← Unit 1: Thermal, nuclear and electrical physics
Topic 1: Heating processes
Describe internal energy, temperature and thermal equilibrium in terms of the kinetic theory of matter, and distinguish heat from temperature
A focused answer to the QCE Physics Unit 1 dot point on internal energy and thermal equilibrium. Defines internal energy as the sum of microscopic kinetic and potential energies, distinguishes heat (energy in transit) from temperature (average translational kinetic energy of particles), and explains how thermal equilibrium establishes a common temperature.
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What this dot point is asking
QCAA wants you to use the kinetic theory of matter to describe internal energy, temperature and the approach to thermal equilibrium, and to distinguish heat (an energy transfer) from temperature (a property of a body). Both ideas appear in EA Paper 1 multiple choice and as the qualitative spine of every Topic 1 problem.
Internal energy
The internal energy () of a substance is the total of the microscopic kinetic and potential energies of all its particles.
- Kinetic part: translation, rotation and vibration of particles (atoms or molecules).
- Potential part: stored energy in bonds and intermolecular forces.
Internal energy depends on the substance, its mass, its temperature, and its phase (solid, liquid, gas).
Temperature
Temperature is a measure of the average translational kinetic energy of the particles in a substance. The relationship for an ideal gas is
where J K is Boltzmann's constant and is absolute temperature in kelvin.
Temperature does not depend on the number of particles. A single hot drop of water and an entire bathtub at the same temperature have the same average particle kinetic energy, but very different total internal energies.
Heat is not temperature
Heat () is energy transferred between bodies because of a temperature difference. Heat is a process, not a property. A body does not "contain" heat; it contains internal energy. Heat flows from hot to cold spontaneously.
Heat has SI unit joule (J). Temperature has SI unit kelvin (K). Mixing the two in language (saying "the room contains a lot of heat") is the most common written-exam slip QCAA penalises.
Thermal equilibrium
Two bodies in thermal contact reach thermal equilibrium when their temperatures are equal. At that point, the average translational kinetic energy per particle is the same in both. There is no net heat flow.
The zeroth law of thermodynamics: if A is in equilibrium with B and B with C, then A is in equilibrium with C. This is what allows a thermometer to measure temperature; it equilibrates with the body and reads the common temperature.
Common traps
Treating heat and temperature as the same thing. A bathtub of warm water has more internal energy than a cup of boiling water, but a lower temperature.
Adding kinetic and potential incorrectly in a state change. During melting or boiling, heat raises potential energy of particles (breaking bonds) without raising kinetic energy. Temperature is unchanged across the phase transition.
Confusing celsius with kelvin in formulas. Use kelvin in the kinetic-theory formula. Convert: .
Treating thermal equilibrium as "no energy". At equilibrium there is still molecular motion. There is no net heat flow, but micro-level energy exchange continues.
In one sentence
Internal energy is the total kinetic plus potential energy of the particles in a substance, temperature is a measure of the average translational kinetic energy per particle, heat is energy in transit between bodies at different temperatures, and thermal equilibrium is the state where bodies in contact share the same temperature and no net heat flows.
Past exam questions, worked
Real questions from past QCAA papers on this dot point, with our answer explainer.
Year 11 SAC3 marksA cup of hot tea and a glass of cold water are placed in a sealed insulated box. Describe what happens in terms of internal energy, heat flow and temperature, until equilibrium is reached.Show worked answer →
Internal energy is the total kinetic and potential energy of all particles. The tea has higher average translational kinetic energy than the water (its temperature is higher).
Heat flows spontaneously from the higher-temperature tea to the lower-temperature water (second law of thermodynamics). Energy is transferred from tea particles colliding with the cup wall to water particles, on average.
The tea cools while the water warms. The two reach the same temperature when their particles have the same average translational kinetic energy; this is thermal equilibrium. No further net heat flow occurs.
Total internal energy of the system is conserved (insulated box). Markers reward the distinction between heat and temperature, the direction of heat flow, and the equilibrium criterion.
Related dot points
- Solve problems involving specific heat capacity ($Q = mc\Delta T$) and specific latent heat ($Q = mL$) of fusion and vaporisation, including state changes
A focused answer to the QCE Physics Unit 1 dot point on specific heat capacity and latent heat. Applies $Q = mc\Delta T$ and $Q = mL$ to heating, cooling and phase-change calculations, and works the QCAA-style multi-stage problem (heating ice, melting, heating water, vaporising) used in EA Paper 1.
- Describe and distinguish between conduction, convection and radiation as mechanisms of heat transfer, with reference to everyday and industrial applications
A focused answer to the QCE Physics Unit 1 dot point on heat transfer mechanisms. Defines conduction (particle-to-particle collisions), convection (bulk fluid motion driven by density differences) and radiation (electromagnetic emission), and works the QCAA-style application question on insulation and energy-efficient homes.
- Thermal energy, temperature and kinetic theory of matter, methods of heat transfer (conduction, convection, radiation), specific heat capacity $Q = mc\Delta T$, and latent heat
A focused answer to the QCE Physics Unit 1 subject-matter point on thermal physics. Kinetic theory of matter, temperature and internal energy, heat transfer mechanisms, specific heat capacity and latent heat calculations.