Heat, Equilibrium & the Zeroth Law
Energy that flows on its own from hot to cold, and the law that lets us define temperature.
Lesson at a glanceShow the key points
- Lost mechanical energy (friction, collisions) becomes internal energy: bodies get hot.
- In contact, a hot body loses internal energy and a cold one gains it, with no work done: energy transferred this way is heat.
- Thermal equilibrium: no heat flows when two bodies touch.
- Zeroth law: if A is in equilibrium with B and with C, then B and C are in equilibrium with each other. It is an observed fact, not obvious.
- It lets us assign temperatures: equal for bodies in equilibrium; heat flows from higher to lower temperature.
- An (ideal) adiabatic wall stops heat flow; a diathermic wall lets it through quickly.
Rub your hands together and they warm up. Brake a bicycle and the brake pads heat. Where does the lost motion go, and what exactly is it that flows from a hot cup of tea into your fingers?
Hot bodies have more internal energy
Whenever mechanical energy seems to disappear (friction, a crash, a sliding block slowing down), the bodies involved get warmer. The energy hasn’t vanished: it has gone into the internal energy of the bodies, the energy of their jiggling atoms and molecules. A hot body has more internal energy than an otherwise identical cold one.
Heat
Put a hot body against a cold one: the hot one cools and the cold one warms. Internal energy passes from one to the other. Nothing is pushed through a distance, so no mechanical work is done. That makes it different from lifting a ball or a spring pushing a block.
Thermal equilibrium and the zeroth law
Two bodies are in thermal equilibrium if no heat flows between them when they are put in contact. Experience shows:
The zeroth law lets us define temperature: bodies in thermal equilibrium have equal temperatures, and a hotter body has a higher temperature. Heat always flows from higher to lower temperature.
Walls that block or pass heat
An ideal wall that lets no heat through. Bodies on either side can stay at different temperatures indefinitely.
A wall that lets heat through quickly. Bodies on either side soon reach the same temperature.
Real materials lie in between: good insulators (wool, foam, the vacuum of a flask) come close to adiabatic; metals are nearly diathermic.
- 1Lost mechanical energy becomes internal energy; hot bodies have more internal energy.
- 2Heat is energy transferred because of a temperature difference, with no work done.
- 3Zeroth law: two bodies each in equilibrium with a third are in equilibrium with each other; this defines temperature.
- 4Heat flows from higher to lower temperature. Adiabatic walls stop it; diathermic walls pass it.
Check yourself
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