AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Energy transfers in everyday appliances: The amount of energy transferred by electrical work can be calculated using the equation:
The energy transferred by electrical work is calculated from the charge that flows and the potential difference across the component.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The energy transferred by electrical work is calculated from the charge that flows and the potential difference across the component. The equation is E = QV, where E is energy transferred in joules, Q is charge in coulombs and V is potential difference in volts. Charge itself is current multiplied by time, Q = It, so the energy equation can also be written as E = VIt. For example, a 230 V heater drawing 4 A for 300 s transfers E = 230 × 4 × 300 = 276 000 J, or 276 kJ. Rearranging the equation allows charge or potential difference to be found when the energy transferred is known.
What earns the marks
- Recall and write the equation E = QV, identifying each symbol and its unit.
- Substitute charge in coulombs and potential difference in volts to calculate energy in joules.
- Use the linked equation Q = It to find charge when current and time are given.
- Rearrange E = QV to calculate charge or potential difference when the other quantities are known.
- Convert units where necessary, such as minutes to seconds or kilojoules to joules, before calculating.
Common mistakes and how to put them right
Mixing up E = QV with E = Pt.
Put it right: E = Pt uses power in watts and time in seconds, while E = QV uses charge and potential difference.
Forgetting to convert time to seconds when using Q = It.
Put it right: Multiply current in amperes by time in seconds to obtain charge in coulombs.
Treating potential difference as energy.
Put it right: Potential difference is energy transferred per unit charge, measured in volts.
Exam tips
- Show the equation, substitution and answer with units in a clear sequence.
- If current and time are given instead of charge, calculate Q = It first, then use E = QV.
- Estimate the size of the answer to check that the calculated energy is reasonable for the appliance.
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: lesson, made from checked teaching notes for this specification point.
Have a go first
A 230 V mains kettle transfers 138 000 J of energy in 60 s. Calculate the current in the kettle.
Listen
The energy transferred by electrical work is calculated from the charge that flows and the potential difference across the component. The equation is E = QV, where E is energy transferred in joules, Q is charge in coulombs and V is potential difference in volts. Charge itself is current multiplied by time, Q = It, so the energy equation can also be written as E = VIt. For example, a 230 V heater drawing 4 A for 300 s transfers E = 230 × 4 × 300 = 276 000 J, or 276 kJ. Rearranging the equation allows charge or potential difference to be found when the energy transferred is known. A strong answer earns marks for these points. Recall and write the equation E = QV, identifying each symbol and its unit. Substitute charge in coulombs and potential difference in volts to calculate energy in joules. Use the linked equation Q = It to find charge when current and time are given. Rearrange E = QV to calculate charge or potential difference when the other quantities are known. A common slip: Mixing up E = QV with E = Pt. Instead, E = Pt uses power in watts and time in seconds, while E = QV uses charge and potential difference. Exam tip: Show the equation, substitution and answer with units in a clear sequence.
Key words
Say what each word means before you turn the card.
Charge, current and time relationships
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Potential difference as energy per unit charge
Try answering aloud or in your head before turning the card.
Unit conversion and calculation accuracy
Try answering aloud or in your head before turning the card.
See every definition
- Charge, current and time relationships
- Charge is current multiplied by time, Q = It, so the energy equation can also be written as E = VIt.
- Potential difference as energy per unit charge
- Potential difference is energy transferred per unit charge, measured in volts.
- Unit conversion and calculation accuracy
- Convert units where necessary, such as minutes to seconds or kilojoules to joules, before calculating.
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