Question

In a superconducting ring, a persistent current has been flowing without decay for years. This is possible because

• A. Resistance is exactly zero and flux is quantized
• B. Resistance is very high and current is quantized
• C. Magnetic field is zero and voltage is maximum
• D. Temperature is very high and resistance is finite

Hint:

Persistent current in a superconducting ring is possible due to zero resistance and magnetic flux quantization.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
Superconductivity is a unique state of matter. The question asks why an electric current can flow indefinitely in a closed loop made of such a material without any external power source.
Step 2: Detailed Explanation:

When certain materials are cooled below a specific critical temperature (\(T_c\)), they undergo a phase transition into a superconducting state.

The most prominent characteristic of this state is that the electrical resistance drops to exactly zero. It is not just "extremely low" but physically non-existent for direct current (DC).

According to Ohm's Law (\(V = IR\)), in a normal conductor, resistance causes a portion of the electrical energy to be lost as heat (Joule heating). In a superconductor, because \(R = 0\), no energy is dissipated as heat, allowing the current to flow forever.

Another key feature is that magnetic flux passing through a superconducting ring is quantized. It can only exist in discrete units called fluxons (\(\Phi_0 = h/2e\)).

This flux quantization provides a "topological lock" on the current. Since the flux cannot change by an infinitesimal amount, the current sustaining that flux remains stable and trapped in the ring.

While magnetic field expulsion (Meissner effect) is a property of superconductors, it describes how a material behaves in an external field, not specifically why a current persists.

Option C is incorrect because superconductivity occurs at temperatures above absolute zero (though usually very cold).

Step 3: Final Answer:
The persistence of current is due to the absolute lack of electrical resistance combined with the quantization of magnetic flux in the ring.