Question

A loop of wire is horizontal. A bar magnet is pushed toward the loop from below, along the axis of the loop, as shown in Figure Q25.15.

a. In what direction is the current in the loop? Explain

b. Is there a magnetic force on the loop? If so, in which direction? Explain

c. Is there a magnetic for on the magnet? If so, in which direction?

Answer 1

Concepts and reason

The concepts used in this question are Lenz’s law and magnetic force. Initially, find the direction of induced magnetic field and then apply right-hand thumb rule to find the direction of the induced current. Then, find the magnitude and direction of the magnetic force on the loop by using the left-hand rule. Finally, find the magnitude and direction of the magnetic force on the magnet.

Fundamentals

The Lenz’s law states that the current induced in a circuit due to change in the magnetic field through the circuit is directed in such a way that it opposes the change in flux through the circuit. This also results in the generation of a mechanical force to oppose the motion or change.

(a)

The magnet is moving upward so that the magnetic flux through the loop is increasing. According to Lenz’s law, this change in the magnetic flux will induce another magnetic field in the loop such that this increase of flux can be compensated.

This can happen only if the induced magnetic field is in the direction opposite to the direction of increasing magnetic field. This induced magnetic field will counter balance the increasing magnetic field so that the change in magnetic field through the loop gets opposed.

To oppose the change in magnetic flux through the loop the magnetic field induced is in downwards direction as the increasing magnetic field is in upward direction.

Thus, the induced current is in Clockwise direction.

(b)

The magnet is moving in upward direction. The north-pole of the magnet is in upward direction. The induced magnetic field is in opposite direction as compared to the magnet. Its north pole is in downwards direction such that the south poles of the magnet and the induced magnetic field face each other.

The likes poles repulse each other so that the force on the loop is repulsive in nature. The repulsive force acts on the loop in upward direction.

[Part b]

Part b

(c)

The force on the loop is in upward direction. This force will exert an opposing force on the magnet in the opposite direction.

Thus, the force on the magnet is in downwards direction.

Ans: Part a

The direction of induced current in the loop is clockwise.

Part b

Yes, the force on the loop is in upwards direction.

Part c

Yes, the force on the magnet is in downwards direction.