Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thebrain13 on 05/09/2007 22:19:55
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Moving a magnet end over end on its poles relative to a wire produces electric current. So theoretically I could just spin a magnet, end over end, near a group of wires and create endless amounts of energy (granted I could create a frictionless aparatus). However I know this is not the case and I dont remember hearing why not. So how does the force responsible for slowing the magnet, relative to the wires, work?
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When you move a magnet near a conductor it, as you rightly say, induces a current in the inductor.
What you have missed is that the opposite is also true, if you run a current in an conductor, it generates a magnetic field (this is what an electromagnet is, and without which you could not have an electric motor).
So, when you move a magnet near a piece of wire, it induces a current in that wire, but that current in the wire also generates a magnetic field, and the magnetic field is exactly such as to oppose the motion of the magnet.
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the
DIRECTION OF the magnetic field is exactly such as to oppose the motion of the magnet.
This is Lenz's law and it is the electrical equivalent to Newton's third law - (action and reaction are equal and opposite). The opposing force is proportional to the amount of induced current. There would be an opposing force, because of this, to your spinning magnet.
But there would be another factor - the equivalent of friction - due to the resistance in the wires. The resistance in the wires would cause energy to be dissipated and the magnet would slow down. It's why you have to do work against the handle of a dynamo in order to light a lamp. This is the effect that dominates, normally.
The Lenz's law force is very big when there is no resistance. When a superconducting metal object is dropped onto a magnet the force caused by the induced currents is enough to suspend (levitate) the object. The (force times distance) work done on the object moving it down toward the magnet represents the energy given to getting the charges moving in the metal. The charges keep going because there is no resistance. But no energy can be got out of the system - or the object falls.
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thanks guys