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), *

**Example****1**. *The following data relate to an electromagnet : *

*Total flux *= 8 × *10 ^{-4} *

*Wb*

*Cross **sectional area of the core **= **200 **mm ^{2} *

*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 mm^{2} = 2 × 10^{-4} m^{2 }

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 : **

i**i. ****Magnetomotive force, m.m.f.****:**

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

**iii. ****Magnetic field strength, H:**

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