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 remain at the point of contact between the rod and the sphere, because resistance in the metal prevents the charges from easily moving.

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 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 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 sphere is a conductor, so the extra electrons move freely and repel one another. In electrostatic equilibrium they migrate to the outer surface. For a symmetric (spherical) conductor the charge spreads uniformly over the outer surface to minimize repulsion and make the electric field inside the conductor zero. (The insulating stand prevents the charge from leaking to ground.) Options A, B and C are incorrect for the reasons discussed: metals have low resistance so charge moves easily, electrons do not become heavier and sink, and they do not “spread into” neutral air unless breakdown/ionization occurs.