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:

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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 quickly spread to the air around the sphere, because it will seek to occupy as large a volume of neutral matter as possible.

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

Answer: D.

Reason: The metal sphere is a conductor. The extra electrons are free to move and, because like charges repel, they flow until electrostatic equilibrium is reached. For a hollow, isolated conducting sphere that symmetry forces the electric field inside the metal and cavity to be zero, so all excess charge resides on the outer surface. By symmetry the surface charge density is uniform on a sphere, so the negative charge spreads evenly over the sphere’s surface.

Why the others are wrong:
- A: Electrons in a metal are not stuck to individual atoms and do not make atoms heavier; they are mobile conduction electrons.
- B: The charge does not “spread into the air” under normal conditions (except by slow leakage or corona discharge); it stays on the conductor’s surface.
- C: Metals have low resistance and the electrons move quickly, so the charge does not remain at the contact point.