Opposition that a circuit presents to electric current .
It includes both resistance and reactance . Resistance arises from collisions of the current-carrying charged particles with the internal structure of the conductor . Reactance is an additional opposition to the movement of electric charge that arises from the changing electric and magnetic fields in circuits carrying alternating current . Impedance in circuits carrying steady direct currents is simply resistance. The magnitude of the impedance Z of a circuit is equal to the maximum value of the potential difference, or voltage V , across the circuit, divided by the maximum value of the current I through the circuit, or simply Z = V / I . The unit of impedance is the ohm.