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Heat is energy too

Energy and power · level 2, lesson 5 of 6

A question to start

A cup of hot cocoa left on the table goes cold. Ice cream left in the sun melts. Where does the cocoa's warmth go, and where does the ice cream's warmth come from?

The idea

Hot things have more thermal energy, or heat energy, than cold ones. Everything is made of tiny pieces that jiggle about; in a hot thing they jiggle faster.

  • Warming something up needs energy: a kettle uses electricity to heat the water.
  • Heat always flows from hotter things to colder ones. The hot cocoa gives its energy to the cooler air around it, until both are the same temperature. The ice cream takes energy from the warm air and the sunshine, and melts.
heat flows outheat flows in

To keep something warm, you slow down how fast heat escapes. A coat, a blanket, a woolly hat and a flask all do this.

On Mars, keeping warm is a big job. It is often −60 °C or colder outside, so heat is always trying to flow out of a home. Thick walls, layers of soil on the roof and heaters all help to keep the energy in.

Worked examples

1. A hot-water bottle. It gives its heat energy to your cold feet, which warm up, while the bottle cools down.

2. A fridge. It uses electricity to pump heat out of the food inside, into the kitchen. That is why the back of a fridge feels warm.

Try it

  1. Which has more thermal energy: a hot bath or the same bath when it is cold?
  2. Which way does heat flow: from hot to cold or from cold to hot?
  3. Why does a coat keep you warm?
  4. Why does a home on Mars need thick walls?
Answers
  1. The hot bath.
  2. From hot to cold, on its own.
  3. It slows down the heat flowing out of your body.
  4. It is very cold outside, so heat keeps trying to flow out; thick walls slow it down.

Common mistake

"A coat makes heat." A coat does not make any heat. It keeps in the heat your body makes. Put a coat on a snowman, and it would keep the cold in.

Where it leads