What is the angle made by the plane of eddy currents with the plane of magnetic lines of force?

The correct option is 90°.

To understand why this is correct, let us break down the concept of eddy currents and magnetic lines of force step-by-step:

1. Definition of Eddy Currents:
When a bulk conductor is placed in a changing magnetic field, or when it moves through a magnetic field, the magnetic flux passing through the conductor changes over time. According to Faraday's Law of Electromagnetic Induction, this change in flux induces circulating currents within the bulk of the conductor. These closed loops of induced current are known as eddy currents.

2. Direction and Plane of Eddy Currents:
According to Lenz's Law, the induced eddy currents will flow in such a direction that the magnetic field created by them opposes the change in the original magnetic field that produced them. Because the changing magnetic flux passes through the surface of the conductor, the induced circulating currents must flow in paths that lie in planes perpendicular to the magnetic lines of force causing the flux change. This orientation ensures that the magnetic field produced by the eddy currents is parallel (or anti-parallel) to the external magnetic lines of force, thereby opposing the change in flux effectively.

3. Mathematical Relation:
Let the magnetic field vector be denoted by $\mathbf{B}$ and the area vector representing the plane of the eddy current loop be denoted by $\mathbf{A}$. The area vector $\mathbf{A}$ is perpendicular to the plane of the loop. For the magnetic flux to pass through the loop, the magnetic lines of force must be perpendicular to the plane of the loop. Therefore, the angle between the plane of the eddy currents and the direction of the magnetic lines of force is 90° (perpendicular).