OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Power: (b) energy transfer; W = V I t
Energy transferred by an electrical component equals power multiplied by time, so W = V I t.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 5 of 6
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Energy transferred by an electrical component equals power multiplied by time, so W = V I t. Power P = VI, and energy W = Pt, giving W = VIt. For a 230 V heater drawing 4.0 A for 300 s, W = 230 × 4.0 × 300 = 276 000 J, or 2.76 × 10⁵ J. The same relationship links to P = I²R and P = V²/R, so W = I²Rt and W = V²t/R are also valid when resistance is known. In multiple-choice questions, watch the time unit: seconds give joules, while hours give watt-hours. Convert minutes to seconds by multiplying by 60 before substituting.
What earns the marks
- States that energy transferred W equals power multiplied by time.
- Uses W = VIt with potential difference in volts, current in amperes and time in seconds.
- Recognises that W = I²Rt and W = V²t/R follow from substituting the power equations.
- Converts time to seconds before calculating energy in joules.
- Interprets the result as energy in joules, not power in watts.
Common mistakes and how to put them right
Using time in minutes without converting.
Put it right: Multiply minutes by 60 to obtain seconds before substituting into W = VIt.
Confusing energy with power.
Put it right: Power is the rate of transfer in watts, while energy is the total transferred in joules.
Omitting one factor from the product.
Put it right: Multiply all three quantities V, I and t together, not just two of them.
Exam tips
- Write the equation, substitute values with units, then evaluate to reduce errors.
- Check whether the question asks for energy in joules or in kilowatt-hours before finalising the answer.
- Estimate the expected order of magnitude to catch decimal-point slips.
Try the lesson
The start of the Power: (b) energy transfer; W = V I t lesson, made from checked teaching notes for this specification point.
Listen
Energy transferred by an electrical component equals power multiplied by time, so W = V I t. Power P = VI, and energy W = Pt, giving W = VIt. For a 230 V heater drawing 4.0 A for 300 s, W = 230 × 4.0 × 300 = 276 000 J, or 2.76 × 10⁵ J. The same relationship links to P = I²R and P = V²/R, so W = I²Rt and W = V²t/R are also valid when resistance is known. In multiple-choice questions, watch the time unit: seconds give joules, while hours give watt-hours. Convert minutes to seconds by multiplying by 60 before substituting. A strong answer earns marks for these points. States that energy transferred W equals power multiplied by time. Uses W = VIt with potential difference in volts, current in amperes and time in seconds. Recognises that W = I²Rt and W = V²t/R follow from substituting the power equations. Converts time to seconds before calculating energy in joules. A common slip: Using time in minutes without converting. Instead, multiply minutes by 60 to obtain seconds before substituting into W = VIt. Exam tip: Write the equation, substitute values with units, then evaluate to reduce errors.
See it
Key words
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Energy transfer in electrical circuits
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Power as rate of energy transfer
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Unit conversion of time
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Application of W = VIt to real components
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See every definition
- Energy transfer in electrical circuits
- Energy transferred by an electrical component equals power multiplied by time, so W = V I t.
- Power as rate of energy transfer
- Power is the rate of transfer in watts, while energy is the total transferred in joules.
- Unit conversion of time
- Convert minutes to seconds by multiplying by 60 before substituting, since seconds give joules while hours give watt-hours.
- Application of W = VIt to real components
- Multiply all three quantities V, I and t together, not just two of them, to find the energy transferred.
Label the diagram
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1Box 1
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