Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): A superconductor placed in a magnetic field pushes the field lines completely out of it.
Reason (R): A superconductor is a perfect conductor of electricity.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
Assertion (A) A superconductor placed in a magnetic field pushes the field lines completely out of it. — True. A superconductor is a perfect diamagnet. For it \( \chi=-1 \) and \( \mu_r=0 \), so no field is left inside it. This is called the Meissner effect.
Reason (R) A superconductor is a perfect conductor of electricity. — True. At very low temperatures, a superconductor shows perfect conductivity as well as perfect diamagnetism.
Why Reason (R) does not explain Assertion (A)?
Pushing out the field is perfect diamagnetism. It is a magnetic property of the material.
Perfect conductivity is a separate property. No classical theory links the two. The BCS theory, a quantum theory, explains both.
NCERT (2026-27):
Diamagnetism – “… These are metals, cooled to very low temperatures which exhibits both perfect conductivity and perfect diamagnetism. Here the field lines are completely expelled! …”
Points to Ponder – “5. There exists a perfect diamagnet, namely, a superconductor. … The external magnetic field is totally expelled. Interestingly, this material is also a perfect conductor. However, there exists no classical theory which ties these two properties together.”