In physics, the magnetomotive force (abbreviated mmf or MMF, symbol F ) is a quantity appearing in the equation for the magnetic flux in a magnetic circuit, Hopkinson's law. It is the property of certain substances or phenomena that give rise to magnetic fields : F = Φ Φ R , where Φ is the magnetic flux and R is the reluctance of the circuit. It can be seen that the magnetomotive force plays a role in this equation analogous to the voltage V in Ohm's law, V = IR, since it is the cause of magnetic flux in a magnetic circuit: