Two point charges $+10 q$ and $-4 q$ are located at $x=0$ and $x=L$ respectively. What is the location of a point on the $x$-axis from the origin, where the net electric field due to these two point charges is zero? $(r = \text{required distance})$

Explain electric field and its source as well as magnetic field and its source.

Three charges $2q, -q, -q$ are located at the vertices of an equilateral triangle. At the centre of the triangle:

What is the electric field at the center of a semi-circular ring of radius $R$ having a linear charge density $\lambda$? $\left( k = \frac{1}{4\pi \varepsilon_0} \right)$

Two large circular discs separated by a distance of $0.01 \ m$ are connected to a battery via a switch as shown in the figure. Charged oil drops of density $900 \ kg \ m^{-3}$ are released through a tiny hole at the center of the top disc. Once some oil drops achieve terminal velocity,the switch is closed to apply a voltage of $200 \ V$ across the discs. As a result,an oil drop of radius $8 \times 10^{-7} \ m$ stops moving vertically and floats between the discs. The number of electrons present in this oil drop is (neglect the buoyancy force,take acceleration due to gravity $g = 10 \ m \ s^{-2}$ and charge on an electron $e = 1.6 \times 10^{-19} \ C$):

Two particles $A$ and $B$ having charges $20\, \mu C$ and $-5\, \mu C$ respectively are held fixed with a separation of $5\, cm$. At what position should a third charged particle be placed so that it does not experience a net electric force?