OCR · GCSE · Physics
Course version: J249 · Version from Sept 2018
Recall and apply: power (W) = potential difference (V) × current (A)
Full specification wording
Simple circuits: recall and apply: power (W) = potential difference (V) × current (A)
Power is the rate of energy transfer, measured in watts (W), where 1 W equals 1 J transferred each second.
Checked against the OCR J249 specification (opens in a new tab) on .
- Point 3 of 14
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Power is the rate of energy transfer, measured in watts (W), where 1 W equals 1 J transferred each second. In a circuit, power equals potential difference in volts (V) multiplied by current in amperes (A): P = V × I. For example, a component with a potential difference of 6 V across it and a current of 2 A through it transfers 6 × 2 = 12 W. Rearranged, V = P ÷ I and I = P ÷ V. In a multiple-choice question, identify which two quantities are given, check their units, then choose the option that multiplies them and gives the answer in watts.
What earns the marks
- Selects the option that multiplies potential difference in volts by current in amperes to give power in watts.
- Applies P = V × I with the correct substitution, for example 6 V × 2 A = 12 W.
- Rearranges the equation correctly when power and one other quantity are given, for example I = P ÷ V.
- Rejects options that confuse the equation with energy = charge × potential difference or that use incorrect units.
Common mistakes and how to put them right
Multiplies charge by potential difference when the question asks for power.
Put it right: Use potential difference × current instead.
Gives power in joules rather than watts.
Put it right: Power is measured in watts.
Divides potential difference by current when the equation requires multiplication.
Put it right: The relationship is a product.
Exam tips
- Recall the equation before looking at the options so you are not misled by plausible wrong answers.
- Check that the answer is in watts, the unit of power.
- Rearrange the equation when the unknown is current or potential difference.
Try the lesson
The start of the Simple circuits: recall and apply: power (W) = potential difference (V) × current (A) lesson, made from checked teaching notes for this specification point.
Have a go first
A heater draws a current of 4 A from a 230 V mains supply. What is the power of the heater?
Listen
Power is the rate of energy transfer, measured in watts (W), where 1 W equals 1 J transferred each second. In a circuit, power equals potential difference in volts (V) multiplied by current in amperes (A): P = V × I. For example, a component with a potential difference of 6 V across it and a current of 2 A through it transfers 6 × 2 = 12 W. Rearranged, V = P ÷ I and I = P ÷ V. In a multiple-choice question, identify which two quantities are given, check their units, then choose the option that multiplies them and gives the answer in watts. A strong answer earns marks for these points. Selects the option that multiplies potential difference in volts by current in amperes to give power in watts. Applies P = V × I with the correct substitution, for example 6 V × 2 A = 12 W. Rearranges the equation correctly when power and one other quantity are given, for example I = P ÷ V. Rejects options that confuse the equation with energy = charge × potential difference or that use incorrect units. Exam tip: Recall the equation before looking at the options so you are not misled by plausible wrong answers.
Key words
Say what each word means before you turn the card.
Electrical power
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Rate of energy transfer
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Using and rearranging equations
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Units and quantities
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See every definition
- Electrical power
- The rate of energy transfer, measured in watts, where 1 W equals 1 J transferred each second.
- Rate of energy transfer
- Power, measured in watts, where 1 W equals 1 J transferred each second.
- Using and rearranging equations
- Applying power equals potential difference multiplied by current, and rearranging it so that potential difference equals power divided by current and current equals power divided by potential difference.
- Units and quantities
- Power is measured in watts, potential difference in volts and current in amperes.
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