Ohm’s law, which states that the current through a conductor is directly proportional to the voltage across it, is a simple and useful concept that applies to many electrical circuits and devices. However, there are situations where Ohm’s law does not hold in its general form.
One reason why Ohm’s law may not hold is that the resistance of the conductor or material may change with temperature, pressure, or other environmental factors. For example, the resistance of a metal wire may increase as its temperature rises, which means that the current flowing through the wire will be less than what Ohm’s law predicts. This effect is known as thermal resistance, and it can be a significant factor in some circuits and devices.
Another reason why Ohm’s law may not hold is that some materials have nonlinear or time-varying resistance. In these cases, the relationship between the voltage and current is not a simple linear one, and Ohm’s law does not apply. For example, in a diode or transistor, the relationship between the voltage and current is highly nonlinear, and a different set of equations is needed to describe their behavior.