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help!: burning question about 'jumping ring'

help!: burning question about 'jumping ring' - Physics Forum

help!: burning question about 'jumping ring' - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.

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Old 10-31-2005, 08:57 PM
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Default help!: burning question about 'jumping ring'

I' have a question about the theoretical explanation of the following

An alternating voltage is placed on the primary coil which surrounds a
twelve-inch long, one-and-a-half-inch diameter iron core. A conducting ring
surrounding this core will have a current induced in it. The force
on this current due to magnetic field lines fringing out from the core will
cause the ring to jump twenty to fifty centimeters in the air.
This experiment is an application of Lenz's Law. Lenz's Law states that
an induced current in a closed conducting loop will appear in such a
direction that it opposes the change that produced it. The actual
explanation of what happens with the ring is as follows: we see an increase
in magnetic flux in the primary coil which induces a current in the ring.
The induced current opposes this change, and sets up its own magnetic
field. This opposition is in effect a repulsion (two like poles facing one
another) and the reason that the ring jumps off.

What I do not understand is the constant repulsion instead of an alternation
of repulsion and attraction. Does that mean that there is a phaseshift of
0.5 between the currents in the primary and the secondary coils? Why? When
the magnetic
flux decreases, then the ring is creating a 'supporting' flux and hence the
current is in the same direction
and there should be attraction instead of repulsion, or am I wrong?

I appreciate your help,

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Old 10-31-2005, 09:44 PM
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Default help!: burning question about 'jumping ring'

Pennink wrote:

Basically, the response time is a lot faster than the 8 ms for an
oscillation to flip. The current induced flips just as fast as the
current in the primary.


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Old 11-05-2005, 07:01 AM
The TimeLord
Posts: n/a
Default help!: burning question about 'jumping ring'

Pennink <[Only registered users see links. ]> wrote in
<4366853e$0$155$[Only registered users see links. ].hccnet.nl> on Monday 31 October 2005
14:57 posted to alt.sci.physics:


That's the usual explanation. However, the details are a bit more involved.

The way I usually explain it is as follows:

Faraday's Law is that when magnetic flux changes in a coil, we get an
induced voltage in the ring of

U = - d/dt (phi) = - d/dt (B*A)

This voltage induces a current of

I = U / R = - 1/R * d/dt (B*A) = - (2 Pi r)/R * dB/dt

by Ohm's Law. However, this current interacts with the magnetic flux from
the coil to produce a Lorentz force (also called Heavyside force in older
books) of

F = I * 2 Pi r * B = - (4 Pi^2 r^2)/R * B * dB/dt

on the ring, which makes the ring jump. From this we can see a couple of
things that seem to be borne out in experiment:

1. Large magnetic fields produce more force.
2. Fast changing magnetic fields produce more force.
3. Coils with low electrical resistance produce more force.
4. Large rings produce more force (up to the point where the magnetic field
is no longer uniform in space).

I guess your idea is not technically wrong. However, I hope that my
explanation sheds some light on the subject.

Take care.

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burning , jumping , question , ring

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