Hello. The skin depth is nothing but measure of penetration depth of EM Wave when it is incident on the conductor. It is well known that the skin depth is decreased when the EM wave frequency increases. But thinking about DC field. We know that when DC is applied to metal (metal is installed between two electrodes) field inside the metal should be zero as charges are distributed according to the external field and the result is not external field and the field due to space charge are cancelled out each other. Thus maybe we can say that skin depth of DC is zero as field can not existed inside the conductor right? This is contrary to typical knowledge of skin depth that it is decreased as frequency raises. And one more question: why skin depth decreases as incident frequency increases in AC? I imagine that in higher frequency, electrons have to respond faster than case of lower frequency thus they feels harder to shield field.