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Superconductivity Explained
Modern Physics
A superconductor is a material that loses all electrical resistance when cooled below a critical temperature. It can carry current forever with no energy loss, and it expels magnetic fields from its interior.
What superconductivity is
Superconductivity was discovered in 1911, when Heike Kamerlingh Onnes found that mercury's resistance vanished below 4.2 K. In a superconductor, electrons pair up into Cooper pairs, which move through the crystal without scattering off vibrations or impurities, so no energy is lost as heat. Each material has a critical temperature, Tc, below which it superconducts, and a critical magnetic field and critical current above which superconductivity is destroyed. Mercury's Tc is 4.2 K, lead's is 7.2 K, and niobium's is about 9.2 K. The phenomenon is a quantum effect visible on a large scale.
The Meissner effect
A superconductor does more than conduct perfectly: it expels magnetic fields from its interior, an effect called the Meissner effect. If a magnet is placed above a superconductor cooled below Tc, the field is pushed out and the magnet is repelled, so it can levitate and stay stably suspended. This is different from a merely perfect conductor, which would trap any field already inside it. The Meissner effect is the defining test of superconductivity, and it makes the levitation of a magnet over a cooled superconductor one of the most striking demonstrations in physics. The expulsion happens because surface currents in the superconductor cancel the field inside.
Where you see it
Superconducting magnets produce the strong, stable fields in MRI scanners and in particle accelerators such as the Large Hadron Collider, where they bend beams of particles travelling near light speed. Maglev trains float on magnetic repulsion and travel with very little friction. Superconducting cables could carry power with no loss, and sensitive detectors called SQUIDs measure tiny magnetic fields from the brain or the heart. The search for a room-temperature superconductor continues, because it would transform power grids, transport and computing, but no material has yet been confirmed to superconduct at room temperature under ordinary pressure.
📝 Worked example: mercury below its critical temperature
Mercury becomes superconducting below its critical temperature Tc = 4.2 K. Liquid helium boils at 4.2 K, so cooling mercury in liquid helium brings it just to its transition. Cool it further, to 3.0 K, and it is well below Tc. At 3.0 K its electrical resistance is zero, not merely small. A current started in a superconducting mercury ring at 3.0 K would keep flowing with no battery and no measurable loss. Warm it above 4.2 K and the resistance returns, and the current decays away.
Common mistakes
- Saying a superconductor has very low resistance: below its critical temperature its resistance is zero, not just small.
- Confusing a superconductor with a perfect conductor: a perfect conductor would trap a magnetic field, but a superconductor actively expels it, which is the Meissner effect.
- Thinking all superconductors need liquid helium: some materials superconduct above the boiling point of liquid nitrogen, 77 K, and are called high-temperature superconductors.
Practice questions
- A material has Tc = 77 K. Is it superconducting at 90 K?
Show answer
No, because 90 K is above its critical temperature of 77 K, so it would be in its normal, resistive state.
- What happens to a current in a superconducting loop with no battery?
Show answer
Its resistance is zero, so the current persists without a voltage to drive it.
- Why can a magnet levitate above a superconductor?
Show answer
The Meissner effect: it expels magnetic field from its interior, so the magnet is repelled and can float above it.
Frequently asked questions
Does resistance fall gradually to zero as a material cools?
No. The resistance drops sharply to zero at the critical temperature, rather than decreasing gradually to zero.
Can a magnetic field destroy superconductivity?
A strong enough magnetic field destroys superconductivity, which is why superconducting magnets have a maximum field as well as a maximum temperature and current.