Superconductivity Basics: 17 MCQs with answers

Zero electrical resistance

The hallmark of a superconductor is exactly zero electrical resistance below the critical temperature, allowing lossless current flow.

critical

The critical temperature (Tc) is the threshold below which the material enters the superconducting state.

MRI (Magnetic Resonance Imaging)

MRI machines use superconducting magnets to generate strong, stable magnetic fields for imaging.

True

Zero resistance is observed only in the superconducting state, which exists below the critical temperature (Tc).

Meissner effect

The Meissner effect is the expulsion of magnetic field lines from a superconductor when it transitions below Tc, a key property distinguishing it from a perfect conductor.

It is expelled

Below Tc, the superconductor actively expels magnetic field lines from its interior, a hallmark of the Meissner effect.

Mercury

Mercury was the first discovered superconductor (1911) and is a classic type-I superconductor, while the others are type-II (cuprates and MgB2).

Critical current density

The critical current density (Jc) is the maximum current per unit area that a superconductor can sustain before normal resistance reappears.

Cooper

BCS theory explains superconductivity via Cooper pairs—two electrons bound together through lattice vibrations (phonons).

Ferromagnetism

Superconductors are perfectly diamagnetic (Meissner effect), not ferromagnetic. Ferromagnetism is a different magnetic ordering.

The maximum magnetic field that destroys superconductivity

Hc is the maximum applied magnetic field at which a superconductor can remain in the superconducting state; above it, superconductivity is destroyed.

YBCO (YBa2Cu3O7)

YBCO has a critical temperature around 92 K, above the boiling point of liquid nitrogen (77 K), making it a high-temperature superconductor.

It is a consequence of perfect diamagnetism

The Meissner effect is the macroscopic manifestation of perfect diamagnetism in superconductors, where the magnetic susceptibility is -1.

Niobium-tin (Nb3Sn)

Nb3Sn is a type-II superconductor used in high-field magnets, whereas lead, mercury, and tin are type-I superconductors.

False

The Meissner effect occurs in both type-I and type-II superconductors, though type-II superconductors allow partial flux penetration above the lower critical field.

The energy needed to break a Cooper pair

The superconducting energy gap (2Δ) is the minimum energy required to break a Cooper pair into two normal electrons, leading to quasiparticle excitations.

The depth to which a magnetic field penetrates a superconductor

The London penetration depth (λ) characterizes how far a magnetic field can penetrate into the surface of a superconductor before decaying to zero.