What is the difference between impedance and resistance?

Resistance is a concept used for DC (direct currents) whereas impedance is the AC (alternating current) equivalent. Resistance is due to electrons in a conductor colliding with the ionic lattice of the conductor meaning that electrical energy is converted into heat. Different materials have different resistivities (a property defining how resistive a material of given dimensions will be). However, when considering AC you must remember that it oscillates as a sine wave so the sign is always changing. This means that other effects need to be considered - namely inductance and capacitance. Inductance is most obvious in coiled wire. When a current flows through a wire a circular magnetic field is created around it. If you coil the wire into a solenoid the fields around the wire sum up and you get a magnetic field similar to that of a bar magnet on the outside but you get a uniform magnetic field on the inside. With AC since the sign is always changing the direction of the field in the wires is always changing - so the magnetic field of the solenoid is also changing all the time. Now when field lines cut across a conductor an emf is generated in such a way to reduce the effects that created it (this is a combination of Lenz's and Faraday's laws which state mathematically that E=N*d(thi)/dt , where thi is the magnetic flux linkage). This means that when an AC current flows through a conductor a small back emf or back current is induced reducing the overall current. Capacitance is a property best illustrated by two metal plates separated by an insulator (which we call a capacitor). When current flows electrons build up on the negative plate. An electric field propagates and repels electrons on the opposite plate making it positively charged. Due to the build up of electrons on the negative plate incoming electrons are also repelled so the total current eventually falls to zero in an exponential decay. The capacitance is defined as the charge stored/displaced across a capacitor divided by the potential difference across it and can also be calculated by the size of the plates and the primitivity of the insulator. read less

A DC circuit will not exhibit any form of Reactance, therefore opposition to current flow will always be measured in Resistance. In addition to Resistance, AC circuits also exhibit Reactance.. Reactance exists in two forms, Capacitive & Inductive. The combination of Reactance & Resistance is called Impedance. Impedance can therefore be thought of as the opposition of electrical current flow in a AC circuit. read less