## Resistance

\[R\]

, 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.