Heat and Where It Comes From

1What is heat?

Heat is what makes a thing hot. A lot of heat, and the thing is hot. Very little heat, and the thing feels cold.

You cannot see heat. You cannot hold it in your hand. But you can feel it, and you can see what it does: it boils water, it cooks beans, it dries your uniform on the line.

2Where does heat come from?

Anything that gives out heat is a source of heat.

• The sun. The biggest source of all, and it is free. It warms the whole of Rwanda every day — the hills, Lake Kivu, the roof of your house, you.
• Fire. Burning firewood, a charcoal jiko, a gas flame, a candle, a match.
• Electric things. A cooker, a kettle, a flat iron, even a bulb that has been on for a while — touch it (no, do not touch it!) and it is hot.
• Rubbing. Rub your two palms together very fast, twenty times. Go on, try it now. Feel that? They are warm. You just made heat with movement.
• Your own body. That is why a room full of children gets warm and stuffy.

3Heat always moves from hot to cold — a worked example

Mutesi's mother leaves a metal spoon standing in the pot of hot porridge. Five minutes later Mutesi grabs the spoon — and drops it, because the handle is hot.

But nobody put the handle in the fire. So how did it get hot?

Here is what happened. The hot porridge touches the bottom of the spoon. Heat leaves the hot porridge and creeps up through the metal, all the way to the cold handle at the top. Now the handle is hot too.

Heat always moves from the hotter thing to the colder thing. That is exactly why the cup warmed Mutesi's cold fingers, and why her fingers cooled the cup a tiny bit.

4Metal is fast. Cloth and wood are slow.

• Some pans have a wooden or plastic handle — it stays cool enough to hold.
• You lift a hot pot with a thick, dry cloth. (Never a wet one! Water carries heat fast, and the wet cloth turns to steam that can burn your hand.)
• A jumper keeps you warm on a cold Musanze morning — it is slow, so it keeps your body heat close to you.

5What your family does with heat every day

• Cooks food. Raw beans are hard; heat makes them soft enough to eat.
• Boils water. Heat kills the germs, so the water is safe to drink.
• Dries things. The sun's heat dries the washing, and dries maize, beans and coffee before they are stored or sold.
• Warms us. A fire on a cold evening; a warm blanket.
• Presses clothes. A hot iron pushes the creases out of your school uniform.
• Hardens things. Heat bakes soft clay pots and bricks until they are hard and strong.

Try it yourself

Work each one out before you look.

1. Name two sources of heat you can find at your own home.
2. Keza puts a metal spoon in hot tea and a plastic spoon in the same cup. After five minutes, which handle is hotter?
3. The flames in the jiko died out an hour ago. Byiringiro wants to move it, so he picks it up by the middle. Is that safe?
4. Umutoni's mother must lift a hot pot off the fire. She reaches for a cloth — but the cloth is soaking wet. Should she use it?
5. Habimana's shirt is soaked from the rain. He hangs it outside in the bright sunshine and it dries. Which source of heat did the work?

6Answers

1. Any two real ones are right. For example: the sun, the charcoal jiko, the cooking fire, a candle, the flat iron, the kettle.
2. The metal one. Heat rushes through metal, so it reaches the handle quickly. Heat crawls slowly through plastic, so that handle stays cooler.
3. No! Charcoal holds its heat for a long time after the flame goes out, and the jiko itself is metal, so it is hot all over. He must let it cool, or ask an adult.
4. No. A wet cloth carries heat straight to her skin and turns to hot steam. She needs a thick, dry cloth.
5. The sun — the biggest source of heat we have, and it costs nothing.