The angle between the electric lines of force and the equipotential surface is:

Electric field lines are always perpendicular to the equipotential surface. This is because no work is done in moving a charge on an equipotential surface ($\Delta V=0$). Since $W=\vec{E}\cdot d\vec{r}=0$, the vector $\vec{E}$ must be perpendicular to the displacement $d\vec{r}$ along the surface. Hence, the angle is $90^{\circ }$.