What it shows
A jet of water hitting spoon-shaped buckets turns a wheel that lifts a weight. The power in the jet is ρ g Q H, and comparing it with the power lifting the weight gives the efficiency.
The science
Water raised through a head H has potential energy, which becomes kinetic energy in the jet, of speed about √(2 g H) (less due to friction). The jet hits the spoons and its momentum change drives the wheel. The power available is P_in = ρ g Q H, where Q is the flow in cubic metres per second. The wheel lifts a weight m through h in time t, so P_out = m g h / t, and efficiency η = P_out / P_in. A real Pelton wheel reaches 80 to 90 percent; a homemade one reaches perhaps 10 to 30 percent.
Materials
Cost: under Rs 500. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A circular plywood or thick plastic disc about 20 cm across
- Twelve plastic spoons or bottle-cap halves as buckets
- A steel rod or long bolt as the axle, and two bearings from bottle caps
- A large bucket or tank on a raised stand, with a tube of 6 mm bore and a clamp as the nozzle
- A tray and a lower bucket to collect the water
- Thread and washers as the load
- A measuring jug, a stopwatch and a tape measure
Safety
For this project: An adult must drill the disc and cut materials; work outdoors or over a tray, and keep any electric device away from water.
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.
Fix the spoons around the rim of the disc with their hollows facing the same way, evenly spaced, and push the axle through the centre.
Mount the axle on the base so the wheel spins freely in a vertical plane, with a catching tray beneath it.
Tie the thread to the axle and the washers to the other end so that the wheel winds the thread and lifts the load.
Place the water tank so that the nozzle surface is at height H = 0.5 m above the wheel and aim the jet at the spoons.
Measure the flow rate Q by catching the water from the nozzle in the jug for 10 seconds, with the wheel moved aside.
Run the jet on the wheel and time how long the load of 30 g takes to rise 40 cm.
Calculate P_in and P_out and the efficiency; repeat with H = 0.8 m and 1.2 m.
Plot P_out and the efficiency against H.
What you should see
A higher head gives a faster jet, so the wheel lifts the load more quickly and P_out rises with H. The efficiency comes to between about 10 and 30 percent for a home-made wheel.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What is the power in a water jet?
P = ρ g Q H, density times g times flow rate times head.
2Why does a higher head give more power?
The water has more energy per kilogram and leaves the nozzle faster.
3Why are spoon-shaped buckets used?
They turn the jet round and give the wheel the largest push from the momentum change.
4Where is the Pelton wheel used?
In hill hydroelectric plants, where water falls from a great height.
Take it further
Replace the lifting load with a small DC motor as a generator and measure the voltage and current to find the electrical output.
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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