Question

A student rubs a rubber rod with fur, producing a negative charge on the rod. The student then touches the rod to a hollow, neutral metal sphere that stands on an insulating plastic stand. This transfers negative charges from the rod to the sphere:

image 63f9196b581c4f17bbe56cfd6ddd781f

Which statement best predicts what will happen to the charge, and why?

a
It will quickly spread to the air around the sphere, because it will seek to occupy as large a volume of neutral matter as possible.
b
It will remain at the point of contact between the rod and the sphere, because resistance in the metal prevents the charges from easily moving.
c
It will distribute itself evenly over the surface of the sphere, because like charges repel and the sphere is a conductor.
d
It will slowly drift to the bottom of the sphere, because the charges rapidly attach to individual atoms, making them slightly heavier than surrounding atoms.

Answer 1

c

In a conductor excess charge moves freely and redistributes until electrostatic equilibrium is reached. Like charges repel, so the electrons spread out as far from each other as possible — for a symmetric conductor that means a uniform distribution over the outer surface. The insulating stand prevents the charge from leaking to ground.

The other choices are wrong: (a) charge does not “spread into the air” unless the air ionizes or provides a conduction path, (b) metals have low resistance so charges move quickly rather than staying at the contact point, and (d) electrons do not make atoms noticeably heavier and charges do not slowly sink to the bottom.