The frequency of direct current (D.C.) is.

Hint: Usually when we have angular velocity we can get frequency from it and when we get frequency the inverse of that frequency gives us the time period. Time period of anything is the amount of time required for it to complete one oscillation. In order have frequency displacement must be there in both positive and negative directions

Complete answer:
Frequency is the number of waves passing through a point within one second. If we take inverse of that frequency, we will get the time period.
In case of currents we have alternating current where wave form of the current will be in sinusoidal form or cosecant form there will be current displacement existing in the both directions and frequency is something like the number of repetitions we get in a second.
But in the case of direct current we don’t have any repetitions because the plot of direct current with time itself is a straight line. Here there is no meaning if we are about to find the repetitions because no particular repeating waveform is present.



Additional Information:
In case of gravitation satellites revolving around any planet we get angular velocity of rotation of that satellite by equating the centripetal force of that satellite to the gravitational force of attraction between satellite and that planet and then we will find out the frequency and we will take the inverse of frequency to get the time period. This frequency is a bit different from the current frequency but overall the meaning of frequency is same in both cases that is number of repetitions per second.

Note:
In case alternating currents there will be angular velocity of a wave and we can find frequency of an alternating wave from that. That frequency determines the capacitive reactance and inductive reactance and total impedance of the AC circuit. Inductor only works in AC circuits but doesn’t work in DC circuits.