Electromagnetic Induction: Faraday, Lenz, Generators and Transformers
This domain covers Faraday's experiments, Lenz's law, a.c. and d.c. generators, and the transformer with high-voltage power transmission.
Induction
An emf is induced only when the magnetic flux linked with a coil changes - a magnet moving in or out, the coil moving, or the current in a nearby coil changing. The induced emf increases with faster motion, a stronger magnet, more turns and a soft-iron core. A strong magnet held at rest inside a coil induces nothing, because the flux is not changing.
Lenz's law
The induced current opposes the change producing it. This follows from conservation of energy: the induced current must do work against whatever is causing the change.
Generators and transformer
For induced current, Fleming's right-hand rule applies: forefinger field, thumb motion, middle finger induced current. An a.c. generator uses slip rings; a d.c. generator uses a split-ring commutator.
For an ideal transformer, V_s/V_p = N_s/N_p and V_p I_p = V_s I_s. A step-up transformer raises the voltage and lowers the current. It works only on alternating current, because a changing flux is essential. Power is transmitted at high voltage so that the current, and hence the I²R loss in the cables, is small.
Worked example
A transformer has 200 primary turns connected to 230 V and 1000 secondary turns. V_s = 230 × 1000/200 = 1150 V, a step-up. If the secondary current is 0.2 A, then I_p = V_s I_s / V_p = (1150 × 0.2)/230 = 1 A.
Traps that cost marks
- Expecting an induced current from a stationary magnet.
- Using the left-hand rule for induced current.
- Writing V_s/V_p = N_p/N_s.
- Claiming a transformer works on direct current.