An interstellar dust grain, roughly spherical with a radius of 3e-07 m, has acquired some negative charges such that the potential on its surface relative to infinity is -16 V.
(a) How many extra electrons has it picked up? (An electron has charge of -1.602 x 10-19 C.)
(b) Assuming that the charge is spread uniformly, what is the surface charge density?
(c) What is the radial component of the electric field at the surface of the dust grain?