R.h. corkscrew rule
Note the direction of the flux is given by Maxwell's 'R.H. corkscrew rule' and that the flux lines are continuousApplying Ampere's Law to a concentric circular path inside the ring gives:
H.l=N.I
so
H=N.I/l
where l is the length of the chosen path inside the former. (If the radius of the former is large compared to the width, then l =2 .pi .r )
Where ℜ=l/µµ_{0 }A is called the reluctance of the magnetic path It has been written in this way to show the identical form of the equation to current flowing in the conductor ie:
I=V/R
where the current I flows through a conductor of resistance R.
Hence the name 'magnetic circuit' Here, Φ is analogous to current IN.I is analogous to V ℜ is analogous to R The analogy is so strong that the product N.I is often referred to as the 'magnetomotive force' (m.m.f) in analogy with the electro-motive force (e.m.f. or voltage) and
(σ = conductivity of the material).So we may write: flux Φ = m.m.f / circuit reluctance (Analogous to: I = voltage/resistance in an electrical circuit). Hence the flux flows more 'easily' through a material with high µ material - i.e. it requires less m.m.f to produce a given flux in the material, just as current flows more easily through a high conductivity material. If an air gap were now introduced into the core,