MadSci Network: Engineering

Re: Electricity generators

Date: Sat May 2 21:54:30 1998
Posted By: William Beaty, Electrical Engineer / Physics explainer / K-6 science textbook content provider
Area of science: Engineering
ID: 892332559.Eg

Greetings Najib!

It sounds like you are trying to build a perpetual motion machine. Your ideas are mostly correct, because a swinging magnet could be made to move frictionlessly for a very long period. And if the magnet was inside a closed loop of wire, the relative motion between the magnet and the wire would cause an electric current to be induced in the circuit. As long as no energy is extracted from the system, in theory the electric current can continue forever.

However, in order for this to keep working forever, the wire must be electrically frictionless. Or in other words, it must have zero resistance, and it must not be connected to any load such as a light bulb, motor, etc. Otherwise we would create an effect called ELECTROMAGNETIC BRAKING, which would slow and stop the moving magnet. In electric circuits, electrical "loads" or resistances cause the moving magnet to feel a type of "friction."

When a magnet approaches a perfectly conducting ring, an electric current appears. Since there is nothing to slow the current, it will continue for as long as the magnet's pole is near the ring. When the magnet is pulled away, the changing field causes the current in the ring to slow and stop.

For an electrically resistive ring the situation is different. The magnet pole approaches, and an electric current appears. But when the magnet stops moving, the wire's resistance slows and stops the current. If the magnet is now pulled away, electric current again appears, but this time the charges circulate in an opposite direction than before.

OK, so the situations are different. But how can the resistive ring act like friction?

It does so because of magnetic forces. When there is a current in the ring, the ring becomes an electromagnet, and it has its own field. When the magnet approaches the perfectly zero-resistance ring, the ring's field repels the magnet. Look up LENZ LAW for more information about this. When the magnet is removed, the repulsion field helps push the magnet away and so all energy is recovered. It took energy to force the magnet into the ring, but the ring gives the energy back when the magnet is removed.

But if you bring the magnet near the RESISTIVE ring, there is repulsion at first, the ring and the magnet fight each other, but then the current dies away and so does the repulsion. If you then remove the magnet, the opposite current is induced, and now the ring ATTRACTS the magnet. It fights you as you remove it. So, for the resistive ring, it takes energy both to insert the magnet pole into the ring, and it also takes energy to withdraw the pole. This is like friction: it fights you no matter which way you try to move.

If you spin a magnet in space and then bring it near a closed loop of copper, the current in the copper will heat the electrically resistive metal, and the magnet will slow and stop. If you use a superconductor wire, then the magnet will not stop. But the electric current in the zero-resistance wire cannot be used for anything. If you run it through a light bulb, then the light bulb resistance will slow the current as with the resistive copper loop, and the magnet will be slowed and stopped.

Magnets essentially act to "pump" the charges in a wire. If the charges can flow frictionlessly, then both the charges and the magnet might keep moving forever. But if the charges experience friction and resist moving, this force will find its way back to the magnet and cause it to feel electromagnetic friction as well.

For info about hands-on "electromagnetic braking" experiments, check out my page about NEODYMIUM MAGNET EXPERIMENTS. If you are interested in hobbyists who pursue strange energy sources and "perpetual motion" effects, see my page about WEIRD SCIENCE, especially the parts about "free energy."

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