Question

In which case we should use ohm's law .

Answer 1

Ohms law is always applicable to any components or circuits except for ideal conductor. Ohms law states that when a voltage is applied across an object, there will be current flowing in that object such that the current is directly proportional to voltage applied and resistance is the proportionality constant. Practically resistance of any material is never constant, always changes with potential difference , current , temperature , pressure , etc. So when current increases or decreases in a circuit where constant voltage is applied , then the reason is resistance is changing, not that ohms law is not applicable. usually when we buy a resistor we observe two parameters, resistance value and maximum power, it means that the resistance of that resistor will be almost constant until the power dissipated by the resistor is less than the indicated maximum value. In diodes , transistors and vacuum tubes it is the resistance that change with voltage which changes the current in the circuit, there is no case that ohms law is not applicable.