Final answer:
The charge on hair when a wool hat is taken off is positive, due to the transfer of electrons from the hair to the wool, causing the strands of hair to have an excess of positive charges.
Step-by-step explanation:
The charge on hair when a wool hat is taken off is usually positive. When materials like hair and wool are rubbed together, electrons can be transferred from one material to another. Wool tends to grab electrons from the hair, leaving the hair with a net positive charge because it has lost some negative electrons. This is a result of the triboelectric effect, where different materials have varying tendencies to gain or lose electrons.
For example, in Figure 18.6 from the study materials, when a person touches a Van de Graaff generator and receives excess positive charge, it causes their hair to stand up. The separation of charges (hair losing electrons to wool or receiving positive charges from the generator) creates an excess of positive charges on each strand of hair, which then repel each other due to like-charge repulsion.