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AQA PhysicsMagnetism and electromagnetism

Electromagnetic induction and transformers

Explain induced potential difference and transformer action.

Start here

The key idea

A changing magnetic field through a conductor can induce a potential difference.

Electromagnetic Induction And Transformers
primary coilsecondary coiliron core

Use the labels to explain the scientific relationship shown.

Revision notes

The bit that matters

Short notes first. Learn the idea, then use the worked example and questions to check it properly.

1

Electromagnetic induction

When a conductor moves through a magnetic field, or when the magnetic field through a coil changes, a potential difference (and a current if the circuit is complete) is induced.This is electromagnetic induction.The size of the induced potential difference increases if the conductor moves faster, the field is stronger, or there are more turns on the coil.If there is no change in the field, no potential difference is induced.

2

The generator effect and Lenz's law

An induced current creates its own magnetic field that always opposes the original change that produced it (Lenz's law).This is why work must be done to keep moving the conductor, and that work is the source of the electrical energy.Reversing the direction of motion or of the field reverses the direction of the induced current.This principle is used in generators and microphones.

3

Generators: alternators and dynamos

); it uses slip rings so the output reverses each half turn.), which gives a potential difference that always acts in the same direction but varies in size.In a microphone, sound waves move a coil near a magnet to induce a varying current that represents the sound.

4

Transformers

A transformer changes the size of an alternating potential difference.It has a primary coil and a secondary coil wound on an iron core; the alternating current in the primary creates a changing magnetic field that induces a potential difference in the secondary.The transformer equation is Vp / Vs = Np / Ns, where V is potential difference and N is the number of turns.For an ideal (100 percent efficient) transformer, power in = power out, so Vp x Ip = Vs x Is.Step-up transformers raise voltage for efficient transmission in the National Grid, reducing current and so reducing energy lost as heat in the cables.

Key terms

Definitions to learn

Electromagnetic induction

Inducing a potential difference by changing the magnetic field through a conductor.

Generator effect

Inducing a current by moving a conductor relative to a magnetic field.

Alternator

A generator that produces alternating current using slip rings.

Transformer

A device that changes the size of an alternating potential difference.

Step-up transformer

A transformer with more secondary turns that increases the potential difference.

Worked example

State two ways to increase the induced potential difference in a coil.

1

Consider the rate of change of magnetic field.

2

Consider the coil.

Final answer

Move the magnet faster, use a stronger magnet or increase the number of turns.

Exam habit

State that a changing magnetic field (from AC) is required for induction.For transformer questions, write Vp/Vs = Np/Ns clearly before calculating.For National Grid questions, link high voltage → low current → less heating → less energy loss.

Watch out

A stationary magnet in a stationary coil does not induce a potential difference.

Examiner tips

How to score full marks

  • 1Induction needs a change: state clearly that the magnetic field through the coil must be changing.
  • 2For transformer calculations keep Vp/Vs and Np/Ns on the correct sides; check whether it steps up or down.
  • 3Explain the National Grid using power = V x I: high voltage means low current, so less heat lost (P = I2 x R).
Practice questions

Try these yourself

Start with the core skill, then open the answer only after you have attempted the full question.

