A farmer uses an electric fence controller to keep livestock in a field. The controller is connected to the mains supply and transfers energy to the fence wire. Explain how the concepts of potential difference, current, and power are related in determining the rate at which the controller transfers energy to the fence wire.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
An electric fence controller is rated at 230 V and draws a current of 0.5 A from the mains supply. The controller is left running continuously to deter animals from crossing the fence boundary.
Model answer (5 marks)
Potential difference (voltage) is the energy transferred per unit charge between two points in the circuit.
Current is the rate of flow of electrical charge through the fence wire.
Power is the rate at which energy is transferred by the controller.
Power equals the product of current and potential difference: P = IV.
A higher power means the controller transfers energy to the fence wire at a faster rate, linking voltage, current and energy transfer rate.
Current is the rate of flow of electrical charge through the fence wire.
Power is the rate at which energy is transferred by the controller.
Power equals the product of current and potential difference: P = IV.
A higher power means the controller transfers energy to the fence wire at a faster rate, linking voltage, current and energy transfer rate.
Examiner tips
- Use the formula P = IV explicitly; examiners look for the relationship. Show the link: voltage → energy per charge, current → charge per time, power → energy per time. Keep the answer concise and use the exact terms from the mark scheme.
Common mistakes
- Confusing power with voltage or current alone. Forgetting to state that power is energy per unit time. Using the wrong equation (e.g., P = V^2/R instead of P = IV).
Mark scheme (5 marks)
- Potential difference is the energy transferred per unit charge (between two points in the circuit)
- Current is the rate of flow of electrical charge (through the fence wire/circuit)
- Power is the rate at which energy is transferred (by the controller)
- Power equals current multiplied by potential difference (P = IV)
- A greater power means the controller transfers energy to the fence wire at a faster rate (linking all three quantities to energy transfer rate)
Key terms in this question
potential difference · current · power
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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