AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
The internal energy of a system is increased when energy is transferred to…
Full specification wording
Thermal energy transfer (A-level only): The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it (and vice versa), eg a qualitative treatment of the first law of thermodynamics.
The first law of thermodynamics is a statement of energy conservation for a system.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 2 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The first law of thermodynamics is a statement of energy conservation for a system. Internal energy increases when energy is transferred to the system by heating or when work is done on the system. It decreases when energy is transferred from the system by heating or when the system does work on its surroundings. Heating transfers energy due to a temperature difference; doing work transfers energy by mechanical, electrical or other means. For example, compressing a gas quickly does work on it, increasing internal energy and raising temperature; a gas expanding and pushing a piston does work on the surroundings, decreasing internal energy. A qualitative treatment means you should describe the direction of energy transfer and the resulting change in internal energy, without necessarily using the equation ΔU = Q + W.
What earns the marks
- Internal energy increases when energy is transferred to the system by heating.
- Internal energy increases when work is done on the system.
- Internal energy decreases when energy is transferred from the system by heating.
- Internal energy decreases when the system does work on its surroundings.
- Heating is energy transfer due to a temperature difference; work is energy transfer by mechanical, electrical or other non-heating means.
- The first law is a statement of conservation of energy applied to a system, linking internal energy change to heating and work.
Common mistakes and how to put them right
Saying that heating always increases temperature.
Put it right by noting that heating can change internal energy without a temperature change during a change of state.
Confusing work done on the system with work done by the system.
Put it right by stating that work done on the system increases internal energy, while work done by the system decreases it.
Treating heating and work as types of energy stored in the system.
Put it right by describing them as energy transfer processes, while internal energy is the store.
Omitting the sign or direction of transfer.
Put it right by stating whether energy is transferred to or from the system and the resulting increase or decrease in internal energy.
Exam tips
- Use the phrase 'energy transferred to the system by heating' or 'work done on the system' to make the direction clear.
- For a gas, link compression to work done on the gas and expansion to work done by the gas.
- If a temperature change is not observed, consider a change of state and refer to potential energy changes.
- Keep the treatment qualitative unless an equation is requested; focus on the direction of transfer and the change in internal energy.
Try the lesson
The start of the Thermal energy transfer (A-level only): The internal energy of a system is increased when energy is transferred to it by heating or when work is done on it (and vice versa), eg a qualitative treatment of the first law of thermodynamics. lesson, made from checked teaching notes for this specification point.
Have a go first
Which change increases the internal energy of a gas in a cylinder?
Listen
The first law of thermodynamics is a statement of energy conservation for a system. Internal energy increases when energy is transferred to the system by heating or when work is done on the system. It decreases when energy is transferred from the system by heating or when the system does work on its surroundings. Heating transfers energy due to a temperature difference; doing work transfers energy by mechanical, electrical or other means. For example, compressing a gas quickly does work on it, increasing internal energy and raising temperature; a gas expanding and pushing a piston does work on the surroundings, decreasing internal energy. A qualitative treatment means you should describe the direction of energy transfer and the resulting change in internal energy, without necessarily using the equation ΔU = Q + W. A strong answer earns marks for these points. Internal energy increases when energy is transferred to the system by heating. Internal energy increases when work is done on the system. Internal energy decreases when energy is transferred from the system by heating. Internal energy decreases when the system does work on its surroundings. A common slip: Saying that heating always increases temperature. Instead, noting that heating can change internal energy without a temperature change during a change of state. Exam tip: Use the phrase 'energy transferred to the system by heating' or 'work done on the system' to make the direction clear.
Key words
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Conservation of energy
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First law of thermodynamics
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Energy transfer by heating and work
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Internal energy changes in gases
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See every definition
- Conservation of energy
- The first law is a statement of this applied to a system, linking internal energy change to heating and work.
- First law of thermodynamics
- A statement of conservation of energy for a system, linking internal energy change to heating and work.
- Energy transfer by heating and work
- Heating is energy transfer due to a temperature difference; work is energy transfer by mechanical, electrical or other non-heating means.
- Internal energy changes in gases
- Compressing a gas quickly does work on it, increasing internal energy and raising temperature; a gas expanding and pushing a piston does work on the surroundings, decreasing internal energy.
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