What it shows
Warm air is lighter than cool air and rises. A paper spiral hung above a warm surface spins as the rising air pushes on its sloping coils, showing convection, the way heat spreads in air and water.
The science
When air is warmed it expands, so the same amount of air takes up more space and becomes less dense than the cooler air around it. The cooler, heavier air sinks and pushes the warm air up, making a steady upward current called a convection current. The coils of the spiral are tilted, so the rising air pushes them sideways as it slips past, and the spiral turns. The same currents carry warm air across a room from a heater and make sea breezes blow on the coast.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A sheet of thin paper or an old magazine page (free)
- Scissors and a pencil
- About 30 cm of thread and a needle or tape
- A cup of hot water, a warm lamp with an ordinary bulb, or a sunny windowsill as the heat source
Safety
For this project: An adult must pour the hot water and the spiral must hang well above it; never hang paper over a flame or a gas stove.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Draw a spiral on the paper, starting from a circle about 15 cm across and winding in to the centre with a gap of about 1.5 cm between the turns.
Cut carefully along the spiral line so you get one long coiled strip with a small circle in the middle.
Tie the thread to the centre of the spiral with a needle or a bit of tape so the coils hang down in a cone.
Hold the thread so the spiral hangs still in a corner of the room with no draughts, and check that it does not turn by itself.
Ask an adult to place a cup of hot water on the table and hold the spiral about 10 cm above it; watch it begin to turn.
Move it higher and lower and find where it spins fastest.
Try it above a warm (not hot) lamp and above a sunny windowsill and compare.
Hold the spiral above a bowl of ice and notice that it does not turn, which shows that only warm air rises.
What you should see
The spiral hangs still in cool air but spins steadily when held above hot water or a warm lamp, and turns faster when closer to the heat.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does warm air rise?
It expands and becomes lighter for its size than the cool air around it, which sinks and pushes it up.
2What makes the spiral turn rather than just lift?
The coils are sloped, so the rising air pushes them sideways as it passes.
3What is this way of carrying heat called?
Convection.
4Give one example of convection in nature.
Sea breezes, or birds soaring on rising warm air.
Take it further
Hold a thin strip of tissue paper beside a hot cup, then above it, to find which way the air is flowing at different places.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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