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davenn is offline
Dec27-12, 03:01 AM
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Quote Quote by entropy15 View Post
Consider a magnet moving towards a coil.
We know that the motion of the magnet will induce a current in the coil and the direction of this induced current is to oppose the motion of the magnet.

Now does the magnet experience resistance to its motion immediately as soon as it starts moving?
Since the magnet and coil are physically separated, it would take a time t (which is equal to the time taken by light to travel from the magnet to the coil)
to induce a current in the coil and an equal amount of time for the effect of this current to travel back to the magnet and oppose its motion.
Hence the total delay appears to be 2t.
for what to travel at the speed of light from the magnet ??
Nothing is travelling from the magnet
Why ? ... because the magnetic field already exists around the magnet whether its moving or not
As soon as the wire encounters the field lines, a current will start to flow in the wire
bringing the magnet even closer just has the effect of more field lines "moving through" the wire and generating a larger current

The 2 velocities would be
1) the velocity that YOU move the magnet towards the coil, then
2) the velocity of the expanding field around the wire ( over a short distance you would be lucky to measure this)

So does the magnet experience resistance immediately or does it have to wait for time 2t?
I would suspect any delay you measured would be more related to the times it takes for the mangetic field around the wire to grow strong enough to have a measurable interaction with the magnets field

as said above.... The magnetic field around the wire doesnt just suddenly appear, it progressively grows stronger as the magnet comes closer