Question

Potential energy of system of four point charges to equal to capital can two equal to small

Answer 1

Gravity is doing work on the object by pulling it towards the Earth, but since you are pushing it in the other direction, the work you do on the box (and therefore the force) is negative. You do negative work when you increase a particle's potential energy

Answer 2

The net force on charge placed at D is given by the superposition of forces of the charges placed at other corners

Force on D due to A

F= Kq^2/r^2

Similarly force on D due to C is

F= kq^2/r^2

Force on D due to B is

F= kq^2/2r^2

Because the distance of diagonal is ✓2r

Now apply superposition of charges and get answer

F(due to A and C)

F= kq^2✓2/r^2

We will add this force to force of B

F+ F(B)=

kq^2/r^2 (✓2+1/2)N is the answer