What it shows
A current flowing through a magnet in a magnetic field feels a sideways force (F = B I L), and here that force makes the magnet and screw spin at hundreds of revolutions per minute. It is the simplest motor that works with no coil and no commutator.
The science
The magnet sticks to the screw head and the screw tip hangs from the cell terminal by magnetic attraction. Touching the wire from the other terminal to the rim of the magnet completes the circuit, and current flows radially through the magnet. The magnet’s own field is along its axis, so the current and the field are at right angles and the Lorentz force F = B I L acts in the tangential direction. That torque spins the magnet and the screw, and reversing either the cell or the magnet reverses the spin.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- One AA cell
- A 5 cm steel screw or nail
- A small disc neodymium magnet, 10 to 15 mm (about Rs 30 from an online shop)
- A 15 cm piece of copper wire with the insulation stripped
- A second magnet and a paper cup for the extension
Safety
For this project: The cell is shorted while the motor runs and gets hot; run it only for 10 to 20 seconds at a time. Neodymium magnets snap together hard and pinch, and small ones must be kept away from young children. Cells only.
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.
Stick the magnet to the flat head of the screw so the screw hangs down from it.
Hold the cell vertically and let the screw tip hang from the bottom terminal; it should hold on by the magnet’s pull through the screw.
Press one end of the bare wire firmly on the top terminal of the cell with your thumb.
Touch the other end of the wire lightly against the edge of the magnet and watch the screw begin to spin.
Flip the magnet so its other face touches the screw and show that the direction of spin reverses.
Reverse the cell and show the spin reverses again; make a small table of cell direction, magnet face and spin direction.
Count revolutions by marking the screw with a white dot and filming in slow motion for 5 seconds, then convert to rpm.
Mount the whole thing inside a cut paper cup on a stand so visitors can see it safely at the exhibition.
What you should see
The screw and magnet spin fast within a second of the wire touching, reverse when the magnet is flipped, and typically reach several hundred rpm in the slow-motion count.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Which force makes the screw spin?
The force on a current-carrying conductor in a magnetic field, F = B I L, acting at right angles to both.
2Why does it need no commutator?
The current direction through the magnet never changes relative to the field, so the torque is always one way.
3Why does the cell get hot?
The circuit resistance is very low, so a large current flows and heats the cell.
4How can the spin be reversed?
Reverse the current (flip the cell) or reverse the field (flip the magnet).
Take it further
Build a wire homopolar motor where a bent copper loop spins around the cell instead of the magnet.
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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