Ohm's law is applicable for a: 

Ohm's law

Ohm’s law states that the voltage across a conductor is directly proportional to the current flowing through it, provided all physical conditions and temperatures remain constant.

V α I

V = R × I

where, V = Voltage

R = Resistance

I = Current

Limitations of Ohm’s law

• It is only applicable to metal conductors and linear elements such as resistor.
• Ohm's Law does is not applicable on non-linear elements such as Zener diode.
• The relation between voltage and current depends on the sign of voltage.
• Ohm's law is applicable when the temperature of the conductor is constant. Resistivity changes with temperature.