Grade 8 physics · Thermodynamics
Thermal motion, temperature and thermal expansion
Temperature links an invisible microscopic world of moving particles to visible changes in thermometers, bridges, rails and wires.
Big idea
Heating usually makes particles move more energetically and increases their average separation, so a body expands.
Learning goals
- Describe thermal motion using the particle model.
- Read and convert Celsius and kelvin temperatures.
- Explain thermal expansion without saying that particles themselves grow.
- Calculate a change in length and explain why engineers leave gaps.
1. Matter is never perfectly still
Atoms and molecules in every substance are in continuous, irregular motion. In a gas they travel between collisions; in a liquid they move past nearby particles; in a solid they vibrate around nearly fixed equilibrium positions. This disordered microscopic movement is thermal motion.
When a body is heated without changing phase, its particles have a greater average kinetic energy. Their individual speeds are not all equal, and any one particle may briefly slow down, but the average energy of the enormous collection increases.