AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Power: power = current ² × resistance
This relationship lets you calculate the power dissipated by a component when you know the current through it and its resistance.
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This relationship lets you calculate the power dissipated by a component when you know the current through it and its resistance. Power is the rate of energy transfer, measured in watts (W), where 1 W = 1 J/s. Current is measured in amperes (A) and resistance in ohms (Ω). Because the current is squared, doubling the current quadruples the power for the same resistance. For example, a 4 Ω resistor carrying 3 A dissipates P = 3² × 4 = 9 × 4 = 36 W. This is useful for heating effects in wires and resistors, and it links to P = VI and V = IR. Rearranged, I² = P ÷ R and R = P ÷ I². Always square the current before multiplying by resistance, and check units are A and Ω to obtain W.
What earns the marks
- State that power is the rate of energy transfer and is measured in watts (W), where 1 W = 1 J/s.
- Identify the equation power = current² × resistance and use it with current in amperes and resistance in ohms.
- Square the current value before multiplying by resistance, for example 3² × 4 = 36 W.
- Rearrange the relationship to find current or resistance when power is known, such as R = P ÷ I².
- Explain that for a fixed resistance, doubling the current increases power by a factor of four because current is squared.
Common mistakes and how to put them right
Multiplying current by resistance before squaring, giving I × R instead of I² × R.
Put it right: Square the current first, then multiply by resistance.
Using current in milliamperes or resistance in kilohms without converting.
Put it right: Convert to A and Ω before substituting.
Treating the equation as P = I × R² and squaring resistance instead of current.
Put it right: The squared term is current, so write I² R.
Exam tips
- Write the equation, substitute values with units, then calculate and give the answer in watts.
- If the current is given in mA, divide by 1000 before squaring to avoid a large error.
- Use the squared relationship to compare two cases: doubling current gives four times the power at constant resistance.
Try the lesson
The start of the Power: power = current ² × resistance lesson, made from checked teaching notes for this specification point.
Have a go first
A resistor has a current of 2 A flowing through it and a resistance of 5 Ω. Using power = current² × resistance, what is the power dissipated by the resistor?
Listen
This relationship lets you calculate the power dissipated by a component when you know the current through it and its resistance. Power is the rate of energy transfer, measured in watts (W), where 1 W = 1 J/s. Current is measured in amperes (A) and resistance in ohms (Ω). Because the current is squared, doubling the current quadruples the power for the same resistance. For example, a 4 Ω resistor carrying 3 A dissipates P = 3² × 4 = 9 × 4 = 36 W. This is useful for heating effects in wires and resistors, and it links to P = VI and V = IR. Rearranged, I² = P ÷ R and R = P ÷ I². Always square the current before multiplying by resistance, and check units are A and Ω to obtain W. A strong answer earns marks for these points. State that power is the rate of energy transfer and is measured in watts (W), where 1 W = 1 J/s. Identify the equation power = current² × resistance and use it with current in amperes and resistance in ohms. Square the current value before multiplying by resistance, for example 3² × 4 = 36 W. Rearrange the relationship to find current or resistance when power is known, such as R = P ÷ I². A common slip: Multiplying current by resistance before squaring, giving I × R instead of I² × R. Instead, square the current first, then multiply by resistance. Exam tip: Write the equation, substitute values with units, then calculate and give the answer in watts.
Key words
Say what each word means before you turn the card.
Power
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Current
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Resistance
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Heating effect of current
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See every definition
- Power
- The rate of energy transfer, measured in watts (W), where 1 W = 1 J/s.
- Current
- A quantity measured in amperes (A) that is squared in the relationship power = current² × resistance.
- Resistance
- A quantity measured in ohms (Ω) that is multiplied by the squared current to give power.
- Heating effect of current
- The effect in wires and resistors that the relationship power = current² × resistance is useful for.
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