3.5.1.5
Superconductors have zero resistivity below a critical temperature
Resistivity & Superconductivity — AQA A-Level Physics
Key Definition
Superconductor — A material whose resistivity drops to exactly zero at and below a characteristic temperature called the critical temperature (or transition temperature).
Key Definition
Critical temperature — The temperature at which a material becomes superconducting. Below this temperature, the resistance is zero. Symbol: T_c.
- Below T_c, resistanceThe opposition to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow. The ratio of potential difference to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).. Measured in ohms (Ω). is exactly zero. Not very small -- zero.
- Since R = 0: $V = IR = 0. No potential differenceThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). across a superconductorA material that has zero resistivityA material property that quantifies how strongly it resists currentThe rate of flow of charge. Measured in amperes (A).. Measured in ohm-metres (Ω m). below its critical (transition) temperature. carrying current$.
- No energyThe capacity to do work. Measured in joules (J). is dissipated as heat. Current flows with no energyThe capacity to do work. Measured in joules (J). loss.
- Mercury was the first superconductorA material that has zero resistivityA material property that quantifies how strongly it resists current. Measured in ohm-metres (Ω m). below its critical (transition) temperature. discovered (1911). Its critical temperature is 4.2 K.
- Most conventional superconductors have critical temperatures below 30 K, requiring liquid helium cooling.
- The resistivityA material property that quantifies how strongly it resists current. Measured in ohm-metres (Ω m).-temperature graph shows a sharp, discontinuous drop to zero at T_c.
Graph of resistivity vs temperature. One curve (normal metal) decreases linearly towards a non-zero value at 0 K. The other curve (superconductorA material that has zero resistivity below its critical (transition) temperature.) follows a similar trend but drops abruptly to zero at T_c, then remains at zero below T_c.
Common Mistake
MEDIUM
Students often: Writing that a superconductor has 'very low' or 'negligible' resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω)..
Instead: A superconductor has exactly zero resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). below T_c. The word 'zero' is required for the mark.
Instead: A superconductor has exactly zero resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). below T_c. The word 'zero' is required for the mark.
Examiner Tips and Tricks
- The exam will not ask you to explain the mechanism of superconductivityThe property of certain materials to have zero resistivity below a critical temperature (the transition temperature). (Cooper pairs, BCS theory).
- You only need the definition, the critical temperature concept, the graph shape, and applications.