Resistance

Electrical - or ohmic - resistance occurs when moving charge carriers collide with Conductor atoms. This impedes uniform charge carrier motion - the current flow. The greater the conductor material's cross-section, the lower the conductor's resistance. This is because the electrons have more space in the conductor with the same current intensity (= same number of charge carriers), so there are fewer collisions with the metallic lattice. Similarly, the resistance increases with the length of the conductor, because a Collision is more likely over a longer distance.

In the case of metallic conductors, resistance increases as the temperature rises (PTC resistor). Semi-conductors are NTC thermistors, i.e. their resistance decreases as the temperature increases.

In mathematical terms, the electrical resistance R expresses the relationship between voltage V and current I. In a circuit with ohmic resistance only, Ohm's law applies: $R = V/I$. In an electrical circuit with inductances or capacitances, the Impedance is derived from the effective resistance and the reactance.