Retrieval Practice
Resistivity & Superconductivity — AQA A-Level Physics
Q1. Define resistivity and state its unit.
- Resistivity is a material property: the resistance of a 1 m length of material with a cross-sectional area of 1 m². ρ = RA/L.
- Unit: Ω m.
Q2. A wire's diameter doubles. What happens to its resistance?
- Resistance drops to one quarter.
- Area = πd²/4, so doubling d quadruples A.
- Since R = ρL/A, R falls by a factor of 4.
Q3. In the resistivity practical, what graph is plotted and what does the gradient give?
- Plot R (y-axis) against L (x-axis).
- The gradient equals ρ/A.
- Multiply the gradient by the cross-sectional area to find resistivity.
Q4. Why must the wire be switched off between readings in the resistivity practical?
- To prevent the wire from heating up.
- Higher temperature changes the resistivity, which would make the results unreliable.
Q5. Explain why the resistance of a metal increases with temperature.
- Higher temperature causes metal ions to vibrate with greater amplitude.
- Conduction electrons collide more frequently with the ions, increasing opposition to current flow.
Q6. Explain why the resistance of an NTC thermistor decreases with temperature.
- Higher temperature provides thermal energy that frees more electrons from atoms in the semiconductor.
- The increase in charge carrier number outweighs the increase in lattice vibrations, so resistance falls.
Q7. Why are metals and thermistors explained by different mechanisms?
- In metals, the number of charge carriers stays constant so increased vibrations dominate (R increases).
- In thermistors, the number of charge carriers increases dramatically so that effect dominates (R decreases).
Q8. Define a superconductor.
A material that has zero resistivity at and below a critical temperature T_c.
Q9. What is the critical temperature?
- The temperature at which a material transitions to the superconducting state.
- Below T_c, resistance is exactly zero.
Q10. State two applications of superconductors and explain why superconductivity is needed.
- MRI scanners and particle accelerators.
- Both need very strong magnetic fields, which require very large currents.
- Only superconducting wires (R = 0) can carry these currents without I²R heating.
Q11. Describe the shape of a resistance-temperature graph for a superconductor.
- It follows the normal metal curve (resistance decreasing with temperature) until the critical temperature T_c, where it drops abruptly and discontinuously to zero.
- Below T_c it remains at zero.
Q12. Why is the diameter measurement the biggest source of uncertainty in the resistivity practical?
- Because area A = πd²/4, any percentage error in d is doubled when calculating A (and therefore ρ).
- A 5% error in d gives a 10% error in ρ.