Experiments with magnets and our surroundings

Twelve Fundamentals of Magnetism

5.   Magnetomotive Force, MMF [ampere-turns, At]

a. Links for this topic


b. What is it?

Similar to the Electromotive Force (EMF) that creates the current in an electric circuit (as used in Ohm's law where EMF = E = I*R, where I is the current in the circuit, and R is the electrical resistance in the circuit), the MMF creates the magnetic flux in a magnetic circuit (MMF = F = Φ*R, where Φ is the flux in the material, and R is the magnetic reluctance of the material). 

c. Its unit

MMF = N*I, where N is the number of turns (of wire around a form) times I, the current flowing in the wire. The actual unit is Amperes, but it is useful to write it as At (ampere-turns) in order to clearly show how it is crated. "Turns" is not a true unit.

d. How do you create an MMF?

This is the one area where the magnetic circuit designer has several ways to get a particular result. For example, if 1000 At is needed, it can be obtained by:
    i. 1000 turns with 1 A, or
    ii. 100 turns with 10A, or
    iii. 10 turns with 100A, or
    iv. 1 turn with 1000A, or
    v.  anything in between

The question is, what kind of current is the designer going to use? 1000A requires a large power supply. 1 A can be obtained from a battery. He could also use 10,000 turns with 0.1A from a solar cell! They will all provide the designer with 1000 At.

e. Copper cross section

Please note that with the different ways to obtain 1000 At, the total amount of copper will be about the same since a wire that can handle 1A is fairly small, so 1000 turns will make a coil with a certain cross sectional area. A wire that can handle 100A has about 100 times the cross sectional area, so 10 turns of that would give you a coil with about the same total cross sectional area. Packing density of the wires does vary, though.