The figure below shows a wire that is made of two equal-diameter segments with resistivities P₁ and P₂. When current I passes through the wire, a thin layer of charge appears at the boundary between the segments. Find an expression for the surface charge density o on the boundary. Give your result in terms of I, P₁, P2, and the wire's cross-sectional area A.

The figure below shows a wire that is made of two equal-diameter segments with resistivities P₁ and P₂. When current I passes through the wire, a thin layer of charge appears at the boundary between the segments. Find an expression for the surface charge density o on the boundary. Give your result in terms of I, P₁, P2, and the wire's cross-sectional area A.