A little while ago I saw a great demonstration by Alom Shaha of an ingeniously simple motor. It was so simple that I immediately thought even I could make it.
Now Alom went to great pains to first talk about the demonstration and get people to think what was going to happen and make a testable prediction. I will not subject you to that. Instead I invite you to watch and wonder, secure in the knowledge that at the end I will offer an explanation of sorts.
The motor works as follows:
- Electric current flows through the copper wire when it touches both the upper terminal of the battery, and the magnets attached to the base of the battery.
- The magnets are metallic and so electric current can flow through the magnets to the lower terminal of the battery.
- Near the magnets, the electric current flows through a region of strong magnetic field. In this region there is a force on the wire.
- The strength of the force is given by
Force (in newtons) = current (in amperes) x length of wire in field (in metres) x magnetic field strength (in tesla)
- The current is probably between 0.1 amperes and 1 ampere (I guess)
- The length of wire in a strong field is around one centimetre (0.01 metres)
- The magnetic field strength is (I guess) somewhere around 0.01 tesla
- Putting these together we get a force in the range 0.000 01 newtons to 0.000 1 newtons
- This is equivalent to the gravitational force exerted on an object weighing a few tens of grams – such as a few twopenny pieces.
- Because the wire does not weigh very much, and is free to rotate with low friction, even this small force is sufficient to set it in motion
- As the slow motion footage shows, the electrical contact of the wire with the magnets is only intermittent. During this contact, the wire will receive a ‘kick’.
- The direction of the force is at right angles to both the direction of the magnetic field and the electric current.
- In this poor implementation of the original Shaha™ design, the force is unlikely to be directed at exactly the correct direction to cause just rotation.
- Some part of the force will cause the wire to move away or towards the magnets which is what causes the unstable rotation – and the occasional loss of contact with the magnets.
I apologise for trying to explain this, but I just wanted to make the point that this is not ‘magic’. It is exactly the same phenomenon that takes place in every motor in every one of the thousands of devices in your life. In an engineered motor, great care is taken to:
- To reduce the friction of the rotating part.
- To put more wire into the magnetic field so that a given current produces the largest force.
- To have the strongest magnetic field created by the lightest components.