A positively charged solid metal sphere is isolated in space. The electric field strength 
is measured for different distances 
from the centre of the sphere. The variation with of the field strength is shown in the graph below
Suggest why, for values of less than 
, the electric field strength is zero.
A point charge of 
moves from a point where 
to a point where 
. Use the graph above to estimate the change in electric potential energy of this point charge.
.
