## Resistance

When a voltage is applied across an object, a current may flow. Every object resists the flow of current through it, and this 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.