What it shows
Falling water pushes on the cups of the wheel and makes it turn. The turning axle winds up a thread and lifts a small weight, so the energy of moving water becomes useful work.
The science
Water held up in a jug has stored (potential) energy. When it falls, that energy becomes movement (kinetic) energy, and the water pushes the caps round. The axle turns with the wheel, winds the thread and raises the weight, so the water has done work against gravity. Real hydroelectric plants use the same idea with a turbine and a generator.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Eight plastic bottle caps (free)
- An old CD or a circle of stiff cardboard
- A pencil or a wooden skewer for the axle
- Two empty matchboxes or a small cardboard frame as stands
- About 50 cm of thread and a bottle cap with a few coins as the weight
- Hot glue or strong tape
- A jug of water and a tray
Safety
For this project: An adult must use the hot glue gun and make any cuts in the plastic.
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.
Glue eight bottle caps around the edge of the CD, all facing the same way round like cups, so the water will catch in them.
Push the pencil through the centre hole of the CD and fix it with glue so the wheel and the axle turn together.
Stand two matchboxes or a cardboard frame on a tray and rest the ends of the pencil on top, so the wheel spins freely.
Tie the thread to one end of the pencil and tie the weighted bottle cap to the other end of the thread.
Pour water slowly from the jug onto the caps at the top of the wheel, a little to one side so it turns one way.
Watch the thread wind round the axle and lift the cap of coins; if it slips, add a drop of glue where the thread meets the pencil.
Try pouring from higher up and from lower down and see which lifts the weight faster.
Add more coins one at a time until the wheel can no longer lift them, and note the heaviest load it managed.
What you should see
The wheel turns steadily under a thin stream of water and winds up the thread, lifting a cap holding two or three coins about 20 to 30 cm. Water poured from higher up lifts it faster.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Where does the energy to lift the weight come from?
From the falling water, which had stored energy because it was held up high in the jug.
2Why are the caps fixed facing the same way?
So every cap catches water on the same side and pushes the wheel in one direction.
3What happens if you pour the water faster?
More water hits the caps each second, so the wheel turns faster and lifts the weight more quickly.
4Where is a water wheel used in real life?
In old flour mills and in hydroelectric power stations, where the wheel is called a turbine.
Take it further
Fit a second, smaller wheel on the same axle and see whether the weight rises faster or slower when the thread winds on the smaller one.
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.
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