Electric Charges and Fields

Four point charges $$q_A=2\mu C, q_B=-5\mu C, q_C=2\mu C$$ and $$q_D=-5\mu C$$ are located at the corners.
A, B, C, D of a square ABCD of side 10 cm. What is the force on a charge of $$1\mu C$$ placed at the centre of the square?
Some spherical equipotential surfaces are shown in figure. The values of the potentials are 100 V, 80 V, 40 V on surfaces of radii 10 cm,12.5 cm and 25 cm. The electric field at a distance r from the common centre is

A particle of mass 1kg and carrying positive charge 0.01 C is sliding down an inclined plane of angle $$30^{0}$$with the horizontal. An electric field E is applied to stop the particle. If the coefficient of friction between the particle an the surface of the plane is$$\dfrac{1}{2\sqrt{3}}$$, E must be:

The adjacent diagram shows a charge $$+Q$$ held on an insulating support S and enclosed by a hollow spherical conductor. O represents the center of the spherical conductor and P is a point such that $$OP = x$$ and $$SP = r$$. The electric field at point P will be
Charge q is divided into two part and is placed at some fixed distance apart. What should be the value of the two parts such that force between them is maximum?