Question

If you pass a comb through your hair and it acquires a charge of -0.1 µC, how many excess electrons does
it carry? What conclusions can you reach by applying the law of conservation of charge to this situation?

Answer 1

1. The charge of one electron is

$$e = -1.6\times 10^{-19}C$$

If the comb acquires a charge of $Q = -0.1\space\mu C = -0.1\cdot 10^{-6}C$, then it carries the following amount of excess electrons:

$$N = \dfrac{Q}{e} = \dfrac{ -0.1\cdot 10^{-6}C}{ -1.6\times 10^{-19}C} = 6.25\times 10^{11}$$

2. By applying the law of conservation of charge we can conclude, that hair also acquiresa a charge wich is opposite to the charge of the comb, namely:

$$Q_{hair} = -Q = 0.1\space\mu C$$

Answer. $N = 6.25\times 10^{11}$.