Resistance is DC only
Impedance = Resistance + j*Reactance [AC can contain a complex "resistance" called reactance]

Since your plug is DC, there is no reactance, hence impedance is resistance only.

And the real Ohm's Law is J=Sigma*Epsilon, where J is current flux density, sigma is conductivity and epsilon is the electric field. J and Epsilon are vectors, sigma is a scalar.

Terminology! I'm EE as well; I couldn't resist posting to this thread. Now, as you can imagine, knowing all this stuff helps impress the ladies!