Define electric potential at a point.
Two small spherical charged particles P and Q may be assumed to be point charges located at their centres. The particles are in a vacuum. Particle P is fixed in position. Particle Q is moved along the line joining the two charges, as illustrated in the figure below.
The variation with separation 
of the electric potential energy 
of particle Q is shown in the graph below.
State how the magnitude of the electric field strength is related to potential gradient.
Use your answer in (i) to show that the force on particle Q is proportional to the gradient of the curve of the graph above.
The magnitude of the charge on each of the particles P and Q is 
. Calculate the separation of the particles at the point where particle Q has electric potential energy equal to 
.
By reference to the graph above, state and explain
whether the two charges have the same, or opposite, sign,
the effect, if any, on the shape of the graph of doubling the charge on particle P.
.
