Assume that the charge of the proton is distributed uniformly throughout the volume of a sphere of radius 10-13 cm. Use perturbation theory to estimate the shift in the ground-state hydrogen-atom energy due to the finite proton size. The potential energy experienced by the electron when it has penetrated the nucleus and is at distance r from the nuclear center is -eQ/4((0r, where Q is the amount of proton charge within the sphere of radius r. The evaluation of the integral is simplified by noting that the exponential factor in ( is essentially equal to 1 within the nucleus.