 Electrical resistance is the opposition to the flow of electrons in a circuit.
Electrical resistance is present in all parts of an electric circuit
The unit of electrical resistance is the ohm. An ohm is equal to one volt/ampere
Ohm’s law, R = V/I, can be used to solve for resistance (R), voltage difference (V) and current (I) in electrical circuits.
Wire resistance in electrical circuits is similar to fluid resistance in pipes.
The total resistance (R_{tot}) of two resistors (R_{1} and R_{2}) connected in series is R_{tot} = R_{1} + R_{2}
The total resistance (R_{tot}) of two resistors (R_{1} and R_{2}) connected in parallel is
or:
Conductors, semiconductors and insulators are material classifications based on resistivities. Resistivity means the capacity to resist electric current.
Resistivity
 Constant for any material
 Resistance is proportional to the resistivity and length of the material, and inversely proportional to the crosssectional area of the material.
 Resistance based on resistivity is given by:
Where:
 R = resistance (W)
 r = resistivity (W·cm)
 l = length (cm)
 A = crosssectional area (cm^{2})

