Forces Between Multiple Charges

Electrostatic field line of force do not form closed loops. If they form closed loops then the work done in moving a charge along a closed path will not be zero. From the above two statements, Can you guess the nature of electrostatic force? (conservative/ non-conservative)

Repulsion is the sure test of charging than attraction. Why?

Two point charges $q_A=3 \mathrm{\mu }\mathrm{C}$ and  $q_{B }=-3 \mathrm{\mu C}$ are located $20 \mathrm{cm}$ apart in vacuum. What is the electric field at the midpoint $\mathrm{O}$ of the line $\mathrm{AB}$ joining the two charges? If a negative test charge of magnitude $1.5\times {10}^{-9} \mathrm{C}$ is placed at this point, what is the force experienced by the test charge? Take, $\frac{1}{4\pi {\in }_0}=9\times {10}^9 {\mathrm{Nm}}^2{\mathrm{C}}^{-2}$

Four-point charges ${\mathrm{q}}_{\mathrm{A}}=2 \mathrm{\mu }\mathrm{C}, {\mathrm{q}}_{\mathrm{B}}=–5 \mathrm{\mu }\mathrm{C}, {\mathrm{q}}_{\mathrm{C}}=2 \mathrm{\mu }\mathrm{C},\mathrm{and} {\mathrm{q}}_{\mathrm{D}}=–5 \mathrm{\mu }\mathrm{C}$ are located at the corners of a square $\mathrm{ABCD}$ of side $10 \mathrm{cm}$. What is the force on a charge of $1 \mathrm{\mu }\mathrm{C}$ placed at the center of the square?

Consider an arbitrary electrostatic field configuration. A small test charge is placed at a null point (i.e., where $\mathrm{E}=0$) of the configuration. Show that the equilibrium of the test charge is necessarily unstable. Verify this result for the simple configuration of two charges of the same magnitude and sign placed a certain distance apart.