How does dc current has freequency 0?

DC is unidirectional, while AC reverses it's direction. We characterise that by assigning the time period i.e. the time it takes to comple one cycle, and hence we can also inverse it to define frequency.

So such cycle is possible in DC. We say that it's time period is very large or infinite.

Hence time period inverse i.e. frequency is zero.

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