Energy Transfers in Appliances
Get to grips with Energy Transfers in Appliances for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Appliances transfer energy from the mains, calculated using E = Pt or E = QV, into useful and wasted energy stores.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
When charge flows through an appliance, work is done and energy is transferred. The amount transferred depends on the power of the appliance and how long it is used for.
Two equations apply. Energy transferred = power × time, and energy transferred = charge flow × potential difference. Both give energy in joules, with time in seconds and charge in coulombs.
Everyday appliances transfer energy from the mains or from a battery to kinetic energy stores in motors, or to thermal energy stores in heating devices. The National Grid steps the potential difference up for transmission and down again for domestic use.
Revision notes
The two equations
Energy transferred = power × time, E = Pt.
Energy transferred = charge flow × potential difference, E = QV. Energy in joules, power in watts, time in seconds, charge in coulombs, potential difference in volts.
What appliances do
Work is done when charge flows through an appliance, transferring energy.
Motors transfer energy to kinetic energy stores. Heating devices such as kettles and heaters transfer energy to thermal energy stores. The amount depends on the power and the time of use.
Potential difference as energy per charge
Potential difference is the energy transferred per coulomb of charge.
This is why E = QV works: multiplying the charge that flows by the energy transferred per unit charge gives the total energy transferred.
Key points
- Energy transferred = power × time.
- Energy transferred = charge flow × potential difference.
- Energy is in joules and time in seconds.
- Potential difference is energy per coulomb.
- Motors transfer energy to kinetic stores.
- Heaters transfer energy to thermal stores.
Worked examples
Example 1
A 2000 W kettle is used for 90 seconds. Calculate the energy transferred. [2 marks]
Model answer
Energy transferred = power × time = 2000 × 90 (1 mark) = 180 000 J (1 mark).
Example 2
A charge of 50 C flows through a potential difference of 12 V. Calculate the energy transferred. [2 marks]
Model answer
Energy transferred = charge flow × potential difference = 50 × 12 (1 mark) = 600 J (1 mark).
Example 3
Explain what is meant by a potential difference of 12 V. [2 marks]
Model answer
It means that 12 joules of energy are transferred (1 mark) for every coulomb of charge that flows (1 mark).
Common mistakes
Using minutes instead of seconds.
Time must be in seconds, so multiply minutes by 60.
Confusing the two energy equations.
Use E = Pt with power and time; E = QV with charge and potential difference.
Saying potential difference is a force.
It is the energy transferred per unit charge.
Giving energy in watts.
Energy is in joules; watts measure power.
Exam tips
- Convert time to seconds before substituting.
- Pick the equation matching the data given.
- Define potential difference as energy per coulomb.
- State joules for energy and watts for power.
Key terms
- Energy transferred
- The work done, measured in joules.
- Charge flow
- The quantity of charge passing, in coulombs.
- Potential difference
- Energy transferred per coulomb of charge.
- Appliance
- A device that transfers energy electrically.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.