Question

An illustration with 2 different galvanometers. An illustration of a galvanometer in milliamps that reads from negative 10 m A to 10 m A. The needle points to 7 m A is labeled as A. An illustration of a galvanometer in milliamps that reads from negative 10 m A to 10 m A. The needle points to negative 7 m A is labeled B.
In situation A, a bar magnet moves toward a stationary coil at a constant speed, and the maximum current is recorded. If the magnet moves again toward the coil in situation B at the same speed, from the same direction, what would explain the different current reading?

moving the coil to the right or left
changing the magnet strength
moving the coil toward the magnet at the same speed as the magnet
reversing the magnet polarity

Answer 1

Short answer: The reading reversal would be explained by either reversing the magnet’s polarity or by changing the relative motion so the flux change is in the opposite sense (i.e. moving the coil so the relative velocity between magnet and coil is reversed).

Why:
- Reversing magnet polarity reverses the direction of the induced EMF and current (Lenz’s/Faraday’s law) — +7 mA becomes −7 mA.
- Moving the coil (right/left) can also reverse the direction of flux change (if you make the relative motion equivalent but opposite), so it can change the sign and/or magnitude of the current.

What would not produce the sign flip in itself:
- Changing the magnet strength changes the magnitude of the induced current but not its sign.
- Moving the coil toward the magnet at the same speed as the magnet would make the relative speed zero, so the induced current would drop to zero (not reverse sign).