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

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 slowly drift to the bottom of the sphere, because the charges rapidly attach to individual atoms, making them slightly heavier than surrounding atoms.

C.
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.

D.
It will distribute itself evenly over the surface of the sphere, because like charges repel and the sphere is a conductor.

Answer 1

D.

In a conductor electrons are free to move, so the excess negative charge will flow until electrostatic equilibrium is reached. At equilibrium the electric field inside the metal is zero, so all excess charge resides on the outer surface of the hollow sphere. Because the sphere is symmetric the like charges repel and settle into a uniform distribution over the surface. The plastic stand insulates the sphere so the charge does not leak away quickly.

(The other options are wrong: air is an insulator so the charge won't "spread into the air" unless breakdown occurs; charges in a metal are mobile, not stuck to atoms making them heavier; and the metal's low resistance lets the charge move, so it does not remain only at the contact point.)