Electricity and magnetism · GCSE Physics
Electromagnetic induction
Teacher-written GCSE Physics revision on electromagnetic induction: induced potential, generators, transformers and V₁/V₂ = n₁/n₂ — fuller on Triple / Physics than Combined.
A changing magnetic field through a coil induces a potential difference. Faster movement, a stronger magnet or more turns increases the induced p.d. Transformers need a.c.
The important bits
What you need to know
- 1
This topic is fuller on GCSE Physics (Triple) than on Combined Science: Combined still needs the national-grid transformer idea; Physics adds generators and the transformer equation in more depth.
- 2
A conductor in a changing magnetic field, or moving across a magnetic field, has an induced potential difference. If the circuit is closed, an induced current flows.
- 3
Increase induced p.d. by moving the magnet or coil faster, using a stronger magnet, or using more turns. Reverse the motion and the induced p.d. reverses.
- 4
A generator rotates a coil in a magnetic field (or rotates magnets past a coil). Slip rings and brushes give a.c. A split-ring commutator can give d.c. from a dynamo.
- 5
A transformer uses two coils on an iron core. An alternating current in the primary produces a changing field, which induces an alternating p.d. in the secondary.
- 6
Transformer equation: V₁ / V₂ = n₁ / n₂, where n is the number of turns. Step-up: more turns on the secondary, larger V, smaller I. Step-down: the reverse.
- 7
For an ideal transformer, input power = output power, so V₁I₁ = V₂I₂. That is why stepping up V steps down I.
- 8
Microphones reverse the loudspeaker idea (generator effect): a vibrating coil in a field induces a p.d. that follows the sound.
Quotations worth analysing
Short evidence. Real method.
“A potential difference is induced across a conductor when it moves in a magnetic field, or when the magnetic field through it changes.”
Change is the requirement. A magnet sitting still in a coil gives a reading of zero after any initial blip.
“V₁ / V₂ = n₁ / n₂”
Turns ratio sets the voltages. Combine with V₁I₁ = V₂I₂ for an ideal transformer to find the currents.
“Transformers only work with alternating current.”
A steady d.c. current produces a steady field, which does not keep inducing a secondary potential difference.
Go deeper
Motors and transformers share one idea: a field and a current
Motor: current already flowing, field already there, force appears, coil turns. Generator: coil turning in a field, induced p.d. appears, current can flow if the circuit is closed. Transformers need a.c. because only a changing current in the primary makes a changing field in the core, which induces a p.d. in the secondary. The turns ratio sets whether V goes up or down. Power is conserved in the ideal case, so if V rises, I falls — which is why the grid uses them. Fleming’s right-hand rule (Physics) finds the induced current direction; left-hand remains the motor. Practise both on paper until the three directions are automatic. Combined students should still be able to say why the grid uses transformers and a.c.
Go deeper
The induced p.d. is about how quickly the field through the coil changes
Push a magnet slowly into a coil and the voltmeter twitches. Push it faster and the twitch is larger. Hold it still inside and the reading returns to zero. More turns intercept more of that changing field. A stronger magnet means more field to change. That is the whole “how to increase induced p.d.” list. In a power-station generator the same idea is engineered: a turbine spins a magnet or a coil at 50 Hz so the induced a.c. matches the grid frequency. Students write “the magnet charges the coil” as if it were static electricity. It is not. No contact is required; the changing field is the cause. If the coil’s circuit is open you still have an induced p.d. but no current.
See the idea in action
A transformer has 500 turns on the primary and 50 turns on the secondary. The primary is connected to 230 V a.c. V₁ / V₂ = n₁ / n₂ so 230 / V₂ = 500 / 50 = 10, hence V₂ = 23 V. It is a step-down transformer: fewer turns on the secondary, smaller V, larger I. If the secondary supplies 4.6 A to a lamp and the transformer is ideal, V₁I₁ = V₂I₂ so 230 × I₁ = 23 × 4.6, I₁ = 0.46 A. Power in = 230 × 0.46 = 106 W; power out = 23 × 4.6 = 106 W. Stepping voltage down has stepped current up, which is why a 23 V lamp can draw 4.6 A while the 230 V primary only draws 0.46 A.
Exam technique
Turn knowledge into marks
Say whether the question is Combined or Physics. Increase induced p.d. with speed, field strength or turns. Write V₁/V₂ = n₁/n₂, then V₁I₁ = V₂I₂ for an ideal transformer. State that a.c. is required because the field must keep changing.
Common mistakes
Do not give these marks away
- 01
Saying a transformer works with d.c., or mixing up step-up with an increase in current.
- 02
Claiming a stationary magnet in a coil keeps inducing a current.
- 03
Swapping n₁ and n₂ in the transformer equation, or forgetting that ideal power in equals power out.
A transformer has 200 primary turns and 800 secondary turns. The primary p.d. is 12 V a.c. What is the secondary p.d.?
A3.0 V
B12 V
C48 V
D1600 V
Show the answer
48 V. V₁/V₂ = n₁/n₂ so 12/V₂ = 200/800 = 1/4, hence V₂ = 48 V. More turns on the secondary means a step-up transformer.
Quick questions
If this is the bit you searched
How do you increase an induced potential difference?
Move the magnet or coil faster, use a stronger magnet, or use more turns on the coil. Reverse the motion to reverse the p.d.
Why do transformers need a.c.?
A changing current in the primary is needed to produce a changing magnetic field in the core, which induces a potential difference in the secondary.
Is electromagnetic induction on Combined Science?
Combined includes the national grid and why transformers use a.c. Full generator detail, F = BIl, and V₁/V₂ = n₁/n₂ in depth are more typical of GCSE Physics. Check your specification.
What is the difference between a motor and a generator?
A motor uses current in a field to produce a force (motion). A generator uses motion in a field to induce a potential difference. They are reverse processes.