AQA · GCSE · Physics
Course version: 8463 · Version from Sept 2016
The amount of energy transferred by electrical work can be calculated…
Full specification wording
Energy transfers in everyday appliances: The amount of energy transferred by electrical work can be calculated using the equation: energy trans f erred = power × time E = P t energy trans f erred = charge f low × potential di f f erence E = Q V energy transferred, E, in joules, J power, P, in watts, W time, t, in seconds, s charge flow, Q, in coulombs, C potential difference, V, in volts, V Students should be able to explain how the power of a circuit device is related to: the potential difference across it and the current through it the energy transferred over a given time.
Two equations do this job and they have to agree.
Checked against the AQA 8463 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
Two equations do this job and they have to agree. E = P t uses the power rating and the running time, while E = Q V uses the charge that has passed and the potential difference it passed through. They connect through P = V I and Q = I t, because P t = V I t = V Q. Take a 230 V appliance drawing 2.0 A for 60 s. Its power is 230 × 2.0 = 460 W, so E = 460 × 60 = 27 600 J. Working the other way, the charge is Q = 2.0 × 60 = 120 C, and E = 120 × 230 = 27 600 J, the same answer. Pick the route that matches the data, keep power in watts, time in seconds and charge in coulombs, and the two methods always agree. Power is simply that energy divided by the time.
What earns the marks
- Selecting the equation that matches the data, E = P t or E = Q V.
- A correct intermediate step such as P = V I or Q = I t.
- Correct substitution with the time in seconds.
- The energy transferred with the unit joules, J.
Common mistakes and how to put them right
Mixing the two routes, for example multiplying power by charge.
Using minutes for the time in E = P t.
Quoting the power of a device as its energy transfer without multiplying by time.
Rounding an intermediate power to one significant figure and carrying that error into the final answer.
Exam tips
- List the quantities you have been given before choosing an equation, because the data shows which route is intended.
- Both equations are on the equation sheet, so check the printed form rather than recalling it under pressure.
- Keep full accuracy through the intermediate steps and round only the final answer.
Try the lesson
The start of the Energy transfers in everyday appliances: The amount of energy transferred by electrical work can be calculated using the equation: energy trans f erred = power × time E = P t energy trans f erred = charge f low × potential di f f erence E = Q V energy transferred, E, in joules, J power, P, in watts, W time, t, in seconds, s charge flow, Q, in coulombs, C potential difference, V, in volts, V Students should be able to explain how the power of a circuit device is related to: the potential difference across it and the current through it the energy transferred over a given time. lesson, made from checked teaching notes for this specification point.
Have a go first
Which equation gives the energy transferred by electrical work in terms of power and time?
Listen
Two equations do this job and they have to agree. E = P t uses the power rating and the running time, while E = Q V uses the charge that has passed and the potential difference it passed through. They connect through P = V I and Q = I t, because P t = V I t = V Q. Take a 230 V appliance drawing 2.0 A for 60 s. Its power is 230 × 2.0 = 460 W, so E = 460 × 60 = 27 600 J. Working the other way, the charge is Q = 2.0 × 60 = 120 C, and E = 120 × 230 = 27 600 J, the same answer. Pick the route that matches the data, keep power in watts, time in seconds and charge in coulombs, and the two methods always agree. Power is simply that energy divided by the time. A strong answer earns marks for these points. selecting the equation that matches the data, E = P t or E = Q V. a correct intermediate step such as P = V I or Q = I t. correct substitution with the time in seconds. the energy transferred with the unit joules, J. Exam tip: List the quantities you have been given before choosing an equation, because the data shows which route is intended.
Key words
Say what each word means before you turn the card.
E = P t
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E = Q V
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power and current
Try answering aloud or in your head before turning the card.
consistent units
Try answering aloud or in your head before turning the card.
See every definition
- E = P t
- The equation used to calculate energy transferred, where energy transferred equals power multiplied by time.
- E = Q V
- The equation used to calculate energy transferred, where energy transferred equals charge flow multiplied by potential difference.
- power and current
- The power of a circuit device is related to the potential difference across it and the current through it.
- consistent units
- Keeping power in watts, time in seconds and charge in coulombs so the two methods always agree.
There is more to this lesson
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Test your understanding of Energy transfers in everyday appliances: The amount of energy transferred by electrical work can be calculated using the equation: energy trans f erred = power × time E = P t energy trans f erred = charge f low × potential di f f erence E = Q V energy transferred, E, in joules, J power, P, in watts, W time, t, in seconds, s charge flow, Q, in coulombs, C potential difference, V, in volts, V Students should be able to explain how the power of a circuit device is related to: the potential difference across it and the current through it the energy transferred over a given time.
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