Two point charges $q_1$ and $q_2$ are $l$ distance apart. If one of the charges is doubled and the distance between them is halved, the magnitude of the force becomes $n$ times, where $n$ is:

Two small conducting spheres of equal radius have charges $+10\,\mu C$ and $-20\,\mu C$ respectively and are placed at a distance $R$ from each other,experiencing a force $F_1$. If they are brought into contact and then separated to the same distance $R$,they experience a force $F_2$. The ratio of $F_1$ to $F_2$ is:

In one model of the electron,the electron of mass $m_e$ is thought to be a uniformly charged shell of radius $R$ and total charge $e$,whose electrostatic energy $E$ is equivalent to its mass $m_e$ via Einstein's mass-energy relation $E = m_e c^2$. In this model,$R$ is approximately ($m_e = 9.1 \times 10^{-31} \, kg$,$c = 3 \times 10^8 \, ms^{-1}$,$1 / 4 \pi \varepsilon_0 = 9 \times 10^9 \, Nm^2C^{-2}$,magnitude of the electron charge $e = 1.6 \times 10^{-19} \, C$).

$A$ point charge $Q$ is placed at the center of the line joining two equal point charges $+q$ and $+q$. The value of $Q$ if the system of the charges is in equilibrium,is

Two positive charges of $20 \, C$ and $Q \, C$ are situated at a distance of $60 \, cm$. The neutral point between them is at a distance of $20 \, cm$ from the $20 \, C$ charge. The value of charge $Q$ is: (in $, C$)

If $\varepsilon_0$ and $\varepsilon$ represent the permittivity of free space and absolute permittivity of a medium,then the relative permittivity of the medium is: