Consider a rod of copper 15cm long joined to another rod (of the same diameter) of an alloy of 60%Cu and 40%Ni that is 5cm long. Assume that the interdiffusivity of the copper and nickel can be described by a constant diffusion coefficient, D. Write an expression for the concentration of nickel in the copper rod as a function of position and time that is valid for short times when both rods can be considered semiinfinite. Be as quantitative as possible. Since neither rod is infinitely long, your solution to problem 4) above is not valid for an infinitely long time. How long is it valid for? Indicate what criteria for "valid" you have chosen to use