Question

A cylindrical bar magnet is kept along the axis of a circular coil. If the magnet is rotated about its axis, then

A

A current will be induced in the coil

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B

No current will be induced in the coil

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C

Only emf will be induced in the coil

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D

An emf and current both will be induced in the coil

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Solution

The correct option is B No current will be induced in the coil According to Faraday's law, "whenever magnetic flux linking on a conductor changes, induced emf (electromotive force) is produced.Induced emf produces an electric current in the conductor.The phenomenon of producing induced emf or electricity in a conductor due to changing magnetic flux is called “electromagnetic induction”.Magnetic flux ($\varphi$) linking on the conductor of area ($A$) placed at an angle ($\theta$) in a magnetic field of strength (B) is given by$\varphi =BA\mathrm{cos}\theta$From the above equation, it is clear the magnetic flux linking the conductor can be changed by changing the magnetic field or area, or inclination.When a cylindrical magnet is rotated about its longitudinal axis, the magnetic field, area and the inclination of the magnet do not change and consequently, the magnetic flux linking on the coil remains the same.But according to the basic crux of Faraday's law, “ Change in the magnetic flux linking on the conductor is the basic prerequisite for the generation of the induced emf (electromotive force) and the electric current”.Due to the constant magnetic flux linking on the coil due to the rotation of the cylindrical magnet about its longitudinal axis, no induced emf is produced in the coil.Since no induced emf is produced, therefore “no electric current” is produced in the coil.Below is the diagram showing a cylindrical bar magnet kept along the axis of a circular coil.Hence, option (B) is correct.

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