Aperson observes that in the winter, his hair stands on end when it is brushed. this is because his hair becomes positively charged. why does the act of brushing make his hair positively charged?

Because of the transfer of electrons from hairs to the brush.

Explanation:

In winters, the hairs on brushing usually stands on the ends because when the hairs are rubbed by the brush separation of electrons take place from the parent atoms' nuclei on the hairs and thus the hairs are left positively charged.

Normally the charges are stay together and rectify the effect of each other as one is positive and the other is negative thus making the atoms neutral.

The process due to which the hairs acquire positive charge is known as charging by friction.