1. The problem statement, all variables and given/known data Calculate the electric energy density at the surface of a 3.4 mm diameter copper wire carrying a 28 A current. 2. Relevant equations u = (1/2) ε0 E2 3. The attempt at a solution With the electric energy density equation, I substituted in the equation for the electric field for an infinite line charge: u = (1/2) ε0 ( λ/ (ε0 2 π r ))2 I have all the information except for the charge density, λ. I know that it's a copper wire, but how can I find what λ is with that information?