# Applying the definition of electrical resistance to a voltage source

The resistance (R) of an object is defined as the ratio of the voltage across it (V) to the current that flows through it (I), so why can't I claim that the internal resistance of a voltage source is: r=V/I? (V is the voltage across the voltage source and I is the current that flows through the voltage source)

$R = \frac VI$ is valid for ohmic materials in many (but not all) situations. A voltage source is not ohmic and you shouldn't expect Ohm's law to apply.