# H-field depends on core material of toroid?

Discussion in 'Physics' started by Papikoss, Dec 23, 2010.

1. ### Papikoss Thread Starter New Member

Dec 23, 2010
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If I got this right, the H-field does not depend on the material it is applied on and is only determined by its sources (free currents).

So, suppose that we have a toroidal coil and some undefined core. As far as the H-filed is concerned, it would not make a difference if that core was air, a ferromagnetic material or some combination of those two. The H-field would remain the same.

However, if we apply Ampere's Law to a simple magnetic circuit, consisting of a ferromagnetic coil toroid having a small air gap, the result would be (taken from theory):

HfLf + HgLg = NI, where f, g refer to ferromagnetic material and air gap respectively.

Obviously here Hg is much greater than Hf

Now I'm confused! I know that the magnetic flux must remain constant throughout the circuit, but still I can't grasp how Hg and Hf could be different (the distribution of free currents did not change)!

Any help much appreciated!

2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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887
The field is affected by the magnetic permeability of the core material.

so flux is used up crossing the gap.

(Like a high value resistor will drop the majority of the voltage across itself, leaving little for the rest of the circuit.)

The air gap behaves like a high value resistor to the (MMF)flux.