Faraday's law

Also called Law of induction

The law of induction: the emf induced around a loop equals the rate at which the magnetic flux through that loop changes. A steady magnetic field, however strong, induces nothing at all.

The AP Physics 2 sheet prints it in absolute-value form only:

E=ΔΦBΔt\left|\mathcal{E}\right| = \left|\frac{\Delta \Phi_B}{\Delta t}\right|

The CED adds the signed version at 12.4.A.4, E=ΔΦB/Δt\mathcal{E} = -\Delta \Phi_B / \Delta t, where the minus sign is Lenz's law. AP Physics C: Electricity and Magnetism writes the same statement as a derivative and as Maxwell's third equation, E=Ed=dΦB/dt\mathcal{E} = \oint \vec{E} \cdot d\vec{\ell} = -d\Phi_B/dt, and prints a separate solenoid form carrying the turn count NN.

Rate of change is the whole content. Not the size of the field, not the size of the flux. Park a loop in the strongest steady field available and the induced emf is zero. Halve the time over which the same flux change happens and you double the emf.

Since ΦB=BAcosθ\Phi_B = BA\cos\theta, there are exactly three levers: change the field strength, change the enclosed area, or rotate the loop. A generator is the third one done continuously.

Two things to keep straight. No turn count appears anywhere on the AP Physics 2 sheet, so an NN-turn coil is not a Physics 2 quantitative case. And an emf is not a current: a loop that is not a closed conducting circuit still has an emf induced around it, with no current to show for it.

Topic 12.4 owns the procedure, including the rod-on-rails case and its derived E=Bv\mathcal{E} = B\ell v.

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