I heard this question from my TA and was not satisfied with the answer. Can someone elaborate for me please? There are two infinite lines of current that are traveling in the same direction. By the right hand rule and ampere's law, we can calculate the magnetic field and calculate the F of one line on the other. The forces are pointed toward each other, so they will get closer and closer. A coaxial line can be modeled as many infinite lines of current bundled together. How come the wires do not close themselves in and therefore the current should be concentrated within the center of the coaxial wire. If we were to send an AC voltage down the line, there should be only surface current, yet our previous analysis indicates that the lines should be pulled closer to each other therefore concentrating the current in the center of the coaxial wire. Any explanation will be much appreciated! Thank you!