Question

A rubber rod rubbed with fur acquires a charge of -4,8×10(-9)C. What is the charge on the fur? How much mass is transferred to rod?

Answer 1

1) The charge left on the fur is equal and opposite to the charge transferred to the rod:
$Q=+4.8 \cdot 10^{9} C$
In fact, when the rod is rubbed with the fur, a net charge of $Q=-4.8 \cdot 10^{-9} C$ has been transferred to the rod, leaving it negatively charged. If we assume the fur was initially neutral, this means that we have now an excess of positive charges on the fur, and the amount of this charge must be equal (in magnitude, but with opposite sign) to the charge transferred to the rod.

2) The mass transferred to the rod is equal to the total mass of the electrons transferred to the rod.
The charge transferred to the rod is
$Q=-4.8 \cdot 10^{-9} C$
The charge of 1 electron is
$e=-1.6 \cdot 10^{-19} C$
So the number of electrons transferred is
$N= \frac{Q}{e}= \frac{-4.8 \cdot 10^{-9} C}{-1.6 \cdot 10^{-19} C}=3.0 \cdot 10^{10}$

The mass of 1 electron is $m=9.1 \cdot 10^{-31} kg$, therefore the total mass transferred to the rod is
$M=Nm=(3 \cdot 10^{10})(9.1 \cdot 10^{-31} kg)=2.73 \cdot 10^{-20} kg$