A Polythene piece rubbed with wool is found to have a negative charge of $1 \mathrm{\mu C}$. Estimate the number of electrons transferred. Is there any transfer of a mass from wool to polythene.

The distance between electron and proton in a hydrogen atom is $0.53 {\mathrm{A}}^0$. Find the speed and frequency of revolution of electron round the proton.

$\left({\mathrm{m}}_{\mathrm{e}}=9.1\times {10}^{-31}\mathrm{kg}\right)$

Spherical conductor having a charge of $100 \mathrm{\mu C}$ is brought near another metallic sphere $\left(\text{of mass} 36 \mathrm{kg}\right)$ resting on earth's surface. What minimum distance of separation should be maintained so as to lift up the metallic sphere with uniform speed. What happens if the separation is more or less than the calculated value.

$\left(\text{assume} g=10 \mathrm{m} {\mathrm{s}}^{-2}\right)$

Two identical charges are placed at a separation of $1.0m$. What should be the magnitude of each charge; so that each one repels the other with a force of $36kg wt.$

$\left(g=10m{S}^{-2}\right)$.