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Problem
Calculate the magnetic and electric energy densities at the surface of a 3.0-mm-diameter copper wire carrying a 15-A current.
Solution
To calculate the electric energy density, we need the electric field at the surface of the wire. The electric field will equal the voltage difference along the wire divided by the length of the wire. We can calculate the voltage drop using Ohm’s law and the resistance from the resistivity and diameter of the wire.