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Induction (Faraday's law)

Magnetic flux is a measure of how much magnetic field passes through a surface. Only a change in the flux induces a voltage in a coil, and the direction of the induced current is always such that it opposes the change (Lenz's law).

Φ=BAcos⁡θ\Phi = BA\cos\thetamagnetic flux
ε=−NΔΦΔt\varepsilon = -N\dfrac{\Delta\Phi}{\Delta t}Faraday's law

Symbols

Φ\Phimagnetic fluxWb
NNnumber of turns
ε\varepsiloninduced voltageV

Example

A coil with N=200N = 200 turns and A=0.01A = 0.01 m² is perpendicular to a field increasing by 0.50.5 T over 0.10.1 s. ε=200⋅0.01⋅0.5/0.1=10\varepsilon = 200\cdot 0.01\cdot 0.5/0.1 = 10 V.

A strong but constant magnetic field induces no voltage – only a change in the flux counts.
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