Question

A cylindrical bar magnet is kept along the axis of a circular coil. On rotation the magnet about its axis, the coil will be induced in it.
A. A current
B. No current
C. Only an e.m.f
D. Both an e.m.f and a current.

Answer 1

Hint: The relative motion between the magnet and the coil that is responsible for the generation of electric current in the coil.

Complete step by step answer:
When a cylindrical bar magnet is kept along the axis of a circular coil and is rotated about its axis then there is no induced emf. Therefore, no current flows through a circular coil.
Because, magnetic field lines remain same at every time during the rotation and the magnetic flux linked to the coil also remains same so that it does not change. Thus there is no current induced in the coil.

According to Faraday's law, whenever there is a change in magnetic flux linked with a coil, an emf is induced in the coil. The emf remains as long as there is change in magnetic flux.
But here, no movement of a bar magnet either forward or backward or else up and down. Here we can see only a rotation, no relative motion between magnet and coil.

So, the correct answer is “Option B”.


Additional Information:
The magnetic flux through any surface held in a magnetic field is defined as the total number of magnetic lines of force crossing the surface normally. It is measured as the product of surface area and the component of magnetic field normal to the surface area.
Magnetic flux through the plane surface area A is given by,
$\phi = BA\cos \theta = \overrightarrow B .\overrightarrow A $
Where $\theta $ is the angle between the magnetic field and normal to the surface area.
According to Faraday’s law, the magnitude of the emf induced in a coil is directly proportional to the rate of change of magnetic flux linked with the coil.
Then, induced emf $\varepsilon \alpha \dfrac{{d\phi }}{{dt}}$
$\varepsilon = - \dfrac{{d\phi }}{{dt}}$

Note:
The phenomenon of production of emf in a conductor or coil when the magnetic flux linking the coil changes is called electromagnetic induction.
Most of the electrical devices like electric generators, transformers etc., are based on this principle.