Direction for: In each of the following questions a statement of Assertion is given followed by a corresponding statement of Reason just below it. Of the statements mark the correct answer as
Assertion: A disc-shaped magnet is levitated above a superconducting material that has been cooled by liquid nitrogen. Reason: Superconductors repel a magnet.A) If both assertion and reason are true and reason is the correct explanation of assertion.
B) If both assertion and reason are true but reason is not the correct explanation of assertion.
C) If assertion is true but reason is false.
D) If both assertion and reason are false.
Correct Answer: A
Solution :
Superconductor in a magnetic field will completely repel all field lines. This is called the Meissner effect and is an example of perfect diamagnetism. The perfect exclusion of a magnetic field can be explained by using Faradays law. The integral over a closed loop Of E with \[\left( \phi E.ds=-\frac{\Delta \phi }{\Delta t} \right)\]is equal to rate of change of magnetic flux. Since no potential difference can exist is a superconductor (there is no resistance to make one possible), then the magnetic field inside a superconductor cannot change. When a magnet is placed above a superconductor and cooled by using nitrogen, then the magnetic field inside superconductor is exactly equal and opposite in direction to applied magnetic field. Hence, they are cancelled within the superconductor so these poles repel each other. Therefore, magnet is repelled due to this force of repulsion.
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