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Yes, that's, what I suggested. It certainly doesn't hold, though, in the case of magnets moving near a coil such as in a motor. It might be true for the case of a projectile, which is not magnetized itself, i.e. only by the surrounding field.
I've found some sort of a proof for that the term I * dL/dt contains the complete backEMF. Therefore we have for the coil voltage:
V = L * dI/dt + I * dL/dt
and for the force:
F = 1/2 * dL/dx * I^2
For e.g. constant coil current 50% of the energy suppiled through backEMF goes into mechanical energy and 50% into the magnetic field energy. If L(x) is known a full simulation could be made inside spice.
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780
thanks guys for the formulas, that makes it all clearer
in the case of a projectile being a moving short, it would be magnetized AND produce a field...
would that field be orthogonal as I think? what would be the interaction with the field of the solenoid? Somehow I can't figure this clearly (I have a sinusitis actually, which could play a role ;) )
would that field be orthogonal as I think? what would be the interaction with the field of the solenoid?
Think of a simple loop of wire carrying a current. It will experience a force trying to widen it. This is a special case of the general rule, that the forces try to increase the inductance. Something similar happens to a solenoid. It will try to widen and also to become shorter. Both of these also would increase inductance. Same thing with a piece of iron near the coil. It will be sucked into it and therefore also increases inductance.
If you have a piece of iron, which at the same time will change the number of turns, if moved, two forces appear: One force will suck the piece of iron into larger B fields, i.e. into the coil. The other force will try to increase inductance by lengthening the coil. That will be a force in the opposite direction. The forces won't necessarily cancel, but the larger force will be weakened.
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Coaxial LC design for pulse compression using a moving short
