Which do you think will keep you warmer; wearing one woollen jumper, or putting on four cotton t-shirts? This experiment might help you to decide.
Warning: This experiment uses hot water. Take care whenever pouring or touching an object containing hot water. Younger readers must have an adult to help. Be aware that opened cans sometimes have sharp edges.
What you need
Scissors
Masking tape
Kettle
Thick card
2 standard size, empty and washed tin or aluminium cans (food cans, not soft drink)
Wool/felt cloth
Cotton or calico cloth
2 thermometers
A watch
Pen and paper
What to do
Cut the wool into a rectangle of cloth 24 cm x 11 cm, and a smaller square of 10cm x 10cm. Do the same with the cotton material so you have four separate pieces which are also 24 cm x 11 cm, and four 10cm x 10cm squares.
Cut the card into two circles large enough to make lids for each of the cans. Poke a hole into the middle of each, taking care not to stab yourself with the scissors. The hole should be just large enough for the thermometer to fit so heat doesn't escape.
On one piece of card, lay the square of wool, again cutting a tiny slit for the thermometer. On the other, stack the four squares of cotton, one on top of the other, cutting a small slit through each for the thermometer.
Wrap the wool rectangle around one can and tape it. Wrap the four cotton rectangles around the other can in layers and tape them. Place both in a convenient place, such as on a table.
Boil water in the kettle and pour it into each can until it is about two-thirds full. Caution: once water is in the can, you won't be able to touch it until it has cooled. Do not pick it up while it is still hot .
Poke a thermometer through each lid and place them fabric-side up on top of the cans, with the thermometer sitting in the hot water. Wait until the temperature stops rising and then write down the temperature of each can as well as the time.
After twenty minutes, which can is the warmest? What about after forty minutes?
What's happening?
Any material that helps slow energy from moving from one place to another is called an 'insulator'. Heat is a form of energy which describes how much particles jiggle about. Particles that are close together, like in solids, can pass heat easily by bumping into colder particles. This is called 'conduction'. Particles spread far apart, like in gases such as air, don't pass or conduct heat easily. Think about it this way – is it easier to bump into another person on a crowded bus, or on a soccer pitch in the middle of a game?
Woollen jumpers keep you warm by holding the air in between its fibres close to your body. Since air doesn't conduct heat easily, the heat stays close to you rather than drifting away. The more air you trap, the warmer you'll be. Birds fluff their feathers to ward off the chill for this very reason.
Layers of cotton will trap more air between them than a woollen jumper; more layers equals more warmth. Earth's atmosphere – a big layer of air – also traps heat. Imagine how cold we'd be if that disappeared!
Try other materials and see which ones work the best at insulating.
