s. (a) State the fraction of a turn that the coil makes between time 0 and 0.05 s. (b) Calculate the number of turns the coil makes in 1.0 s. (c) The handle is then turned so that the coil rotates twice as fast. The e.m.f. again starts at a peak at time 0. State the times of the first two zeros.6 marks
A bicycle dynamo is a simple a.c. generator. A magnet turns inside a coil that stays still. The coil is connected to a lamp. The magnet makes 4 turns for each turn of the bicycle wheel. The wheel makes 3 turns in 1.0 s. (a) Explain why this dynamo needs no slip rings or brushes. (b) Calculate the number of times the current in the lamp changes direction in 1.0 s. (c) A second dynamo has twice as many turns of wire on its coil. A cyclist says that, at the same wheel speed, its current changes direction twice as often. State and explain whether the cyclist is correct.7 marks
Describe the pattern of the magnetic field around a long straight wire that carries a current.2 marks
A student sprinkles iron filings around a wire that carries a current. State what the iron filings show and what they cannot show.2 marks
Describe how to find the direction of the magnetic field at several points around a solenoid that carries a current.3 marks
The statements describe how a relay switches on a motor. Write the letters in the correct order. A The contacts are pushed together. B There is a current in the coil. C The soft-iron armature is attracted. D A magnetic field forms around the coil.2 marks
The output of a logic gate is a low voltage. It must switch on a motor that runs from a higher voltage. Name the one component used and give the reason.2 marks
A student says that a loudspeaker works because the cone receives sound from the air. Correct the student.2 marks
There is a current in a long straight wire. State where the magnetic field is stronger, 2 cm or 6 cm from the wire. Describe how the field strength and the spacing of the field lines change with distance from the wire.3 marks
A plotting compass is just outside the right-hand end of a solenoid. With a current of 1 A, the needle points to the right. The current is reversed and increased to 2 A. State the direction in which the needle now points, what happens to the field strength at the compass, and which change to the current made the needle turn.3 marks
A vertical wire passes through a horizontal card. The current in the wire is 4 A downwards. A student looks down on the card, with north at the top. Plotting compass P is 2 cm east of the wire and compass Q is 6 cm west of it. Ignore the magnetic field of the Earth. (a) State the direction of the field lines as the student sees them, and the direction in which each needle points. (b) State which compass is in the stronger field. Give a reason. (c) The current is changed to 2 A upwards. State two changes to the field at P and the direction in which its needle now points.8 marks
A greenhouse fan runs from a 230 V supply. A 5 V sensor circuit is connected to the coil of a relay with a soft-iron armature. The relay contacts are in the fan circuit. (a) Explain how a current in the coil switches the fan on. (b) A student says that current from the sensor circuit passes through the armature into the fan circuit. State whether this is correct, and why a relay is used here. (c) The armature is replaced by a steel one. With the fan running, the current in the coil is switched off. State and explain what happens to the fan. (d) With the soft-iron armature, the field of the coil is too weak to switch the fan on. State the change to the current that is needed, and what reversing the current would do to the field.9 marks
A thin copper wire rests on two copper rails connected to a d.c. power supply. The wire lies between a N pole and a S pole. State what happens to the wire when the supply is switched on.1 mark
A wire carrying a current experiences a force in a magnetic field. State two changes that each reverse the direction of the force.2 marks
The force on a wire is to the right. The current is reversed and the poles of the magnet are swapped. State the direction of the force now.1 mark
A student wants a bigger force on a wire in a magnetic field. State two changes she can make.2 marks
A student reverses the current in a wire and also uses a stronger magnet. Describe how the force on the wire changes.2 marks
A wire carries a current up the page. The magnetic field is directed to the right across the page. State the direction of the force on the wire, and how it relates to the field and the current.2 marks
A wire carries a current to the right across the page. The magnetic field is directed into the page. State the direction of the force on the wire.1 mark
A beam of electrons travels to the right across the page through a magnetic field directed out of the page. (a) State the direction in which the beam is deflected and explain how you decided. (b) An electron is at rest in the same field. State the magnetic force on this electron.3 marks
In a vacuum tube, a beam of electrons travels up the page. A magnetic field deflects the beam to the left of the page. (a) A student says the magnetic field is directed to the left of the page. Explain why the student is wrong. (b) Determine the direction of the magnetic field. (c) The electrons are replaced by positively charged particles travelling up the page. State the direction in which these particles are deflected.4 marks
A student looks down on a copper rod resting on two horizontal metal rails. The current in the rod is up the page. A magnet gives a magnetic field directed into the page. (a) Determine the direction of the force on the rod. Explain your answer. (b) The student reverses the connections to the power supply, turns the magnet over so that the field is out of the page, and increases the current. Compare the force on the rod now with the force in (a). (c) The rod is removed. A beam of electrons travels up the page through the field in (a). State and explain the direction of the force on the electrons.6 marks
State one way to increase the turning effect on a current-carrying coil in a magnetic field.1 mark
A coil in a magnetic field carries a current. A student wants a greater turning effect without changing the coil or the magnets. State the change she should make.1 mark
A student says: “A stronger magnet makes the coil spin faster, so turning effect means speed of spinning.” Explain what is wrong with this.2 marks
A coil in a magnetic field is connected to a constant p.d. The resistance of the coil is increased. State and explain what happens to the turning effect.2 marks
Name the part of a d.c. motor that changes the direction of the current in the coil every half turn.1 mark
The split-ring commutator is removed from a d.c. motor, so the current in the coil never reverses. Describe what happens to the coil.2 marks
A student says the split-ring commutator stops the wires getting tangled. Give the real purpose of the commutator.2 marks
A student says the brushes carry current from the coil to the circuit. Correct this and state the purpose of the brushes in a d.c. motor.2 marks
A student describes the d.c. motor in an electric drill: “A pair of slip rings reverses the current in the coil. The current is reversed once every full turn. The brushes transfer current from the coil to the circuit. Bigger magnets make the turning effect faster.” Identify and correct the five mistakes in this description.5 marks
A model crane uses a d.c. motor to wind up a load. The motor is connected to a battery that gives a constant p.d. (a) The brushes wear away until they no longer touch the split-ring commutator. State and explain what happens to the turning effect on the coil. (b) New brushes are fitted. The coil is then rewound with more turns of thinner wire, so its resistance is greater. Explain why the turning effect on the coil may not increase.5 marks
Name the material used for the core of a simple transformer.1 mark
A transformer has 200 turns on its primary coil and 50 turns on its secondary coil. State whether it is step-up or step-down. Give a reason.2 marks
A transformer has 400 turns on its primary coil and 50 turns on its secondary coil. The primary coil is connected to 240 V a.c. Calculate the voltage across the secondary coil.3 marks
A transformer has 40 turns on its secondary coil. The primary voltage is 230 V and the secondary voltage is 11.5 V. Calculate the number of turns on the primary coil.3 marks
State two advantages of transmitting electricity at a high voltage.2 marks
A transformer at a power station has 1000 turns on its primary coil and 12 000 turns on its secondary coil. The primary voltage is 25 kV. Calculate the voltage that is transmitted and state the type of transformer.3 marks
Explain how an alternating current in the primary coil of a transformer produces a voltage across the secondary coil.3 marks
A 100% efficient transformer supplies 60 W to a 15 V lamp. Its primary coil is connected to a 240 V supply. Calculate the current in the primary coil.3 marks
A 100% efficient transformer supplies a string of garden lamps from the 230 V a.c. mains. It has 2300 turns on its primary coil and 120 turns on its secondary coil. Each lamp that is added increases the current in the secondary coil by 0.50 A. The current in the primary coil must not be greater than 0.50 A. (a) Calculate the voltage across the secondary coil. (b) Calculate the greatest number of lamps that the transformer can supply.6 marks
A wind turbine generates 180 kW of electrical power at 600 V a.c. The power is sent to a village through a cable of total resistance 0.50 Ω. (a) Calculate the power lost in the cable when the power is transmitted at 600 V. (b) A 100% efficient transformer is connected between the turbine and the cable. It has 150 turns on its primary coil and 4500 turns on its secondary coil. Calculate the power now lost in the cable. (c) The transformer makes the voltage 30 times greater. State how many times smaller the power loss becomes and explain this number.8 marks

