Final answer:
The comb gains negative charge due to excess electrons transferred through friction in the combing process. This principle is similar to the operation of a Van de Graaff generator, where electrons are transferred to a larger surface. The result is observable when the hair, acquiring a positive charge, stands up or the comb attracts small paper bits.
Step-by-step explanation:
When hair is combed with a hard rubber comb, the comb becomes negatively charged due to the transfer of excess electrons from the hair to the comb. This process is an example of triboelectric charging, where friction between two materials results in electrons being transferred from one material to another. In this case, the hair gives up electrons to the comb, and as a result, the comb holds extra electrons and becomes negatively charged whereas the hair becomes positively charged due to the loss of electrons.
Using a Van de Graaff generator as an analogy, we can understand this phenomenon. The generator utilizes a rubber belt that transports electrons from a lower to an upper comb through conduction, where they are eventually transferred to a conductive globe, replicating the process of the electrons moving from the hair to the comb. The result of this transfer is that objects with like charges will repel each other, as seen with the strands of hair standing up when they become positively charged and repel each other.
In Figure 5.3, we notice that a charged comb can attract small paper strips or bend a stream of water, which is due to the electric force that arises from the excess electrons on the comb creating an attractive or repulsive interaction with other charged or neutral objects.