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Magnetomotive force (m.m.f.). Magnetomotive force drives or tends to drive flux through a magnetic circuit and corresponds to electromotive force (e.m.f.) in an electric circuit. The m.m.f. of a coil product of current in the coil and the number of turns of the coil and has the unit of ampere turns (AT). Magnetic field strength (H). The magnetic field strength (H) is defined as the m.m.f. per meter length of magnetic circuit i.e.,

where,  N = number of turns of a coil,

l = current (amperes), and

I = length of the magnetic circuit tm),

Example1. The following data relate to an electromagnet :

Total flux = 8 × 10-4 Wb

Cross sectional area of the core = 200 mm2

Number of turns = 100

Magnitude of current = 2 A

Length of the magnetic circuit = 400 mm

Calculate the following:

i.            Flux density in the coil,

ii.            Magnetomotive force, and

iii.            Magnetic field strength.     

Solution Total flux, ɸ = 8 × 10-4 Wb

Area of the coil, A = 200 mm2 = 2 × 10-4 m2

Number of turns, N = 100

Current,         I = 2A

Length of the magnetic circuit, l = 400 mm = 0.4 m.

i.        Flux density in the coil, B : 

ii.        Magnetomotive force, m.m.f.:

m.m.f. = NI = 100 × 2 = 200 AT. (Ans.)

iii.        Magnetic field strength, H: