Answer:
Electrons have a negative charge, so your hair has a small positive charge when a few electrons leave it. Positive charges repel each other, so your charged hairs repel each other and stand out from your head.
Explanation:
When turned on, the dome of the Van de Graaff generator becomes charged (either positive or negative depending on its design). Since like charges repel, the hairs are pushed away from each other and the similarly-charged dome, causing them to stand up.
