Potential difference acts like a pressure difference that makes things flow, while the resistance of a conductor is a measure of how difficult it is to get charges to flow through the conductor. Electrical resistance is similar to friction. For water flowing through a pipe, a long narrow pipe provides more resistance to flow than does a short fat pipe. The same applies for flowing currents: long thin wires provide more electrical resistance than do short thick wires.

The resistance R of a material depends on its length l, cross-sectional area A, and the resistivity r, a number that depends on the material:

R

=

r l A

The unit for resistance is the ohm (W).

1 W = 1 V/A

Resistivity and conductivity are inversely related. Good conductors have low resistivity, while poor conductors (insulators) have resistivities that can be 20 orders of magnitude larger.

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