Question

The diagram shows a setup for inducing a current in a wire.

A wire connected to the red and blue terminals on a grey box, which contains a white box with a needle pointing to the right with the number 0 above the center of the white box. The wire passes through a 3 sided trough with a bar magnet running on each side of the top inside of the trough. There the wire is labeled with an upward pointing arrow and the words Direction of movement of wire.

Which change would cause the needle on the ammeter to point to the left of the zero?

making the wire thicker
adding coils to the wire
disconnecting the wire from one end of the ammeter
moving the wire downward through the magnetic field

Answer 1

Moving the wire downward through the magnetic field.

Reason: the induced emf (and thus current) direction is set by v × B (or Fleming's right‑hand rule). Reversing the wire's motion reverses the sign of the emf and so reverses the ammeter deflection. Making the wire thicker only changes resistance (magnitude), adding coils changes magnitude (and may increase emf) but not the sense, and disconnecting it produces no current.