Two positive point charges of unequal magnitude are placed at a certain distance apart. A small positive test charge is placed at null point, then

If two charges of $1$ coulomb each are placed $1 \,km$ apart, then the force between them will be ……….. $N$

Three charges each of magnitude $q$ are placed at the corners of an equilateral triangle, the electrostatic force on the charge placed at the center is (each side of triangle is $L$)

A charge $Q$ is placed at each of the opposite corners of a square. A charge $q$ is placed at each of the other two corners. If the net electrical force on $Q$ is zero, then  $\frac{Q}{q}=$ ______