Resistance is the opposition to current flow and occurs in AC and DC circuits. Resistance is measured in Ohms and represented by the Greek letter “omega” or symbol Ω.

Impedance is the total opposition to current flow in an AC circuit. Just like resistance, impedance is measured in Ohms but often symbolized by the letter “Z”.

Impedance in an AC circuit includes both resistance and reactance. Reactance is a term used to describe amplitude and phase changes of sinusoidal current going through an AC circuit.

In DC circuits, there is only resistance since reactance does not occur in a DC circuit.

Factors such as wire size, length and material all have an impact on the final amount of resistance in a circuit. A longer run of wire has greater resistance than a shorter run of the same wire. The larger the wire, the lower the resistance. Copper is a more noble metal and better conductor than aluminum and therefore has less resistance.

Different materials vary in their level of resistance and can be classified into one of the following three categories:

• Conductors -Material that easily conducts electricity and has a low resistance.
• Semiconductors – Materials that fall somewhere between conductors and insulators.
• Insulators – Materials with a very high electrical resistance (conductor insulation for example)

In a series circuit, the total resistance is found by summing up all the resistors.

In a parallel circuit, the total resistance equals the reciprocal of the sum of the reciprocals in each branch resistance. In a parallel circuit, the total resistance is always less than any of the branch resistances.