Strictly speaking Ohm's law is _not_ always true, its a property of certain conductors and intrisic

semiconductors.

Ohm's law states that the resistance of such a material is independent of current, ie that

voltage across a resistor is in linear proportion to current through it. (Or put another way,

R is a constant at constant temperature)

This clearly doesn't hold for a diode, a battery, a transistor, an insulator, a vacuum, etc etc,

its a property of certain materials that exhibit resistance like metals, carbon, electrolytes and

uniformly doped semiconductors.

People often think that the equation V = IR is Ohm's law, and it isn't, its just the definition of

resistance. This is a very common misconception, but its worth remembering when you encounter

materials that dont obey Ohm's law, like voltage-dependent resistors.

People often loosely refer to the equation V=IR as Ohm's law, and we usually know what they

mean, but don't be confused into thinking its a law of nature.

Compare with distance = velocity x time. That's the definition of velocity, so its always true,

but its not a law of nature because you can't independently test it with an experiment,

its a logically tautology. Laws of nature have to be discovered by interactiing with nature!