Why Heat Only Flows One Way
- Physics
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- Heat flows from hotter to colder until everything reaches the same temperature.
- The energy is conserved either way, so conservation alone does not explain the direction.
- No engine can turn all of its heat into work, and the limit depends on the temperatures.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
The direction energy chooses
Pour a kilogram of water at 80 °C into a kilogram at 20 °C, and you end up with two kilograms at about 50 °C. Now picture the reverse: the lukewarm water separating itself into a hot half and a cold half. Nobody has ever seen it happen, yet the total energy would be exactly the same either way. So conservation of energy cannot explain why one direction happens and the other does not. Something else sets the direction, and physicists call it the second law of thermodynamics. Heat flows from hotter to colder by itself, and never the other way round.
What an engine can and cannot do
A heat engine takes in heat and turns some of it into useful work, but it can never turn all of it. Some heat must always be dumped into something colder, which is why power stations have cooling towers or sit beside rivers. How much can be used depends on the two temperatures. The larger the gap between the hot source and the cold sink, the larger the possible share. A steam power station working between about 550 °C and the outside air could not exceed about two thirds even in theory, and in practice it reaches about 40%. A refrigerator is the same machine run backwards: it moves heat from cold to warm, and that is exactly why it needs electricity.
Curriculum links: NGSS HS-PS3-4 · NGSS HS-PS3-2