Describe thermal equilibrium and temperature sensors
Explain the direction of thermal-energy transfer, define thermal equilibrium and compare physical properties used to measure temperature.
A Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 14. These original explanations and calculations are study aids; full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use temperature differences, proportional reasoning, energy, mass and SI units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In which direction is net thermal energy transferred between regions at different temperatures?
What you will learn
- Describe thermal-energy transfer from higher to lower temperature
- Recognise thermal equilibrium at equal temperature
- Give physical properties that vary with temperature and can measure it
Temperature difference drives net thermal transfer
Thermal energy is transferred from a region at higher temperature to a region at lower temperature. If two objects can exchange energy, the net transfer continues while their temperatures differ.
When two regions reach the same temperature, they are in thermal equilibrium: there is no net thermal-energy transfer between them. Microscopic interactions continue, but energy transfers in both directions balance overall.
A thermometer uses a changing physical property
A physical property that varies with temperature can act as a thermometer signal. Examples include the density of a liquid, the volume of a gas kept at constant pressure, the electrical resistance of a metal, and the e.m.f. of a thermocouple.
A thermometer is useful when the property changes measurably and can be calibrated. The property is a way to indicate temperature; it is not itself the definition of thermodynamic temperature.
Pause and explain
When are two regions in thermal equilibrium?
Worked example
A hot metal block is placed in cooler water in an insulated container. Describe the energy transfer and the final equilibrium condition.
Show the worked solution
- Compare the initial temperatures.
- State the direction of net thermal-energy transfer.
- Describe the condition when net transfer stops.
Answer Thermal energy transfers from the hotter block to the cooler water. They approach the same temperature, where they are in thermal equilibrium and there is no net transfer.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A 70 °C liquid is poured into a 20 °C liquid in an insulated beaker. State the direction of net energy transfer immediately after mixing.
Give me a hint
Compare temperatures before describing the direction of net thermal transfer.
Compare my reasoning
- The first liquid is at higher temperature.
- The second liquid is at lower temperature.
- Net thermal energy transfers from the 70 °C liquid to the 20 °C liquid.
Net thermal energy transfers from the hotter liquid to the cooler liquid.
Look for these in your work
- Initial temperatures compared
- Direction correct
- Thermal-energy transfer named
Give four physical properties that can vary with temperature and be used to measure temperature.
Give me a hint
Think of a liquid, a gas at controlled pressure, a metal sensor and a thermocouple.
Compare my reasoning
- A liquid's density can vary.
- Gas volume at constant pressure and metal resistance can vary.
- A thermocouple's e.m.f. can vary with temperature.
Examples: liquid density, gas volume at constant pressure, metal resistance and thermocouple e.m.f.
Look for these in your work
- Four properties given
- Gas condition included
- Thermocouple signal identified
A student says two objects in thermal equilibrium must contain the same amount of thermal energy. Explain the error.
Give me a hint
Separate the condition for equilibrium from the total energy stored by objects of different mass or material.
Compare my reasoning
- Thermal equilibrium requires equal temperature.
- Thermal energy also depends on the amount and type of material.
- Therefore equal temperature does not imply equal total thermal energy.
Equal temperature means no net thermal transfer, but objects can still contain different total thermal energies.
Look for these in your work
- Equilibrium condition stated
- Energy factors identified
- Claim corrected
Common mistakes
- Saying thermal energy flows from cold to hot without external work
- Defining equilibrium as equal energy rather than equal temperature
- Treating electrical resistance as the temperature scale itself
What can you explain now?
In which direction is net thermal energy transferred between regions at different temperatures?
Compare with the explanation
From higher temperature to lower temperature
The warmer region transfers energy to the cooler region.
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Make it stick
What condition defines thermal equilibrium, and which way does net energy move before it is reached?
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