Why ohm's law is applicable for steady current?

Answer 1

It's an immutable law of physics! It applies to a.c. too but only if the circuit is purely resistive.

Answer

Ohm's Law is not a universal law, and only applies to a limited range of conductors or loads, called 'linear' or 'ohmic' conductors or loads. Basically, it only applies when the ratio of voltage to current is constant for variations in applied voltage. If this ratio changes for variations in voltage, then the conductor (e.g. tungsten) or load (e.g. diode) does NOT obey Ohm's Law and is termed 'non-linear' or 'non-ohmic'.

The ratio of voltage to current is called 'resistance', and the ratio will ALWAYS tell you what the resistance happens to be for that particular ratio (if the ratio varies with voltage, then so too does the resistance), whether the conductor/device obeys Ohm's Law or not!

So Ohm's Law really has nothing to do with a circuit having a 'steady current'; it's all to do with whether or not the ratio of the circuit's voltage to current is constant for variations in voltage. Nothing more!