Suppose u(x, t) measures the concentration of a substance in a moving stream (moving with velocity v). suppose the concentration u(x, t) changes both by diffusion and convection; derive the equatiorn from the fact that at any instant of time, the total mass of the material is not created or destroyed in the region [x, x + ax] hint write the conservation equation change of mass inside [x, x + ax] = change due to diffusion across the boundaries + change due to the material being carried across the boundaries