In many materials the current through a conductor is proportional to the potential difference across it. This is summarized in:

Ohm's Law: ΔV = IR

The resistance, R, is a measure of how difficult it is for charges to flow. The resistance of a object depends on its length L, cross-sectional area A, and the resistivity ρ, a number that depends on the material:

R

=

ρ L A

The unit for resistance is the ohm (Ω).

1 Ω = 1 V/A

The unit for resistance is the ohm (Ω). 1 Ω = 1 V/A

Resistivity values cover an incredibly wide range, as you can see from the table below.