Temperature and Thermal Equilibrium
PhysicsΒ· Unit 20: Thermal Physics, Topic 1Β· 10 min read
1. Defining Temperature and Thermal Contactβ βββββ± 3 min
Temperature is one of the most fundamental concepts in thermal physics. It determines the direction of net thermal energy transfer between any two bodies that can exchange energy.
Temperature
(Celsius), (Kelvin)
An intensive physical quantity that indicates the direction of net thermal energy transfer between bodies in thermal contact, proportional to the average kinetic energy of particles in a substance
Example:
A hot pan has a higher temperature than cold water, so net energy flows from pan to water.
A 1 kg block of copper at 60Β°C is placed next to a 10 kg block of copper at 30Β°C. In which direction does net thermal energy flow?
- 1
Net energy flow depends only on temperature difference, not total mass or internal energy.
- 2
The 1 kg block has a higher temperature ().
- 3
Net thermal energy flows from the 1 kg block at 60Β°C to the 10 kg block at 30Β°C.
2. Thermal Equilibrium: Core Conditionsβ β ββββ± 4 min
When two bodies are in thermal contact, energy will continue to transfer until a balanced state is reached. This balanced state is called thermal equilibrium.
Thermal Equilibrium
A steady state where two bodies in thermal contact have no net transfer of thermal energy between them, which occurs when their temperatures are equal
Example:
A thermometer left in a patient's mouth reaches thermal equilibrium when it stops changing temperature.
Explain when a mercury thermometer placed in hot water reaches thermal equilibrium with the water.
- 1
Initially, the thermometer is cooler than the water, so net energy flows from water to the thermometer.
- 2
Energy transfer causes the mercury to expand and its temperature to rise.
- 3
When the mercury temperature equals the water temperature, there is no net energy transfer. This is thermal equilibrium, so we can take an accurate reading.
Exam tip:
This is a common 2-3 mark explanation question in CIE Paper 2, always mention both equal temperature and zero net energy transfer.
3. Key Distinction: Temperature vs Thermal Energyβ β ββββ± 3 min
One of the most common mistakes in A-level Physics is confusing temperature and thermal energy (heat). These are completely different quantities, with key differences:
Temperature is intensive: it does not depend on the amount of substance
Thermal energy (heat) is extensive: it depends on the amount of substance
Temperature measures average kinetic energy per particle; thermal energy measures total energy transferred
Explain why a 10 kg block of ice at 0Β°C has more internal energy than a 1 kg block of ice at 0Β°C, even though their temperatures are equal.
- 1
Temperature depends on average kinetic energy per particle, which is identical for both blocks at 0Β°C.
- 2
Internal energy is the total kinetic and potential energy of all particles in the block.
- 3
The 10 kg block has 10 times more particles than the 1 kg block, so total internal energy is higher.
- 4
There would be no net energy transfer between the two blocks because their temperatures are equal.
4. Common Pitfalls
Wrong move:
Claiming bodies in thermal equilibrium have equal internal energy
Why:
Internal energy depends on mass, substance and state, as well as temperature. Only temperature is guaranteed equal at equilibrium
Correct move:
State that thermal equilibrium means equal temperature and zero net thermal energy transfer between bodies in contact
Wrong move:
Claiming net energy flows from higher internal energy to lower internal energy
Why:
Energy flow direction depends on temperature difference, not total internal energy difference
Correct move:
State that net thermal energy always flows from higher temperature to lower temperature, regardless of total internal energy
Wrong move:
Thinking thermal equilibrium requires physical contact between bodies
Why:
Bodies only need to be able to exchange energy (thermal contact), which can happen via radiation across a vacuum
Correct move:
Define thermal equilibrium by equal temperature and zero net energy transfer, regardless of physical contact
Wrong move:
Claiming temperature is a measure of total heat in a body
Why:
Temperature measures average kinetic energy per particle, not total energy
Correct move:
Define temperature as a quantity that determines the direction of net thermal energy transfer, proportional to average particle kinetic energy
5. Quick Reference Cheatsheet
Concept | Core Definition | Key Property |
|---|---|---|
Temperature | Determines direction of energy transfer | Intensive (independent of mass) |
Thermal Contact | Allows energy exchange between bodies | Does not require physical contact |
Thermal Equilibrium | Zero net energy transfer in thermal contact | Equal temperature |
Thermal Energy (Heat) | Total energy transferred between bodies | Extensive (depends on mass) |
6. Frequently Asked
Is thermal equilibrium the same as equal internal energy?
No. Two bodies in thermal equilibrium only require equal temperature, not equal internal energy. Internal energy depends on mass, substance, and state, as well as temperature.
Can two bodies be in thermal equilibrium without touching?
Yes. They only need to be in thermal contact (able to exchange energy, e.g. via radiation across a vacuum). No physical contact is required.
When this came up on past exams
AI-estimated based on syllabus patterns β cross-check with official past papers for accuracy. Use only as revision-focus signals.
- 2022 Β· 1
Thermal equilibrium concept MCQ
- 2023 Β· 2
Explain thermometer equilibrium
- 2021 Β· 1
Temperature vs energy MCQ
Going deeper
- syllabusCIE 9702 Official Syllabus
- pre-readingInternal Energy BasicsRequired prerequisite for this topic
What's Next
Temperature and thermal equilibrium are the foundation of all thermal physics topics for CIE A-Level. These concepts underpin every calculation and explanation you will do in this unit, from measuring specific heat capacity to analysing ideal gas behaviour. Mastering the distinction between temperature and energy now will help you avoid common mistakes in later calculation questions. Next, you will learn how to construct temperature scales and define absolute zero, then move on to calculation-based topics for thermal processes. The concept of thermal equilibrium will also be central when you study heat transfer and thermal processes in engines.