Two charged metal spheres A and B are situated in a vacuum, as illustrated in the figure below.
The shortest distance between the surfaces of the spheres is 
. A movable point P lies along the line joining the centres of the two spheres, a distance 
from the surface of sphere A. The variation with distance of the electric field strength 
at point P is shown in the graph below.
Use data from the graph above to state the value of at which the rate of change of electric potential is maximum. Give the reason for the value you have chosen.
.
