1. The problem statement, all variables and given/known data If the current density in a wire or radius R is given by J = kr, 0 < r < R , what is the current in the wire? 2. Relevant equations I used j = I/A, the definition of current density: current per unit cross sectional area. the formula for the area of a circle and for a cylinder 3. The attempt at a solution First I tried to do the cross sectional area (a circle) times the current density, but I got none of the answer that were displayed, and by just using a circle, I'm only finding the current for a small section of the wire. So I found the area of a cylinder. A = 2*pi*R*L (L is the length of the cylinder, not given in the problem) j = kr so A*j = 2*pi*R*L*k*r if you set L and r = to R (no idea why you'd do this), then you get 2*pi*R^3*k, but the correct answer is (2*pi*k*R^3)/3? What am I missing here, because this should be a pretty straight forward question... i think.