ELECTRO MEGNETIC

Magnetic Field
The magnetic field at a point is defined as being equal to the force acting on a unit magnetic
pole placed at that point.
[Unit of magnetic field is ampere per meter (A/m)]

Magnetomotive force (mmf)
Magnetomotive force is the flux producing ability of an electric current in a magnetic circuit.
[It is something similar to electromotive force in an electric circuit].
[Unit of magnetomotive force is ampere (A)] -
 Note: Although some books use the term
ampere-turns, it is strictly not correct as turns is not a dimension]
mmf  ℑ = Σ I

Consider a coil having N turns as shown.
It will link the flux path with each turn,
so that
total current linking with the flux would be
Σ I = N.I

Thus from Ampere’s Law, the mmf produced by a coil of N turns would be N I,
and N I = H l.

Field produced by a long straight conductor
If a circular path of radius r is considered around the conductor carrying a
current I, then the field Hr along this path would be constant by symmetry.
∴by Ampere’s Law
, 1.I = Hr .2π r  or Hr=I/2πR
at a radial distance r from the conductor