Thermal equilibrium
Section: Temperature | Syllabus: Cambridge A Level Physics (9702)
Temperature vs. Thermal Energy
We use the term temperature to describe how hot or cold an object is. However, temperature and thermal energy are not the same thing.
Key Distinction- Temperature: Determines the direction of energy transfer.
- Thermal Energy: The total kinetic energy of all the particles in a substance.
Compare two cups of tea at the same temperature (T). Cup A has a larger volume than Cup B. Because it has more molecules, Cup A has more thermal energy, even though the temperatures are identical.
Thermal Contact and Equilibrium
For two objects in thermal contact, thermal energy flows from the object at higher temperature to the object at lower temperature.
Concept: Metal in WaterA hot piece of metal is dropped into cold water. Energy is transferred from the metal (high T) to the water (low T). This continues until their temperatures become equal.
Definition: Thermal EquilibriumWhen the temperatures of two objects in contact become equal, there is no net transfer of thermal energy between them. They are in thermal equilibrium.
This principle also applies within a single object. If one end of a metal rod is heated, energy transfers to the cooler end until the entire rod reaches thermal equilibrium at a uniform temperature.
Summary of Key Ideas
- Energy flows from high T to low T.
- Equilibrium = Same temperature = No net energy transfer.
- Thermal energy is the total kinetic energy of molecules.
Interactive revision notes, videos and practice questions load below.