Points to Ponder

The deep link between electricity and magnetism was uncovered in two stages. First, early 19th-century work by Oersted, Ampère, and others showed that moving charges (an electric current) create a magnetic field. Then, around 1830, Faraday and Henry demonstrated the reverse: a moving magnet can generate an electric current in a circuit.

In a closed loop, the induced current always flows in a direction that opposes the change in magnetic flux through the loop — this is Lenz’s law, which follows from energy conservation. For an open circuit, no current flows, but an emf still appears across its ends. This induced emf is given by $\mathcal{E} = -\frac{d\Phi_B}{dt}$, where $\Phi_B$ is the magnetic flux through the loop. …