When we touch a Van de Graaff generator, our hair may stand up if it is dry. This happens only if the strands of our hair do what?

Repel each other.

Explanation:

Van de Graaff generator is a device that was invented in 1929, by an American physicist called Robert J. Van de Graaff.

A Van de Graaff generator refers to an electrostatic generator that typically involves the use of a moving belt to accumulate electric charge on a hollow globe that is made from metal placed on top of an insulated column and thereby, creating very high electric potentials i.e very high voltage-direct current (DC) electricity at low current levels.

Generally, when we touch a Van de Graaff generator, our hair may stand up if it is dry. This happens only if the strands of our hair repel each other because they are like charges i.e having the same potentials.