The diagram shows the variation of the gravitational force with distance between two masses. What does the shaded area represent $?$

The diagram shows equipotential surfaces due to two spherical masses.

Using the diagram, explain how it can be deduced that masses are unequal.

The diagram shows equipotential surfaces due to two spherical masses.

Copy the diagram and draw in the gravitational field lines due to two masses.

The diagram shows equipotential surfaces due to two spherical masses.

Explain why the equipotential surfaces are spherical very far from the two masses.

Four equal charges $5.0 \mathrm{\mu c}$ are placed at the vertices of a square side of $10 \mathrm{cm}$.

Calculate the value of the electric potential at the centre of the square. (use $K=9\times {10}^9$)

Four equal charges $5.0 \mathrm{\mu c}$ are placed at the vertices of a square side of $10 \mathrm{cm}$.

Determine the electric field at the centre of the square.