|MadSci Network: Engineering|
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
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."
Try the links in the MadSci Library for more information on Engineering.