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.