\[R\]
, measured in Ohms \[\Omega\]
is expressed as the ratio of the driving force - the voltage \[V\]
, measured in Volts, V - to the current \[I\]
, measured in Amps, A: \[R=\frac{V}{I}\]
.The resistance
\[R\]
of an object is itself proportional to the length \[l\]
of the object and inversely proportional to the cross sectional area \[A\]
.We write
\[R=\rho \frac{l}{A}\]
, where \[\rho\]
is a constant called the resistivity of the material, measured in Ohm metres - specific to each material.Good insulators - bad conductors - have high resistivity, and bad insulators - good conductors - have low resistivity. Semiconductors are intermediate between these two.
Increasing the temperature tends to increase the resistivity of good conductors and decrease the resistivity of insulators and semiconductors.