What is the difference between resistance and impedance?

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 resistivity (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 buildup 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 permittivity of the insulator.

So simply resistance and impedance have different fundamental origins even though the

calculation for their value is the same:

R=V/I