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.

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