Superconductivity is a state in which certain materials carry electric current with zero electrical resistance below a specific critical temperature. This means a current can flow without losing energy as heat, unlike in ordinary wires. The effect matters because it could make power systems, magnets, sensors, and transportation much more efficient.
It also reveals how quantum physics can appear on a large, visible scale.
Key Facts
- Superconductivity means electrical resistance drops to zero below the critical temperature Tc.
- Ohm's law is V = IR, so if R = 0 a steady superconducting current can flow with V = 0.
- The Meissner effect is the expulsion of magnetic field from the inside of a superconductor.
- Magnetic levitation occurs when expelled magnetic fields create forces that can support a magnet above a superconductor.
- A superconductor stops being superconducting if temperature T > Tc, magnetic field B > Bc, or current I > Ic.
- Magnetic flux through a loop is Φ = BA cos θ, and superconductors strongly restrict changes in magnetic flux.
Vocabulary
- Superconductor
- A material that has zero electrical resistance and expels magnetic fields when cooled below its critical temperature.
- Critical temperature
- The temperature below which a material enters the superconducting state.
- Meissner effect
- The expulsion of magnetic field from the interior of a superconductor as it becomes superconducting.
- Cooper pair
- A pair of electrons that move together through a superconductor in a coordinated quantum state.
- Critical magnetic field
- The maximum magnetic field a superconductor can withstand before losing superconductivity.
Common Mistakes to Avoid
- Thinking zero resistance means infinite current in every situation is wrong because the current is limited by the circuit, the power source, and the critical current of the material.
- Confusing perfect conductivity with the Meissner effect is wrong because zero resistance alone does not explain why magnetic fields are expelled from a superconductor.
- Assuming all materials become superconductors if cooled enough is wrong because superconductivity depends on the material's electronic structure and interactions.
- Ignoring critical limits is wrong because a superconductor can return to a normal resistive state if its temperature, magnetic field, or current becomes too large.
Practice Questions
- 1 A copper wire has resistance 2.0 Ω and carries 3.0 A. What voltage is needed across it? If the same current flows in a superconducting wire with R = 0 Ω, what voltage is needed to maintain the current?
- 2 A material has a critical temperature Tc = 92 K. Convert this temperature to degrees Celsius using °C = K - 273.15. Is it superconducting at the temperature of liquid nitrogen, 77 K, if no other critical limit is exceeded?
- 3 Explain why a magnet can levitate above a cooled superconductor and why this levitation is evidence of more than just ordinary zero resistance.