What you need (the build)
Gather: one AA or D battery, about 1 m of thin enamelled (varnished) copper wire, two safety pins or large paper clips for the stands, one strong magnet (a flat disc or ring magnet), sticky tape and a little sandpaper.
- Wind the wire 8 to 10 times round the battery to make a round coil. Slip it off and leave two straight ends of about 3 cm sticking out, one on each side, in line with each other. These ends are the axle.
- Take off the enamel. On one end, scrape off all the varnish. On the other end scrape it off only on the top half. This is the commutator.
- Tape the two pins to the battery ends so they stand up like two stands. Rest the coil ends on the pins. Put the magnet on the battery under the coil.
- Give the coil a small flick. It should spin.
Safety: do not leave the battery joined for long. The wire and the battery can get warm. Keep magnets away from phones and cards.
Why the coil gets pushed
A wire that carries current makes its own magnetic field. When it sits in the field of a magnet, the two fields push on each other. The wire feels a force. The force is biggest when the wire is at right angles to the field.
In a coil, the current goes one way along one side and the opposite way along the other side. So one side is pushed up and the other is pushed down. Together they turn the coil. Use Fleming's left-hand rule: first finger = field (N to S), middle finger = current, thumb = force.
The size of the force on one wire is F = B × I × L.
The commutator trick
When the coil becomes upright, its two sides are pushed straight against each other. The push no longer turns it and it stops. If it went past that point, the push would turn it back.
The fix is to reverse the current just as the coil passes the upright position. A split-ring commutator does this in a real motor: two half rings touch two fixed carbon brushes. In your home-made motor, the half-scraped end does the same job. For half of each turn the wire touches the pin and current flows. For the other half it does not touch, and the coil coasts through on its momentum. So it is always pushed the same way.
Make it faster and fix problems
Three things raise the force and so the speed: a bigger voltage (more current), more turns in the coil, and a stronger magnet. Flip the battery and the motor spins the other way. Flip the magnet and it also turns the other way.
- Does not start: check the scraped ends touch the pins, the coil is balanced, and the magnet is close.
- Shakes but does not turn: the coil is not balanced or the ends are not in line.
- Gets hot: the contact is shorting. Disconnect and fix it.
A real motor never wastes its energy fully: some becomes heat and sound. Most of the electrical energy becomes spinning energy.
Try it
Predict, then check. Before each test, say what will happen. (1) Use two batteries in a row: faster or slower? (2) Use 4 turns instead of 10. (3) Turn the magnet upside down. (4) Scrape the enamel from both halves: does it still spin? Write your guess, test it and note what you saw. Count how many turns it makes in 10 seconds for each test.
Key formulas and definitions
- F = B × I × L (force on a wire in a magnetic field)
- Motor energy: electrical energy → kinetic (movement) energy + a little heat
- Left-hand rule: first finger = field, middle finger = current, thumb = force
- Speed goes up with: voltage, number of turns, magnet strength
Worked examples
1. A wire 5 cm long carries 2 A in a field of 0.1 T. Find the force.
L = 5 cm = 0.05 m. F = B × I × L = 0.1 × 2 × 0.05 = 0.01 N.
2. Your coil has 10 turns, and each turn's side feels 0.01 N. What is the force on that side of the coil?
The 10 turns all add up on the same side: 10 × 0.01 = 0.1 N.
3. A motor takes 0.5 A from a 3 V battery for 10 s. How much electrical energy does it use?
E = V × I × t = 3 × 0.5 × 10 = 15 J.
4. The coil makes 25 turns in 10 seconds. How many turns per second?
25 ÷ 10 = 2.5 turns per second (150 turns per minute).
5. You add a second battery so the voltage goes from 1.5 V to 3 V. The coil resistance is 3 Ω. How does the current change?
I = V/R. First 1.5/3 = 0.5 A, then 3/3 = 1 A. The current doubles, so the force doubles and the motor spins faster.
6. Why does the motor not start if the magnet is far away?
The force F = B × I × L depends on the field B. Far from the magnet B is very small, so the force is too small to move the coil.
Common mistakes
- Scraping the enamel off both halves of the second end. Then current never switches off and the coil gets stuck upright.
- Leaving the enamel on the ends. The varnish blocks the current, so nothing happens.
- Using an unbalanced coil or bent axle. It wobbles and stops. Make the coil round and the ends in line.
- Thinking the magnet moves the coil by touching it. The force comes from the current in the magnet's field, with no contact.