## Physics for Scientists and Engineers: A Strategic Approach with Modern Physics (4th Edition)

(a) Electrons have been removed from the sphere and transferred to the rod. (b) $2.5\times 10^{10}$ electrons were transferred.
(a) Since it is normally quite easy for electrons to move, we can assume that it is the electrons which are transferred when there is a transfer of charge. The rod's net charge changes by $-4.0~nC$. In this case, we can assume that electrons have been removed from the sphere and transferred to the rod. (b) The change in the charge of the rod is $-4.0~nC$. Each electron has a charge of $-1.6\times 10^{-19}~C$. We can find the number of electrons that were transferred. $\frac{-4.0\times 10^{-9}~C}{-1.6\times 10^{-19}~C} = 2.5\times 10^{10}~electrons$ $2.5\times 10^{10}$ electrons were transferred.