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Read the notes

1. Inducing an e.m.f.#

2. An experiment to show induction#

3. Factors affecting the induced e.m.f.#

4. The direction of the induced e.m.f.#

5. Force, field and induced current#

6. A simple a.c. generator#

7. Slip rings and brushes#

8. Graph of e.m.f. against time#

9. Changing the output#

10. Field around a straight wire#

11. Field around a solenoid#

12. Investigating the field pattern#

13. Relays#

14. Loudspeakers#

15. Strength and direction of the field#

16. Showing the force on a wire#

17. Changing the size of the force#

18. Directions of force, field and current#

19. Beams of charged particles#

20. The turning effect on a coil#

21. The split-ring commutator#

22. The brushes#

23. Which parts belong to a d.c. motor#

24. Construction and terms#

25. The transformer equation#

26. High-voltage transmission#

27. How a transformer works#

28. Power in equals power out#

29. Why high voltage cuts power loss#

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Workbook

Chapter at a Glance

Free notes are just the start.

Related chapters

1. Inducing an e.m.f.

2. An experiment to show induction

3. Factors affecting the induced e.m.f.

4. The direction of the induced e.m.f.

5. Force, field and induced current

6. A simple a.c. generator

7. Slip rings and brushes

8. Graph of e.m.f. against time

9. Changing the output

10. Field around a straight wire

11. Field around a solenoid

12. Investigating the field pattern

13. Relays

14. Loudspeakers

15. Strength and direction of the field

16. Showing the force on a wire

17. Changing the size of the force

18. Directions of force, field and current

19. Beams of charged particles

20. The turning effect on a coil

21. The split-ring commutator

22. The brushes

23. Which parts belong to a d.c. motor

24. Construction and terms

25. The transformer equation

26. High-voltage transmission

27. How a transformer works

28. Power in equals power out

29. Why high voltage cuts power loss