AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Basic principles and uses of heat pumps and refrigerators.
Full specification wording
Reversed heat engines (A-level only): Basic principles and uses of heat pumps and refrigerators.
A heat pump or refrigerator is a reversed heat engine: work input moves thermal energy from a cold region to a hot region.
Checked against the AQA 7408 specification (opens in a new tab) on .
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A heat pump or refrigerator is a reversed heat engine: work input moves thermal energy from a cold region to a hot region. A refrigerator extracts energy Q_C from a cold interior at T_C and rejects Q_H = Q_C + W to the warmer surroundings at T_H. A heat pump delivers Q_H to a warm space, taking Q_C from a colder outside region. The coefficient of performance (COP) measures useful energy transfer per unit work input: for a refrigerator, COP = Q_C/W; for a heat pump, COP = Q_H/W. For example, a refrigerator with COP 4 removes 4 J of thermal energy from its interior for every 1 J of electrical work supplied.
What earns the marks
- State that a heat pump or refrigerator uses work input to transfer thermal energy from a cold region to a hot region.
- Apply the energy balance Q_H = Q_C + W to a reversed heat engine.
- Calculate the coefficient of performance of a refrigerator using COP = Q_C/W.
- Calculate the coefficient of performance of a heat pump using COP = Q_H/W.
- Describe practical uses such as domestic refrigerators, air conditioners and heat pumps for space heating.
Common mistakes and how to put them right
Treating a refrigerator as a device that creates coldness.
Put it right by explaining that it removes thermal energy from the cold interior and rejects it to warmer surroundings.
Using the heat pump COP expression for a refrigerator or vice versa.
Put it right by matching the useful energy transfer to the device: Q_C for a refrigerator and Q_H for a heat pump.
Assuming COP must be less than 1.
Put it right by noting that COP can exceed 1 because work input moves additional thermal energy rather than creating it.
Exam tips
- Draw and label an energy flow diagram showing Q_C, W and Q_H before substituting into equations.
- Check whether the question asks for refrigerator COP or heat pump COP and select the matching expression.
- Use energy conservation Q_H = Q_C + W to find any missing energy value before calculating COP.
Try the lesson
The start of the Reversed heat engines (A-level only): Basic principles and uses of heat pumps and refrigerators. lesson, made from checked teaching notes for this specification point.
Have a go first
A refrigerator is a reversed heat engine. What is the main energy transfer that a refrigerator is designed to produce?
Listen
A heat pump or refrigerator is a reversed heat engine: work input moves thermal energy from a cold region to a hot region. A refrigerator extracts energy Q_C from a cold interior at T_C and rejects Q_H = Q_C + W to the warmer surroundings at T_H. A heat pump delivers Q_H to a warm space, taking Q_C from a colder outside region. The coefficient of performance (COP) measures useful energy transfer per unit work input: for a refrigerator, COP = Q_C/W; for a heat pump, COP = Q_H/W. For example, a refrigerator with COP 4 removes 4 J of thermal energy from its interior for every 1 J of electrical work supplied. A strong answer earns marks for these points. State that a heat pump or refrigerator uses work input to transfer thermal energy from a cold region to a hot region. Apply the energy balance Q_H = Q_C + W to a reversed heat engine. Calculate the coefficient of performance of a refrigerator using COP = Q_C/W. Calculate the coefficient of performance of a heat pump using COP = Q_H/W. A common slip: Treating a refrigerator as a device that creates coldness. Instead, explaining that it removes thermal energy from the cold interior and rejects it to warmer surroundings. Exam tip: Draw and label an energy flow diagram showing Q_C, W and Q_H before substituting into equations.
Key words
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Reversed heat engines
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Coefficient of performance
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Practical applications of thermodynamics
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See every definition
- Reversed heat engines
- A heat pump or refrigerator, in which work input moves thermal energy from a cold region to a hot region.
- Coefficient of performance
- A measure of useful energy transfer per unit work input, given by Q_C/W for a refrigerator and Q_H/W for a heat pump.
- Practical applications of thermodynamics
- Uses such as domestic refrigerators, air conditioners and heat pumps for space heating.
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