The foil strip expands away from the heat.
Make a curious device that doesn’t like being put near a candle flame. When it gets hot, it curls away!
Safety: This activity uses a candle. Ask an adult to help. Make sure you have fire safety equipment nearby.
First aid: If you burn yourself, put the burn under cool, running water for 20 minutes. If needed, seek medical advice.
You will need
- Candle
- Matches or lighter
- Aluminium foil
- Mailing label
- Scissors
- Metal tray
What to do
Cut a piece of aluminium foil and place it flat on a table.
Take a mailing label and stick it to the foil.
Cut out the mailing label with scissors. This will create a strip with paper on one side and foil on the other.
Cut another piece of foil the same size as the first strip.- Carefully peel off another mailing label and put it with the other strips. Make sure it doesn’t stick to anything!
Put the candle on a metal tray and place the tray away from anything flammable.
Light the candle.
Next you will be testing the strips. If a strip starts to smoke or change colour, take it away from the flame and put it down on the tray. The reaction is likely to occur quickly, so do not leave a strip heating for more than 10 seconds.
Hold the plain aluminium strip about 5 centimetres above the candle flame. Move the strip back and forth to heat a larger area, while keeping your fingers a safe distance from the flame. Do you notice anything happening?- Next, try a mailing label with the glue side down. Move the strip back and forth to heat a larger area, while keeping your fingers a safe distance from the flame. Do you notice anything this time?
Hold the double-sided strip, aluminium side down, about 5 centimetres above the candle flame. Move the strip back and forth to heat a larger area, while keeping your fingers a safe distance from the flame. What do you notice happening?
What’s happening?
No, your strip isn’t trying to escape a burn – it can’t feel pain. It’s curling away because it’s getting bigger!
Many things get bigger when they are heated. You might notice that the elements in your toaster get longer when they’re glowing, or maybe you’ve seen a section of footpath that bows upwards when the weather is hot. This effect, called thermal expansion, happens in most materials.
A piece of foil that’s made of aluminium will expand reasonably evenly, but that changes when it’s attached to paper. Not all materials expand the same amount when they get hot.
In this activity, the aluminium expands more than the paper does. That means the aluminium strip will become longer than the sticker. But the glue on the sticker keeps the two materials stuck together.
The strip curls because the outside of a curve is slightly longer than the inside. That way, the two materials can stay stuck together, even though they are different lengths.
Real-life science
When a strip is made of two different materials it’s called a bimaterial strip. They’re often made with two different metals and are then called bimetallic strips.
Since metal conducts electricity, it’s quite easy to turn bimetallic strips into switches that turn on and off depending on the temperature. You might find a bimetallic strip inside an old thermostat or a dial thermometer.
Bimetallic strips are also used as a safety feature in gas ovens. If the flame goes out unexpectedly, you don’t want an oven filling with flammable gas. A bimetallic safety valve near the flame will cool and close soon after the flame goes out, preventing more gas from being released, and stopping an oven from exploding.
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