A charged point mass is situated in a vacuum. A proton travels directly towards the mass, as illustrated in the figure below.
When the separation of the mass and the proton is 
, the electric potential energy of the system is 
. The variation with of the potential energy is shown in the graph below.
Use the graph above to state and explain whether the mass is charged positively or negatively.
The gradient at a point on the graph above is 
. Show that the electric field strength 
at this point due to the charged point mass is given by the expression
where 
is the charge at this point.
Use the expression in (a)(ii) and the graph above to determine the electric field strength at a distance of 
from the charged point mass.
.
