Two small charged metal spheres A and B are situated in a vacuum. The distance between the centres of the spheres is 
, as shown in the figure below.
The charge on each sphere may be assumed to be a point charge at the centre of the sphere. Point P is a movable point that lies on the line joining the centres of the spheres and is distance 
from the centre of sphere A. The variation with distance of the electric field strength E at point P is shown in the graph below.
State the evidence provided by the graph above for the statements that
the spheres are conductors,
the charges on the spheres are either both positive or both negative.
State the relation between electric field strength 
and potential gradient at a point.
Use the graph to state and explain the distance at which the rate of change of potential with distance is
maximum
minimum
.
or 
to 
or 
.
