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

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 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

B.

In a conductor excess charge moves freely and, because like charges repel, the negative charges spread out over the outer surface until electrostatic equilibrium is reached. For a hollow metal sphere the charges reside on the outside surface and (by symmetry) distribute uniformly. The insulating stand prevents the charge from leaking away.