Lab: Magnetic Fields and Electromagnetic Induction

A coil hooked to a voltmeter and nothing else still shows a signal whenever the magnetic field through it changes, because changing flux induces an emf. No battery is needed because the coil itself is not the source of energy, the motion of the magnet is. Faraday's law says the induced emf equals the rate of change of flux through the coil, so a faster-moving magnet or a field that swings more violently produces a bigger reading. Lenz's law tells you the direction: the induced current always opposes the change that caused it, which is why dropping a magnet through a coil gives a voltage pulse that flips sign as the magnet enters and then exits.

Mapping a bar magnet's field with a compass works because the compass needle itself is a tiny magnet that aligns with whatever field it sits in. Placing it at many points around the magnet and tracing the direction it points, then connecting those directions, reconstructs the field lines running from the north pole, out and around, into the south pole. For a solenoid, increasing the current increases the field strength inside in direct proportion, which you can verify by placing a small sensor or compass near the coil and recording field readings at several current values, then checking that the plotted relationship is linear.

These labs sit together because they all extract a physical law from a graph: straighten the data, read off the slope, and that slope is the constant in the equation. The drop-the-magnet lab produces a voltage-versus-speed graph whose slope is itself a useful quantity tied to the coil's geometry and magnet strength.