Basic Units

Resistance

Conductors do not conduct perfectly. The electrons interact with the material and this results in energy being lost and this energy is released as heat, as has been said. This property is called resistance and the units of resistance (R) are ohms and this is often written as W. Resistance is defined as the ratio of voltage across the conductor to the current flowing through it. That is:

 

This equation is known as Ohm's law*.
From this we can also work out alternative ways of expressing power namely:

 

*Named after Georg Simon Ohm, 1787-1854
(see: http://www-gap.dcs.st-and.ac.uk/~history/Mathematicians/Ohm.html)