What exactly does Ohm's law say? Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the constant of proportionality, the resistance, R one arrives at the usual mathematical equation that describes this relationship:
$$V=IR.$$
Then why the assertion that V=IR is a statement of Ohm's law is not true?
 A: $V = RI$ is a statement of Ohm's law, provided the resistance $R$ is a constant, i.e. independent of the voltage $V$ or the current $I$. Ohm's law is valid to a good accuracy for a wide range of materials (called ohmic materials), but does not apply to all materials.
A: Just check here: Why V=IR is not Ohm’s Law, and why that matters
What is V=IR?
This formula is the definition of electrical resistance (often stated as R=VI, but it’s the same thing, just rearranged). The SI units of the quantities voltage, current and resistance are volts (V), amperes (A) – amps for short, and ohms (Ω), respectively. So if a voltage of 10 V causes a current of 2 A in a resistor, then its resistance is 5 Ω.
Why is it not the Ohm's Law?
Resistance doesn't have to be always constant as in an old-style filament light bulb, it can vary due to factors such a temperature. A light bulb just has a resistance that changes with current, unlike the constant resistance of a resistor. So in each instant of time V=IR, but Ohm's Law does not hold due to R being variable.
A: The Ohm’s Law is usually just shown as:
V = I•R
and most people end up saying voltage (voltage is rather an incorrect word in case of physical terminology; we use potential difference) is directly proportional to current.
But the most important thing is, in the equations in physics, the constants introduced are of two types (as I surmise): independent and dependent. The independent constants are never changeable, as Newton tried for his Universal Gravitational Constant. But the dependent constants are themselves variable or changeable as in Ohm’s Law—you can easily change the resistance of a conducting device. So, in all, Ohm’s Law actually states that:
The potential difference is directly proportional to electric current only if resistance is constant and not fluctuating or variable.
Hope this helped you!
