What it shows
Turning a small DC motor by hand makes a voltage, so the motor acts as a generator. Faster cranking gives higher voltage and brighter light, showing that mechanical energy changes into electrical energy.
The science
When a coil of wire turns between magnets, a voltage is induced in the coil. A DC motor has such a coil and magnets, so spinning its shaft makes electricity instead of using it. The faster the coil cuts the magnetic field, the larger the voltage.
Materials
Cost: under Rs 200. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A small DC toy motor
- A cardboard or plastic wheel as the crank
- A wooden or cardboard base
- A rubber band and a thread reel or bottle cap as the small pulley
- Two red LEDs and a 330 Ω resistor
- A multimeter
- Wires and tape
- A small handle made of a bolt or pencil
Safety
For this project: Use only the motor and LEDs; never connect mains. An adult must supervise cutting and any soldering. Always keep a resistor in series with an LED, and keep fingers clear of the rubber band belt.
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.
Step 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 motor firmly onto the base with tape or glue so it does not rock.
Mount the wheel on a short axle fixed to the base, fix the handle near its edge, and loop the rubber band round the wheel rim and the small pulley glued on the motor shaft, so the shaft turns several times for each turn of the crank.
Connect the multimeter across the two motor terminals in voltage mode.
Turn the handle slowly and note the reading, then turn faster and note again.
Count crank turns in ten seconds and read the peak voltage for slow, medium and fast cranking.
Disconnect the meter and connect an LED in series with the 330 Ω resistor, with the long leg of the LED towards the positive terminal.
Crank until the LED glows and note the speed at which it first lights; a red LED needs about 2 V, so crank faster if it stays dark.
Swap the motor wires to see that the LED lights only one way round, and record the reading.
What you should see
The meter and the LED show that more turns per second give a higher voltage. A table links crank speed to voltage and to LED brightness.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
01What energy change occurs?
Mechanical to electrical energy.
02Why does the LED light only one way?
An LED conducts in one direction only.
03What law explains the voltage?
Faraday’s law of electromagnetic induction.
04How is this useful in a power cut?
Hand-crank torches and radios use the same idea.
Take it further
Use a smaller pulley or add a second gear stage from bottle caps to raise the motor speed and see the extra voltage.
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
Science projects
Physics Working Models Explained
Twenty classic models, how each works and how to make it
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