Electrical generator

An electrical generator converts mechanical work into electrical energy. In an induction generator, relative motion changes how a conducting circuit interacts with a magnetic field. For an ideal coil with $N$ turns and area $A$, rotating at angular velocity $\omega$ in a uniform field of magnitude $B$, the linked magnetic flux is $NBA\cos \omega t$. Faraday's law gives the electromotive forcewith sign determined by the chosen loop orientation. Connecting a load lets this electromotive force drive an electric current; the resulting electromagnetic torque opposes the driving motion, consistent with conservation of energy.

Michael Faraday's rotating-disc apparatus makes the conversion visible without a many-turn coil. A copper disc rotated between magnetic poles, with an external circuit contacting its axle and rim. The motion-dependent Lorentz force separates charge between those contacts. This homopolar generator maintains one output polarity for a fixed field and rotation direction, unlike the alternating output of the rotating-coil example.^[ref]

An original quantitative demonstration compares the open-circuit output at different rotation rates while retaining the same coil and field geometry. The ideal prediction is proportionality to $\omega$. Under load, losses and the required driving torque supply additional tests of the conversion model.