Faraday and Lenz

[mgopalan]

Have a thought question I am having problems with.

If I have a coil of wire...say something simple wound around a paper towel roll or something. And I move a permanent magnet inside it. Completely inside it.

If the magnet is a lot shorter than the length of the coil of wire, any movement of the magnet inside the coil should product a net potential of zero at the terminals right?

I think the potential created by the Field coming out of the North pole of the magnet cutting the coils is opposed equally by the potential created by the Field of the South Pole cutting the wires.Isn't that what Lenz's law says? The North pole moving vreates current in one direction and the South pole creates an equal current in the other direction. All things being equally symmetric...I should see nothing at the terminals...

If I am right, then all those experiments they made me do in Junior High and High school only worked because the magnets were never moved completely inside the coil.

Any confirm or reject this? I've tried to read up on this as much as I can and yet I see contradictions in the few web sources I have seen that discuss this.

 

[logbook]

Your null result in this thought experiment seems right to me.

Rather than thinking in terms of north and south poles, one would normally think of the amount of flux linkages being changed. If we reduce the experiment down to one coil in front of the magnet and one coil behind, and if the rate of change of flux in the one coil is exactly equal but opposite to that in the other coil the induced EMFs will cancel.

Induced EMF= time rate of change of flux in a coil (or a turn), summed over all coils.

 

[Warpspeed]

I agree it should in theory be zero, but it probably never will be exactly central, especially if it moves.

The field of the magnet extends to infinity, so there is always likely to be some extremely small(maybe unmeasurably small) residual voltage on the coil whenever the magnet moves.

 

[mgopalan]

Thanks for the reply...Heres some counters that make my confusion greater.

Imagine a single coil of wire...with a magnet placed exactly in the center of the coil..If the magnet is stationary, nothing happens, we all agree there...

What happens If I move the magnet in either direction? By my previous argument, the output should be zero...but that does not seem right...My intuition says that you will see a potential on the coil.

Heres another counter to this...

Let us say we have a coil of wire with two loops seperated by say 2 inches. Lets take a 1 inch long magnet and put it in the middle equidistant from each loop. Lets say that at this location, both loops see a field of magnitude 500 Gauss. The loop closest to the north will see 100 Gauss while the loop closest to the south pole sees an equal but opposite field of -500 Gauss.

If I move the magnet in one direction, say towards the north of the magnet, the loop closest to the north magnet sees an increasing field as it is getting closer to the pole. Say that we moved 0.1" and the loop sees a jump from 500 to 600 gauss.

Well at the same time, the south pole is getting farther away from the coil. So its field is also changing. Its field is going from -500 to -400 Gauss.

Well the north coil has seen a net change of +100 Gauss and the south coil has seen a change of +100 gauss. By this argument, the effect is additive and you WILL see a net change in the output of the coils...

So which one is right?

MG

 

[Warpspeed]

It is all about flux linkages cutting the wire in an equal and symmetrical manner around the coil and the induced voltages canceling in various sections around the coil.

In theory this should be possible, in practice not likely. In theory you should be able to sit one steel ball bearing on top of another at the exact balance point.

The Earths magnetic field is pretty even provided there are no nearby objects of high permeability to distort it. You can move an air cored coil around all you want and the voltages will cancel.

A small magnet is more like a point source with a highly variable field strength with distance. What should be possible in theory, and what happens in practice might differ considerably.

Another theoretically possible experiment that does not work is getting one magnet to hover over another where the like poles will strongly repel.

 

[Barry1961]

I would think the resistance/impeadance of the wire would have a affect on the voltage measured at the terminals. If you were exactly in the middle of the coil in all planes and the coil was symetrical in all planes the impeadance would be the same to each terminal. But then you would be stationary and not generating any voltage to not measure.

Barry1961

 

[VEBill]

"If the magnet is a lot shorter than the length of the coil of wire, any movement of the magnet inside the coil should product a net potential of zero at the terminals right?"

I don't believe that this is correct.

You could mentally divide the long coil into three parts and take the two end parts far away and call them simple series inductors, L1 and L3, with no significant effect.

This leaves the active middle part (call it L2) being just the right size to act as a generator.

I'm open to correction if my gut reaction isn't right.

 

[maged541946]

Hi,To understand somthing we may need to reverce, uce AC or else, so ..
*)Just assume the opposite, if the magnet is free and a current is applied, if similar poles are on one side, the magnet will be pushed inwards and settle, but if opposite poles, the magnet will be pulled both ways too but stability is vulnrable, any vibration will cause the magnet tp MOVE towared one end. this motion is energy and work done and according to DC machine theory if you reverce action you get reverce results i.e move the magnet you get EMF.
*) How to figure out this EMF will be, well , now use AC.
Assume the magnet is replced by another coil with AC current in it, it will generate an emf. Its envelop tells what a permenant magnet's result will be.
At the center, the filed will generate equal and opposite fileds on the coil, and the result is zero. SHIFTING (not moving , i.e a little displacement and stop to measure) will cause emf to increase in one phase while shifting toward the other end increases EMF in the opposite phase which lend it to be a position sensor device (actually used in early devices using inductors or capacitors). The EMF increase toward the end.
Back to our permenant magnet. it is shown that moving inside the coil from one end to another will generate a voltage from zero far outside the coil , as the magnet approaches, the voltage will increase to a maximum at a point near the enterance end, then decrease gradually to zero at the middle, then incease in reverse polarity to the same maximum value near the exit end then decays to zero far away. At any moment it will generate a constant Voltage/turn value in every turn, the number of turns befor and after the magnet are changing .
Voltage/turn depend on coil geometry, nu of flux lines crossed.
I hope that explanes all the questions

 

[Warpspeed]

A moving permanent magnet is not the same as an ac field.

An ac field rises and collapses to zero then rises in the opposite direction. Of course it will induce a voltage in a surrounding coil, that is how a transformer works.

On the other hand, a moving permanent magnet has a constant steady field. Moving it within a coil will cause the field to change relative position, and that is entirely different. It might get stronger at one part, and weaker in another part of the same winding.

 

[maged541946]

I know that but remember I said THE envelop of the ac out represent the the out..., also he frequency you use repesent thr speed of magnet motion.
these are some examples of how you would manipulate things to get an easy replication of a phenomena and see the expected output outline as measuring the actual out would reqire a machanical setup to move the magnet in a repeated forma or a storage oscilloscope to get a steady picture.