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

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

C.
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.

D.
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.

Answer 1

B.

In a conductor excess charge moves freely and repels itself, so in electrostatic equilibrium the negative charge spreads out over the outer surface. For a symmetric hollow metal sphere that means it distributes uniformly over the surface. The other choices are incorrect: electrons do not “stick” and get heavier (A), they do not diffuse into neutral air under ordinary conditions (C), and they do not remain localized at the contact point because metals have mobile charge carriers (D).