Another problem of the same OP:
Energy stored within an electric field: .
I'm helping someone in PHF but as Latex is down there, I'm posting here only for the latex. I might use this thread if the OP needs more explanations on the same problem.
So Biot-Savart's law is . Thus . Now go back to PHF.
I'll be using this post for the same problem and other posts for others...