| r L |
 |
| A |
In many materials the current through a conductor is proportional to the potential difference across it. This is summarized in:
Ohm's Law: DV = 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 r, a number that depends on the material:
| R | = |
|
The unit for resistance is the ohm (W).
1 W = 1 V/A
The unit for resistance is the ohm (W). 1 W = 1 V/A
Resistivity values cover an incredibly wide range, as you can see from the table below.
| Material | Resistivity (W m) |
|---|---|
| Copper | 1.7 x 10-8 |
| Tungsten | 5.6 x 10-8 |
| Nichrome | 1.0 x 10-6 |
| Silicon | 640 |
| Hard Rubber | 1 x 1013 |
| Teflon | 1.0 x 1016 |