Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- Particles are always moving. At any moment they have a spread of kinetic energies.
- Temperature measures the average kinetic energy of the particles. The particles in a warmer body have a greater average kinetic energy than those in a cooler body.
- When two bodies are in thermal contact, their particles collide. On average, energy passes from the faster particles to the slower ones, so energy moves from the warmer body to the cooler body. This is heat transfer (also called heat exchange, or transfer of energy as heat).
- The warmer body cools, the cooler body warms, and the net transfer slows as the gap closes.
- At thermal equilibrium the average kinetic energies, and so the temperatures, are equal. Particles still move and collide, but energy passed each way is equal: no net transfer.
- In an insulated system, energy lost by the warmer body = energy gained by the cooler body, so the final temperature lies between the two starting temperatures.
Key relationships
| Relationship | Meaning | Use it to |
|---|---|---|
| KE = ½mv² | kinetic energy of one particle of mass m and speed v | explain why lighter particles move faster at the same temperature |
| average KE ∝ T (in K) | doubling the Kelvin temperature doubles the average kinetic energy | compare two temperatures: KE₂ / KE₁ = T₂ / T₁ |
| T(K) = T(°C) + 273.15 | Kelvin conversion | convert before any ratio |
| net heat flow: higher T → lower T | direction of transfer | say which body gains energy |
| equilibrium: T₁ = T₂ | equal average kinetic energy | say when transfer stops |
Assumptions behind the model
- The two bodies are insulated from everything else, so no energy is lost to the air or the container.
- Energy moves only as heat; no work is done, and there is no phase change or reaction.
- Each body has a single, uniform temperature at any moment (stirring helps make this true).
Mistakes to avoid
- “Cold moves.” Only energy moves, warmer → cooler.
- “Particles stop at equilibrium.” They keep colliding; only the net transfer stops.
- Same temperature ≠ same speed. It means the same average kinetic energy.
- Same temperature ≠ same total energy. A bigger body has more particles.
- Celsius ratios. Convert to kelvin before comparing kinetic energies.
- Saying “heat” for temperature. Temperature is a property of a body; heat is energy in transit.
Quick self-check
- A gas is heated from 200 K to 400 K. By what factor does its average kinetic energy change? (It doubles.)
- A gas is heated from 100 °C to 200 °C. Does its average kinetic energy double? (No: 373.15 K → 473.15 K, a factor of 1.268.)
- Two blocks at 30 °C touch. Is there heat transfer between them? (No net transfer: their average kinetic energies are equal, although particles still collide.)
Next: practice questions.