PD Modules
Teaching About Heat and Thermal Energy

Temperature vs. Energy: The Motion of Particles

Anything made of molecules has thermal energy, even cold things. It's just that the molecules in warmer and colder objects move at different speeds.

Understanding the concepts of temperature and thermal energy means reconciling your day-to-day experiences with what is happening at a tiny scale. All matter is made of particles—atoms and molecules—that are in constant motion. These particles have kinetic energy, the energy of motion.

  • Temperature is a measure of the average kinetic energy of particles within matter and does not depend on the number of particles.
  • Thermal energy is the total amount of kinetic energy of all particles in a sample of a material or object; thermal energy depends on the motion of the particles as well as the amount and type of particles.

Consider these two scenarios:

Scenario 1

If you compare two glasses with the same amount of water but each at a different temperature, the particles in the warmer water are moving faster than the particles in the cooler water, and the glass of warmer water has more thermal energy than the glass of cooler water.

Scenario 2

If you compare two glasses with different amounts of water but both at the same temperature, the average kinetic energy of the particles is the same in both containers, but the glass with more water has more thermal energy.

The dynamics can get more interesting with both differences in amount and temperature. For example, a large pool of cool water can have more thermal energy than a glass of warm water. Why? Because the pool contains more water.

Take Notes

  • Which contains more thermal energy, a cup of hot tea or a pitcher of iced tea? Which liquid would require more heat to change its temperature? Explain your reasoning for both.
  • How would you describe the relationship between heat and cold? When a substance cools down, describe what happens to it at the particle level.