AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Isothermal, adiabatic, constant pressure and constant volume changes.
Full specification wording
Non-flow processes (A-level only): Isothermal, adiabatic, constant pressure and constant volume changes.
A non-flow process changes the state of a fixed mass of gas in a cylinder.
Checked against the AQA 7408 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A non-flow process changes the state of a fixed mass of gas in a cylinder. Isothermal means constant temperature, so internal energy of an ideal gas stays fixed and any heat added equals work done by the gas. Adiabatic means no heat enters or leaves, so work done changes internal energy and temperature. Constant pressure (isobaric) heating lets the gas expand, doing work pΔV while temperature rises. Constant volume (isochoric) heating does no work because ΔV is zero, so all heat raises internal energy and temperature. Link each name to the quantity held fixed, then to the first law.
What earns the marks
- Identifies the quantity held constant in each process: temperature for isothermal, heat flow for adiabatic, pressure for isobaric, volume for isochoric.
- Applies the first law ΔU = Q − W to each process, noting W = pΔV only when pressure is constant.
- Explains that for an ideal gas isothermal implies ΔU = 0, so Q = W, while adiabatic implies Q = 0, so ΔU = −W.
- Distinguishes constant volume, where W = 0 and Q = ΔU, from constant pressure, where both Q and W are non-zero.
- Uses a p–V diagram to represent each path: isothermal and adiabatic curves, horizontal isobar, vertical isochore.
- Relates the microscopic picture, such as molecular speed distribution, to temperature changes in each process.
Common mistakes and how to put them right
Saying an adiabatic process must be at constant temperature.
Put it right: Adiabatic means Q = 0, so temperature generally changes unless the process is also isothermal.
Using W = pΔV for every process.
Put it right: This expression applies only when pressure is constant, so use the area under the p–V curve otherwise.
Assuming constant volume means constant pressure.
Put it right: At constant volume the gas is sealed, pressure can rise as temperature rises.
Exam tips
- State the fixed quantity first, then write the first law with the correct term set to zero.
- Sketch the p–V path and shade the work area to justify the sign of W.
- Use the words system and surroundings to make clear where heat and work cross the boundary.
Try the lesson
The start of the Non-flow processes (A-level only): Isothermal, adiabatic, constant pressure and constant volume changes. lesson, made from checked teaching notes for this specification point.
Have a go first
A fixed mass of ideal gas is heated slowly in a cylinder while the piston is free to move so that the pressure stays constant. Which name is given to this non-flow process?
Listen
A non-flow process changes the state of a fixed mass of gas in a cylinder. Isothermal means constant temperature, so internal energy of an ideal gas stays fixed and any heat added equals work done by the gas. Adiabatic means no heat enters or leaves, so work done changes internal energy and temperature. Constant pressure (isobaric) heating lets the gas expand, doing work pΔV while temperature rises. Constant volume (isochoric) heating does no work because ΔV is zero, so all heat raises internal energy and temperature. Link each name to the quantity held fixed, then to the first law. A strong answer earns marks for these points. Identifies the quantity held constant in each process: temperature for isothermal, heat flow for adiabatic, pressure for isobaric, volume for isochoric. Applies the first law ΔU = Q − W to each process, noting W = pΔV only when pressure is constant. Explains that for an ideal gas isothermal implies ΔU = 0, so Q = W, while adiabatic implies Q = 0, so ΔU = −W. Distinguishes constant volume, where W = 0 and Q = ΔU, from constant pressure, where both Q and W are non-zero. A common slip: Saying an adiabatic process must be at constant temperature. Instead, adiabatic means Q = 0, so temperature generally changes unless the process is also isothermal. Exam tip: State the fixed quantity first, then write the first law with the correct term set to zero.
See it
Key words
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Isothermal
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Adiabatic
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Isobaric
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Isochoric
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See every definition
- Isothermal
- A process in which temperature is held constant, so the internal energy of an ideal gas stays fixed and any heat added equals work done by the gas.
- Adiabatic
- A process in which no heat enters or leaves, so work done changes internal energy and temperature.
- Isobaric
- A constant pressure process in which heating lets the gas expand, doing work pΔV while temperature rises.
- Isochoric
- A constant volume process in which heating does no work because ΔV is zero, so all heat raises internal energy and temperature.
Label the diagram
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1Box 1
2Box 2
3Box 3
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