1Explain why transformers require alternating current.
Mark scheme
  1. 1.Consider the changing magnetic field.
Alternating current creates a changing magnetic field, which induces a potential difference in the secondary coil.
2A transformer has 200 primary turns and 800 secondary turns. The primary voltage is 12 V. Find the secondary voltage.
Mark scheme
  1. 1.Use Vp / Vs = Np / Ns.
  2. 2.Rearrange for Vs.
48 V
3Explain why the National Grid uses a high transmission voltage.
Mark scheme
  1. 1.Use power and current.
  2. 2.Link current to heating losses.
For the same power, a higher voltage means a lower current, reducing energy loss by heating in cables.
4State two ways to increase the size of the potential difference induced in a coil.[2 marks]
Mark scheme
  1. 1.Move faster / stronger field.
  2. 2.More turns.
Move the magnet/coil faster or use a stronger magnetic field (1); increase the number of turns on the coil (1).
5Explain why no potential difference is induced when a magnet is held still inside a coil.[2 marks]
Mark scheme
  1. 1.Induction needs a change in field.
  2. 2.No movement means no change.
Electromagnetic induction requires a change in the magnetic field through the coil (1); a stationary magnet produces no change in field, so no potential difference is induced (1).
6A transformer has 200 turns on the primary coil and 50 turns on the secondary coil. The primary potential difference is 240 V. Calculate the secondary potential difference.[3 marks]
Mark scheme
  1. 1.Use Vp / Vs = Np / Ns.
  2. 2.Rearrange and substitute.
  3. 3.Evaluate.
Vp / Vs = Np / Ns, so Vs = Vp x Ns / Np = 240 × 50 / 200 (1) = 240 × 0.25 (1) = 60 V (1).
7State whether this is a step-up or step-down transformer and explain how you can tell.[2 marks]
Mark scheme
  1. 1.Compare turns / voltages.
  2. 2.Identify type.
It is a step-down transformer (1) because the secondary has fewer turns than the primary, so the output potential difference (60 V) is lower than the input (240 V) (1).
8Explain how step-up transformers help the National Grid transmit electricity efficiently over long distances. Refer to current, power loss and the role of step-down transformers.[4 marks]
Mark scheme
  1. 1.Step-up raises voltage, lowers current.
  2. 2.Lower current means less heat loss in cables (P = I2 R).
  3. 3.Power transmitted at high V efficiently.
  4. 4.Step-down lowers voltage for safe use.
Step-up transformers raise the potential difference for transmission; for the same power, a higher voltage means a lower current (P = V x I) (1). A lower current greatly reduces the energy wasted as heat in the cables, since power loss = I2 x R (1), making transmission more efficient over long distances (1). Step-down transformers then lower the potential difference again so electricity can be used safely in homes (1).
9A step-up transformer has 500 primary turns and 4000 secondary turns. The primary is connected to a 25 V a.c. supply. Calculate the secondary voltage. If the secondary current is 0.10 A and the transformer is ideal, calculate the primary current.[3 marks]
Mark scheme
  1. 1.Vs = Vp x Ns / Np.
  2. 2.For ideal transformer: Vp x Ip = Vs x Is; rearrange for Ip.
Vs = 25 × 4000 / 500 = 200 V (1); for ideal transformer, Vp x Ip = Vs x Is; Ip = (Vs x Is) / Vp = (200 × 0.10) / 25 (1) = 0.80 A (1)
10Explain the difference between an alternator and a dynamo. State what type of current each produces and describe the key structural difference.[3 marks]
Mark scheme
  1. 1.Alternator uses slip rings: output reverses each half turn — produces a.c.
  2. 2.Dynamo uses split-ring commutator: output always in same direction — produces d.c.
  3. 3.Slip rings vs split-ring commutator is the key structural difference.
An alternator uses slip rings that maintain a continuous connection to each end of the rotating coil; as the coil rotates, the current through the external circuit reverses each half turn, producing alternating current (a.c.) (1); a dynamo uses a split-ring commutator that swaps the connections to the external circuit each half turn, so that even though the current in the coil reverses, the current in the external circuit always flows in the same direction, producing direct current (d.c.) (1); the key structural difference is therefore the connection mechanism — slip rings for the alternator and a split-ring commutator for the dynamo (1)
11A student pushes a bar magnet into a coil connected to a galvanometer. Describe and explain what the student observes, and state what happens when (a) the magnet is pushed in faster, (b) the magnet is held still inside the coil, and (c) the magnet is pulled out.[4 marks]
Mark scheme
  1. 1.Pushing in: changing field induces current; galvanometer deflects.
  2. 2.(a) Faster: larger induced e.m.f.; larger deflection.
  3. 3.(b) Still: no change in field; no deflection.
  4. 4.(c) Pulling out: field decreases; current reverses; deflection in opposite direction.
As the magnet enters the coil the magnetic field through the coil increases; this changing field induces a potential difference and a current, causing the galvanometer to deflect in one direction (1); (a) pushing faster increases the rate of change of magnetic field, inducing a larger potential difference and a greater deflection (1); (b) holding the magnet still means there is no change in the magnetic field, so no potential difference is induced and the galvanometer reads zero (1); (c) pulling the magnet out decreases the field through the coil; the induced current reverses direction (by Lenz's law, opposing the decrease), so the galvanometer deflects in the opposite direction (1)
12Explain Lenz's law and use it to determine the direction of the induced current when a bar magnet's north pole is moved towards a coil. Describe how this is consistent with conservation of energy.[4 marks]
Mark scheme
  1. 1.Lenz's law: induced current opposes the change causing it.
  2. 2.North pole approaching: induced current creates a north pole at near end of coil to repel.
  3. 3.Use right-hand grip rule to find current direction in coil.
  4. 4.Work must be done to push magnet against repulsion — this work becomes electrical energy.
Lenz's law states that the direction of any induced current is such that it opposes the change in magnetic flux that caused it (1); when the north pole of a magnet approaches a coil, the induced current flows in a direction that makes the near end of the coil a north pole — this creates a repulsive force on the approaching magnet, opposing its motion (1); to continue pushing the magnet in, an external force must do work against this repulsive force; by conservation of energy, the mechanical work done by the person becomes the electrical energy of the induced current (1); if Lenz's law did not hold, the coil would attract the magnet and accelerate it, generating ever-increasing current without any energy input — a perpetual motion machine, which is impossible (1)
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