Induced potential, generators, microphones and transformers

Physics · Magnetism and electromagnetism. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A potential difference is induced across a conductor when it moves relative to a magnetic field, or when the field through it changes. If the conductor is part of a complete circuit, a current flows. This is the generator effect. No change means no induced p.d., however strong the magnet.

The induced current makes its own magnetic field, and this field always opposes the change that caused it.

An alternator and a dynamo both rotate a coil in a magnetic field. A transformer uses the same effect without moving parts: an alternating current in the primary coil produces a changing magnetic field in the iron core, which induces an alternating p.d. across the secondary coil.

Rules to remember

Larger induced p.d.: faster movement, stronger magnetic field, more turns on the coil.
Reversing the direction of movement or the poles of the magnet reverses the induced p.d.
Alternator: slip rings, output is ac. Dynamo: split-ring commutator, output is dc that varies in size but not direction.
Microphone: sound makes a diaphragm and coil move in a magnetic field, inducing a p.d. that varies like the sound.
V_p ÷ V_s = n_p ÷ n_s (p.d. ratio = turns ratio).
100% efficient transformer: V_p × I_p = V_s × I_s (power in = power out).
Step-up: more turns on the secondary, p.d. increases. Step-down: fewer turns on the secondary, p.d. decreases.

Common mistake

Students think a step-up transformer increases the power. The power stays the same at best, so when the p.d. goes up the current goes down.

Worked example

A 100% efficient transformer has 1200 turns on the primary coil and 60 turns on the secondary coil. The primary p.d. is 240 V and the secondary current is 2.0 A. What are the secondary p.d. and the primary current?

V_s = V_p × n_s ÷ n_p = 240 × 60 ÷ 1200 = 12 V.
Power out = V_s × I_s = 12 × 2.0 = 24 W, and power in is the same.
I_p = 24 ÷ 240 = 0.10 A.

Answer: Secondary p.d. 12 V; primary current 0.10 A

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