*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.