**Question:**

A polythene piece rubbed with wool is found to have a negative charge of $3 \times 10^{-7} \mathrm{C}$.

(a) Estimate the number of electrons transferred (from which to which?)

(b) Is there a transfer of mass from wool to polythene?

**Solution:**

(a) When polythene is rubbed against wool, a number of electrons get transferred from wool to polythene. Hence, wool becomes positively charged and polythene becomes negatively charged.

Amount of charge on the polythene piece, $q=-3 \times 10^{-7} \mathrm{C}$

Amount of charge on an electron, $e=-1.6 \times 10^{-19} \mathrm{C}$

Number of electrons transferred from wool to polythene $=n$

$n$ can be calculated using the relation,

$q=n e$

$n=\frac{q}{e}$

$=\frac{-3 \times 10^{-7}}{-1.6 \times 10^{-19}}$

$=1.87 \times 10^{12}$

Therefore, the number of electrons transferred from wool to polythene is $1.87 \times 10^{12}$.

(b) Yes.

There is a transfer of mass taking place. This is because an electron has mass,

$m_{e}=9.1 \times 10^{-3} \mathrm{~kg}$

Total mass transferred to polythene from wool,

$m=m_{e} \times n$

$=9.1 \times 10^{-31} \times 1.85 \times 10^{12}$

$=1.706 \times 10^{-18} \mathrm{~kg}$

Hence, a negligible amount of mass is transferred from wool to polythene.