A level Physics · Edexcel 9PH0 · Topic 7D

Electromagnetic induction

Key points

An e.m.f. is induced when a conductor cuts magnetic field lines or when the magnetic flux linkage $N\Phi$ through a coil changes.
Faraday's law: induced e.m.f. equals the rate of change of flux linkage, $\varepsilon = -\frac{d(N\Phi)}{dt}$ .
Lenz's law: the induced current flows in a direction that opposes the change producing it (the minus sign); it follows from conservation of energy.
Increase the induced e.m.f. by moving faster, using a stronger magnet or adding more turns.
A coil rotating in a field (a generator) produces a sinusoidal alternating e.m.f.
For a sinusoidal a.c., $I_{rms} = \frac{I_0}{\sqrt{2}}$ and $V_{rms} = \frac{V_0}{\sqrt{2}}$ ; mean power $= I_{rms}V_{rms}$ .
Transformers use changing flux in an iron core to step voltages up or down; an oscilloscope can show the peak value and period of an a.c. signal.

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Electromagnetic induction is how almost all of our electricity is generated: a changing magnetic field produces an e.m.f. These notes cover Faraday's and Lenz's laws, generators, alternating current and transformers.

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