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 small but finite
• C. The ring is at absolute zero temperature
• D. Magnetic field lines are expelled

Hint:

Zero Resistance = Zero Energy Loss = Infinite Flow.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Superconductivity is a state of matter where a material exhibits exactly zero electrical resistance and the expulsion of magnetic fields (Meissner effect) when cooled below a characteristic critical temperature.
Step 2: Key Formula or Approach:
According to Ohm's Law, \( V = IR \). If the resistance \( R \) is exactly zero, then the potential difference \( V \) required to maintain a current \( I \) is zero. Consequently, energy is not dissipated as heat (\( P = I^2R = 0 \)).
Step 3: Detailed Explanation:
When a current is induced in a superconducting loop, there is no "friction" (resistance) to slow down the electrons. Therefore, the kinetic energy of the charge carriers remains constant indefinitely. Additionally, in a closed superconducting loop, the magnetic flux through the ring remains trapped and is quantized in units of the fluxoid, which stabilizes the persistent current.
Step 4: Final Answer:
Persistent current is possible because the resistance is exactly zero and the magnetic flux is quantized.