Retrieval Practice
Current Electricity — AQA A-Level Physics
Q1. Define electric current.
Electric current is the rate of flow of charge.
Q2. What does an electric current of one ampere represent?
A rate of flow of charge of one coulomb per second.
Q3. Define potential difference.
- The work done per unit charge.
- V = W/Q.
Q4. What is resistance and how is it defined?
- A measure of how much a component opposes the flow of current.
- R = V/I.
- Unit: ohm (Ω).
Q5. State Ohm's law.
The current through a metallic conductor is directly proportional to the p.d. across it, provided the physical conditions do not change.
Q6. What is an ohmic conductor?
- A conductor that obeys Ohm's law.
- It has a constant resistance, giving a straight-line I–V graph through the origin.
Q7. Describe the I–V characteristic for a filament lamp.
- A curve through the origin with decreasing gradient.
- Resistance increases because higher current raises the filament temperature, increasing lattice vibrations and electron collisions.
Q8. Describe the I–V characteristic for a diode.
- In forward bias: negligible current until the threshold voltage (~0.6 V), then current increases steeply.
- In reverse bias: current is approximately zero.
Q9. What is a negative temperature coefficient (NTC) thermistor?
A component whose resistance decreases as temperature increases, because higher temperature releases more charge carriers.
Q10. What is a superconductor?
- A material that has zero resistivity at and below a critical temperature.
- Current flows with no energy loss.
Q11. Why are superconductors useful?
- Current has no heating effect so no energy is lost.
- Used in MRI scanners, particle accelerators, and efficient power transmission cables.
Q12. What is resistivity, and what is its unit?
- A material property: the resistance of a 1 m length with cross-sectional area 1 m². ρ = RA/L.
- Unit: Ω m.