Nib asked
Why does heat flow from hot to cold?
Heat is energy carried by jiggling atoms. When hot and cold things touch, random bumps spread the energy out, and the evening-out is overwhelmingly more likely than the reverse. No cold flows anywhere.
6 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the puzzle
Put a metal spoon in a mug of hot tea. The part in the tea is hot, the handle sticking out is cooler, and within a minute the handle warms up too. We never see a hot spoon get hotter by drawing heat out of a cold hand. Heat has a one-way direction, from the hotter thing to the colder thing.
Heat only ever flows from the hotter object to the colder one.
2no cold flows
There is no substance called cold. Heat is the energy that moves from one object to another because of a temperature difference. A cold object simply has less thermal energy per atom, so it has less to give. When your hand feels the cold, it is losing energy to the colder thing, not receiving cold.
Heat is energy moving. Cold is not a thing that flows; it just means less energy.
a common mix-upCold does not flow into a warm place. Warmth leaves it.
When an ice cube cools your drink, what is really moving?
Show the answer
Thermal energy moves from the warm drink into the ice. No cold goes into the drink.
3jiggling atoms
Inside any solid, atoms vibrate in place. Temperature measures the average energy of that jiggling. In the hot block the atoms vibrate hard. In the cold block they vibrate gently. Put the two blocks in contact and atoms at the boundary start bumping into each other.
Temperature is the average jiggle energy of the atoms.
4the bumps
At the boundary a fast atom collides with a slow one. Any single bump could go either way, but on average the faster atom loses some energy and the slower one gains some, in the same way a fast billiard ball shares its motion with a slow one. Atom by atom, energy trickles across the contact.
At the boundary, bumps pass energy from fast atoms to slow ones on average.
5evening out
Plot the two temperatures against time and they glide toward each other and meet in the middle, then stay together. A hot object has far more ways to share its energy out than to keep it concentrated, so evening out is overwhelmingly more likely than the energy bunching up again. That is why we never see heat flow back by itself.
Energy spreads until temperatures match, and the reverse is overwhelmingly unlikely.
Why does heat stop flowing between two objects?
Show the answer
Once their temperatures match, bumps pass energy both ways equally, so there is no net flow.
6the whole idea
In the tea the atoms jiggle hard. They bump the atoms of the spoon, which bump their neighbours, and the energy passes up the metal to your fingers. Nothing cold travels; the energy simply evens out, because spread-out is overwhelmingly more likely than concentrated. It stops when everything is the same temperature.
Energy moves from fast jigglers to slow ones until they match, so heat flows from hot to cold.
Heat has a direction because jiggling atoms share their energy by bumping, and sharing evenly is overwhelmingly likelier than un-sharing.