The magnetomotive force can be expressed in terms of flux and magnetic reluctance as
where the magnetic reluctance is
Therefore, for the ring without a gap the MMF is
When we have the airgap, the reluctance changes:
where the length of the steel ring will be, of course, 1 mm less than in the previous case.
Calculations give us the answer of
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An air-gap between two pole pieces is 20 mm in length and the area of the flux path across the gap is 5 cm2. If the flux required in the air-gap is 0.75 mWb. Find the mmf necessary.