Electromagnetic Induction: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 21 · Read time: ~10 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
Inducing an e.m.f.
A changing magnetic field through a coil, or a conductor cutting through magnetic field lines, induces an e.m.f. If the circuit is complete, a current flows.
No change, no e.m.f. A magnet held still inside a coil induces nothing, however strong it is.
Factors that increase the induced e.m.f.: moving the magnet or coil faster, using a stronger magnet, and using more turns on the coil.
Direction (Lenz's law): the induced current flows in a direction that opposes the change producing it. Push a north pole into a coil and the coil's near end becomes a north pole, repelling the magnet.
Trap: Lenz's law questions often ask for the pole at the end of the coil. It opposes the change: a magnet moving in is repelled (same pole), a magnet moving out is attracted (opposite pole). It does not simply copy the magnet.
The a.c. generator
A coil rotates in a magnetic field (or a magnet rotates inside a coil). As the coil cuts the field lines, an alternating e.m.f. is induced.
Slip rings and carbon brushes connect the rotating coil to the outside circuit without the wires twisting.
The output is a sine-shaped voltage-time graph. The e.m.f. is maximum when the plane of the coil is parallel to the field (cutting lines fastest) and zero when it is perpendicular to the field.
Spinning the coil faster gives a higher peak voltage and a higher frequency (more cycles per second).
Trick: Don't mix up the a.c. generator with the d.c. motor. A generator uses slip rings and turns motion into electricity. A motor uses a split-ring commutator and turns electricity into motion.
Transformers and power transmission
A transformer has a primary coil and a secondary coil wound on a soft iron core. An alternating current in the primary creates a changing magnetic field in the core, which induces an alternating e.m.f. in the secondary.
Step-up transformers have more turns on the secondary (Ns > Np) and raise the voltage. Step-down transformers have fewer secondary turns and lower it.
Transformers only work with a.c. A steady d.c. creates an unchanging field, so nothing is induced in the secondary.
Why transmit at high voltage: for the same power, P = VI, a higher voltage means a smaller current. Power lost as heat in the cables is P = I²R, so a smaller current wastes far less energy.
Trap: A step-up transformer does not create energy. If the voltage goes up, the current goes down by the same factor, so Vp Ip = Vs Is.
Worked Example
A transformer has 200 turns on its primary coil and 4,000 turns on its secondary coil. The primary is connected to a 12 V a.c. supply and draws a current of 2.0 A. Assume the transformer is ideal.
(a) Is this a step-up or step-down transformer? Explain. [1]
Step-up, because the secondary has more turns than the primary.
(b) Calculate the output voltage. [2]
Vs = Vp × Ns / Np
= 12 × 4,000 / 200
= 240 V
(c) Calculate the current in the secondary coil. [2]
Vp Ip = Vs Is
12 × 2.0 = 240 × Is
Is = 24 ÷ 240 = 0.10 A
(d) Explain why the transformer would not work if the primary were connected to a 12 V battery. [2]
A battery gives a steady d.c., which produces a constant magnetic field in the core.
With no change in the field, no e.m.f. is induced in the secondary coil.
Why this question is a good test of the topic: students who only learn the turns ratio often assume the current goes up too. Part (c) shows that stepping up voltage steps down current, and part (d) tests why a.c. is essential.
Frequently Asked Questions
What three factors increase the induced e.m.f.?
Moving the magnet or coil faster, using a stronger magnet, and increasing the number of turns on the coil.
What does Lenz's law say?
The induced current always flows in a direction that opposes the change producing it.
Why do transformers only work with a.c.?
They need a changing magnetic field to induce an e.m.f. A steady d.c. produces a constant field, so nothing is induced.
Why is electricity transmitted at high voltage?
A higher voltage means a smaller current for the same power, and power lost in cables (I²R) depends on the current squared.
What do slip rings do in an a.c. generator?
They keep the rotating coil connected to the external circuit without the wires becoming twisted.
Transformer questions still confusing?
Small group O Level Physics classes at TGC Academy's Bishan, Bukit Timah and Potong Pasir centres, helmed by Andrew Seah, MOE Award-Winning Teacher and Marshall Cavendish textbook author.