The force required to create a magnetic field is called the magnetomotive force, abbreviated to m.m.f. with a general symbol Fm.
The magnetic field is the result of the magnetomotive force. Like an electric circuit’s electromotive force, a magnetic circuit requires the application of a force known as the magnetic moment of force (m.m.f.).
The magnetic field of a solenoid is proportional to the number of turns, and the magnetic force of a conductor is proportional to the current flowing through it. This means that the magnetic field produced by a coil grows in direct proportion to the square of the current flowing through it multiplied by the number of turns.
Magnetomotive Force Unit & Formula
Because the number of turns in a coil or solenoid is a dimensionless quantity, the ampere is the m.m.f. unit in pure SI units. However, the number of twists must be factored in.
In the electrical industry, m.m.f. values are typically expressed as ampere-turns (IN) with the units specified as ampere-turns (At).
Fm = IN
Where:
Fm = m.m.f. or magnetomotive force, in ampere turns
I = current flowing in amperes
N = number of turns in coil
Example
Determine the value of the magnetic field strength (m.m.f.) produced by a 120-turn coil carrying a 5 A current.
Fm = IN (1)
= 5 x 120 (2)
= 600 at (ans) (3)