In fair weather, the electric field in the air at a particular location immediately above the
Earth’s surface is 120 N/C directed downward. (a) What is the surface charge density on
the ground? Is it positive or negative? (b) Imagine the surface charge density is uniform
over the planet. What then is the charge of the whole surface of the Earth? (c) What is the
Earth’s electric potential due to this charge? (d) What is the difference in potential between
the head and the feet of a person 1.75 m tall? (Ignore any charges in the atmosphere.)
(e) Imagine the Moon, with 27.3% of the radius of the Earth, had a charge 27.3% as large,
with the same sign. Find the electric force the Earth would then exert on the Moon.
(f) State how the answer to part (e) compares with the gravitational force the Earth exerts
on the Moon.