AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Superconductivity as a property of certain materials which have zero…
Full specification wording
Resistivity: Superconductivity as a property of certain materials which have zero resistivity at and below a critical temperature which depends on the material.
Superconductivity is a state, not a coating: below a material-specific critical temperature T_c, certain materials lose all resistance to current.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 4 of 6
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Superconductivity is a state, not a coating: below a material-specific critical temperature T_c, certain materials lose all resistance to current. Resistivity ρ falls to exactly zero, so a steady current persists with no potential difference and no heating. Above T_c the same sample behaves normally, with ρ rising as temperature rises. T_c is fixed for a given material: mercury near 4.2 K, niobium alloys far higher, ceramic cuprates above 77 K. Because ρ = 0, R = 0, so V = IR gives zero volts across the superconductor for any current. Current is limited by a critical current and critical field, not by resistance.
What earns the marks
- Defines superconductivity as zero resistivity at and below a critical temperature, not merely very low resistivity.
- States that the critical temperature is a fixed property of each material and differs between materials.
- Links zero resistivity to zero resistance, so no potential difference is needed to maintain a steady current.
- Explains that above the critical temperature the material reverts to normal resistive behaviour.
- Uses ρ = 0 with R = ρL/A to show a sample of any length or area has zero resistance.
- Applies V = IR to show zero volts across a superconductor carrying a steady current.
Common mistakes and how to put them right
Saying resistivity becomes very small rather than exactly zero.
Put it right by stating ρ = 0 at and below T_c.
Treating T_c as a universal constant such as 0 K.
Put it right by naming a material and its own T_c.
Confusing resistance with resistivity.
Put it right by noting resistivity is a material property while resistance also depends on length and cross-sectional area.
Exam tips
- Quote the definition with both conditions: zero resistivity and at or below the critical temperature.
- When comparing materials, name each material and its own T_c rather than giving one value for all.
- Use ρ = 0 and V = IR to justify statements about zero potential difference and no heating.
Try the lesson
The start of the Resistivity: Superconductivity as a property of certain materials which have zero resistivity at and below a critical temperature which depends on the material. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes the resistivity of a superconducting material when its temperature is at or below its critical temperature?
Listen
Superconductivity is a state, not a coating: below a material-specific critical temperature T_c, certain materials lose all resistance to current. Resistivity ρ falls to exactly zero, so a steady current persists with no potential difference and no heating. Above T_c the same sample behaves normally, with ρ rising as temperature rises. T_c is fixed for a given material: mercury near 4.2 K, niobium alloys far higher, ceramic cuprates above 77 K. Because ρ = 0, R = 0, so V = IR gives zero volts across the superconductor for any current. Current is limited by a critical current and critical field, not by resistance. A strong answer earns marks for these points. Defines superconductivity as zero resistivity at and below a critical temperature, not merely very low resistivity. States that the critical temperature is a fixed property of each material and differs between materials. Links zero resistivity to zero resistance, so no potential difference is needed to maintain a steady current. Explains that above the critical temperature the material reverts to normal resistive behaviour. A common slip: Saying resistivity becomes very small rather than exactly zero. Instead, stating ρ = 0 at and below T_c. Exam tip: Quote the definition with both conditions: zero resistivity and at or below the critical temperature.
See it
Key words
Say what each word means before you turn the card.
Superconductivity
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Critical temperature
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Resistivity
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See every definition
- Superconductivity
- A property of certain materials which have zero resistivity at and below a critical temperature which depends on the material.
- Critical temperature
- A temperature which depends on the material, at and below which certain materials have zero resistivity.
- Resistivity
- A material property which falls to exactly zero at and below the critical temperature in certain materials.
Label the diagram
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1Box 1
2Box 2
3Box 3
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