Copyright © Michael Richmond. This work is licensed under a Creative Commons License.

# Resistance and Resistivity

• Electric resistance is measured in ohms:
```                      1 volt
1 ohm = ­---------
1 amp
```
• The resistance of a wire or rod to the flow of electric current depends both on its geometry and its composition.
• Wires with large cross-sections (thin ones) have small resistance; thick ones have large resistance.
• Short wires have small resistance; long ones have large resistance.
• Different materials have different resistivities. The units of resistivity are ohm-meters.
• The resistance of a wire or rod of uniform cross section can be calculated as
```                            (resistivity) * (length)
Resistance = ------------------------
(cross section area)
```
• The resistivity of a material depends on its temperature. Most metals have lower resistance as the temperature drops.
• The temperature coefficient of resistivity describes the change in resistivity as a function of temperature:
```
resistivity (T) = resistivity (T0) * [1 - a*(T - T0)]

where
a  =      temperature coefficient of resistivity
T0 =      reference temperature
```
• Some materials become superconductors when they fall below a critical temperature. They offer zero resistance to the flow of current.

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Copyright © Michael Richmond. This work is licensed under a Creative Commons License.