Question

What would you predict would occur if you were able to place a rotating magnet near a coil of wire?

Answer 1

You would produce an induced voltage (and, if the coil is a closed circuit, an induced current). This is electromagnetic induction — the rotating magnet changes the magnetic flux through the coil, and Faraday’s law says the coil develops an emf equal to −N dΦ/dt (N = turns, Φ = flux). The polarity of the induced emf is such that the induced current opposes the change in flux (Lenz’s law), so the coil’s magnetic field resists the magnet’s motion — you feel a braking torque if current flows.

Key points and practical consequences:
- If the coil is open you get an emf but essentially no current; if it’s closed through a load you get an AC current.
- The waveform and frequency depend on how the magnet is oriented and how it rotates. For a simple dipole whose axis sweeps relative to the coil, the flux through the coil varies roughly sinusoidally and the voltage is AC at the rotation frequency (or a harmonic thereof). If you rotate the magnet about an axis that keeps the flux through the coil constant (e.g., spinning a bar magnet about its dipole axis with the coil centered on that axis), you will produce little or no changing flux and little or no emf.
- The induced voltage magnitude depends on magnet strength, distance, coil area, number of turns, and rotation speed (rate of change of flux).
- If current is allowed, mechanical energy is converted into electrical energy and the magnet experiences a retarding torque — this is the generator principle (same effect used in dynamos and alternators).
- You can measure the induced voltage with an oscilloscope or galvanometer; you will see alternating polarity and amplitude depending on the setup.