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Q & A: low self-inductance

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Most recent answer: 09/25/2013
Q:
Hi, inductance is the property of a conductor by which a change in current in the conductor "induces" (creates) a voltage (electromotive force) in both the conductor itself (self-inductance). --wikipedia From what I understand, the inducing happens due to the resulting change in magnetic field from the change in current from a change in voltage. Also, from what I understand, the greater the current, the greater the magnetic field. However, the inductance of a coil is not dependent on the current. My reasoning behind that is that for every unit of current in the coil, there is a proportional amount of magnetic field which induces an opposing voltage (and in turn current. So if current increases, a proportional amount of opposing current will be there, therefore it cancels out in the formula just like this guy explains: watch?v=Ab0dJLdmApg My question comes here: If you have a second source of changing magnetism (external source), is it possible that the second magnetic source and the primary magnetic source (coming from the coil itself) have their net flux cancelled out within the same coil? If so, the self inductance of the coil would be zero and the coil would act as a regular wire with no inductance. No net flux, no induction right? This question has been really bugging me since I've learned in school that inductance of a coil cannot change no matter what. Thanks
- kevin (age 19)
Canada
A:

You're right that fields can cancel and reduce the self inductance. Think of a coil with a large field inside (per current) threading all the loops in the same way. That makes a big inductance. Now make a second coil around it, wound the opposite way. The fields cancel. So there's very little self-inductance.

I say very little rather than zero. Another way to think of the inductance L is as the coefficient in the expression for the magnetic field energy: LI2/2, where I is the current. The field energy is proportional to the spatial integral of the square of the field. Even for our counter-wound coils there will be little regions with non-zero fields when current flows, so L won't be zero. In fact, even for a straight wire there is a field around the wire so a straight wire has some self-inductance.

Mike W.


(published on 09/25/2013)

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