# 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?

• The quoted Wikipedia article says, "More specifically, Ohm's law states that the R in this relation is constant, independent of the current." Between that and your posted direct quote (which should be added to the question body, not posted as an answer), what are you unsure of? – J. Murray Sep 2 at 12:56
• There would be much less confusion about this subject if the thing were called Ohm's Rough Guideline for Ohmic Materials when the Current is not too High. – Oscar Bravo Sep 2 at 13:30
• But these things are true for almost all scientific "laws". Gravity, pressure, expansion, elasticity..... That's kind of a given..... – Stilez Sep 2 at 17:45

$$V = RI$$ 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.