An oil drop carries six electronic charges,has a mass of $1.6 \times 10^{-12} \text{ g}$ and falls with a terminal velocity in air. The magnitude of the vertical electric field required to make the drop move upward with the same speed as it was formerly moving is ........$kN/C$.

The Van de Graaff generator is not based on

The figure represents a crystal unit of cesium chloride, $CsCl$. The cesium atoms, represented by open circles, are situated at the corners of a cube of side $0.40 \, nm$, whereas a $Cl$ atom is situated at the centre of the cube. The $Cs$ atoms are deficient in one electron while the $Cl$ atom carries an excess electron.
$(i)$ What is the net electric field on the $Cl$ atom due to eight $Cs$ atoms?
$(ii)$ Suppose that the $Cs$ atom at the corner $A$ is missing. What is the net force now on the $Cl$ atom due to the seven remaining $Cs$ atoms?

Two identical conducting spheres $A$ and $B$ carry equal charge. They are separated by a distance much larger than their diameter,and the force between them is $F$. $A$ third identical conducting sphere,$C$,is uncharged. Sphere $C$ is first touched to $A$,then to $B$,and then removed. As a result,the force between $A$ and $B$ would be equal to

Two identical charged particles,each having a mass $10 \, g$ and charge $2.0 \times 10^{-7} \, C$,are placed on a horizontal table with a separation of $L$ between them such that they stay in limiting equilibrium. If the coefficient of friction between each particle and the table is $0.25$,find the value of $L$ in $cm$. [Use $g = 10 \, m/s^2$]

Two point charges $Q$ and $-Q/4$ are separated by a distance $x$. Then: