When a voltage is applied across an object, a current may flow. Every object resists the flow of current through it, and this resistance  
, measured in Ohms  
  is expressed as the ratio of the driving force - the voltage  
, measured in Volts, V - to the current  
, measured in Amps, A:  
The resistance  
  of an object is itself proportional to the length  
  of the object and inversely proportional to the cross sectional area  
We write  
\[R=\rho \frac{l}{A}\]
, where  
  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.

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