In Example 29-8 we calculated the magnetic field inside and outside a wire of radius R carrying a uniform current I. Consider a filament of current at a distance r from the center of the wire. Show that this filament experiences a force directed toward the center of the wire and that, therefore, the current distribution cannot be truly uniform. This so-called pinch effect depends on the magnitude of the current, but is generally negligibly small. Consider a copper wire of 2.0 cm diameter that carries a nominally uniform current of 400 A. Calculate the force per meter exerted on a 0.1-mm-thick annular region of current at the periphery of the wire. Assume that the current is carried by the conduction electrons. How far toward the center of the wire would this annular current region have to move so that the electrostatic force between the electrons and the fixed positive ions at

the periphery just balances the magnetic force?