Question -
Answer -
Length of the side of a cube = b
Charge at each of its vertices = q
A cube of side b is shownin the following figure.
d =Diagonal of one of the six faces of the cube
l =Length of the diagonal of the cube
The electric potential (V) at thecentre of the cube is due to the presence of eight charges at the vertices.
Therefore, the potential atthe centre of the cube is 
The electric field at the centre of thecube, due to the eight charges, gets cancelled. This is because the charges aredistributed symmetrically with respect to the centre of the cube. Hence, theelectric field is zero at the centre.