A garden irrigation pump is connected to a mains power supply. The pump operates at a potential difference of 230 V and draws a current of 2 A. After one hour of continuous use, the pump stops working. A technician finds that the resistance of the pump motor has increased significantly compared to when it was first switched on. Explain why the resistance of the pump motor increases during use, and describe how this affects the power transferred by the pump.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A garden irrigation pump is connected to a mains power supply. The pump operates at a potential difference of 230 V and draws a current of 2 A. The pump runs continuously for one hour before the technician inspects it.
Model answer (5 marks)
1. When the pump is switched on, electrons flow through the windings and collide with the metal atoms.
2. Each collision transfers energy to the atoms, increasing their vibrational motion – the windings heat up.
3. The increased atomic vibration makes it harder for electrons to move, so the electrical resistance of the windings rises.
4. Power delivered by the mains is P = IV. With the supply voltage fixed at 230 V, an increase in resistance reduces the current drawn.
5. Because the current is lower, the power transferred to the pump decreases, so the pump’s mechanical output falls.
2. Each collision transfers energy to the atoms, increasing their vibrational motion – the windings heat up.
3. The increased atomic vibration makes it harder for electrons to move, so the electrical resistance of the windings rises.
4. Power delivered by the mains is P = IV. With the supply voltage fixed at 230 V, an increase in resistance reduces the current drawn.
5. Because the current is lower, the power transferred to the pump decreases, so the pump’s mechanical output falls.
Examiner tips
- Use the formula P=IV and explain the effect of a higher R on I; mention that V is constant; link heating to resistance increase; keep answer concise and structured.
- Show the chain: current → collisions → heating → higher R → lower I → lower P.
- Use UK spelling (e.g., ‘colour’).
Common mistakes
- Assuming resistance decreases with heating; confusing voltage with current; not linking resistance change to current change; omitting the P=IV relationship.
Mark scheme (5 marks)
- As current flows through the motor, electrons collide with atoms in the resistor/motor windings
- These collisions transfer energy to the atoms, causing them to vibrate more / the motor gets hotter
- Increased vibrations make it more difficult for electrons to flow, so resistance increases
- Power is equal to current multiplied by potential difference (P = IV), so if resistance increases and potential difference stays the same, current decreases
- Therefore power transferred decreases because power depends on current and a lower current means less power is transferred
Key terms in this question
resistance · potential difference · power · current
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Work, energy and power questions
- A student is investigating two electric heaters, Heater A and Heater B, connecte…
- A rechargeable electric bicycle motor transfers energy to the wheels. The motor …
- A building uses two types of electric heaters in its entrance hall. Heater X is …
- A cinema uses two types of lighting in its foyer: a set of filament lamps and a …