When a PD of V is applied across a resistor, a current I flows through the resistor.
It turns out that V is proportional to I for a lot of common components used in electrical circuits. This relationship is called Ohm’s Law.
where the constant of proportionality R has come to be known as resistance.
A word of caution: The symbol V is used to denote an electric potential in the Electric Field chapter. Here the same symbol V is used to denote a potential difference. In a less messed up world Ohm’s Law should be written as 
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xmPhysics 