When is Ohm's Law not true?

Answer 1

Hardly ever.

Ohm's Law is not an universal law, and is only true when the ratio of voltage to current is constant over a wide range of voltages and, then, only for that range of voltages. In other words, if the voltage to current ratio results in a straight-line graph. If the graph is curved, then the ratio of voltage to current is not constant, and Ohm's Law doesn't apply.

So Ohm's Law only applies to metal conductors, for example, providing their temperatures remain constant for changes in current -a situation that usually doesn't occur in real life. And it most certainly doesn't apply to the non-linear parts of the voltage/current characteristic curves for diodes, etc.

Answer 2

When you are using semi conductors or the conductor is at a very high temperature.

Answer

Ohm's Law is not a universal law, and only applies to a limited range of conductors. For Ohm's Law to apply, the ratio of voltage to current must remain constant for variations in voltage; if the ratio varies, then Ohm's Law does not apply. Materials for which Ohm's Law applies are called 'linear' or 'ohmic'; those for which it doesn't apply are called 'non-linear' or 'non-ohmic'. There are far more 'non-linear' materials than there are 'linear'; these include the metal tungsten, electrolytes, gases, and semiconducting devices, such as diodes, etc.

The ratio of voltage to current is termed 'resistance', and applies to both linear and non-linear devices. However, the equation, R = E/I, does NOT represent Ohm's Law; it is derived from the definition of the ohm. This equation applies for both linear and non-linear devices. But, for non-linear devices, it only applies at a particular voltage, as the ratio varies with changes in voltage.