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 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 the excess charges are free to move and repel one another, so they move until they reach electrostatic equilibrium. For an isolated, hollow metal sphere that equilibrium is a uniform distribution of the excess negative charge over the outer surface (the electric field inside the conductor is zero). The other options are incorrect: the charge doesn't spread into the air (air is an insulator unless ionized), it doesn't stay at the contact point because metals have low resistance and charges move quickly, and charges don't "sink" to the bottom by becoming heavier.