Draw the electric field lines produced by a negative point charge and calculate the electric field strength at a distance of 50mm from a point charge of size -30nC.

The diagram should show a negative point charge with the electric field lines pointing towards the point charge. The separation of the lines should decrease as you get closer to the point charge. The lines should be straight and perpendicular to the equipotential lines which circle the point charge.

In order to calculate the electric field strength, the equation for the force on a charge due to Coulomb's law and the equation for the electric field strength must be combined to give an equation for the electric field strength in terms of the size of the point charge;
F = Q₁ Q₂ / 4πε₀ r²
E = F / Q₁
E = Q₂ / 4πε₀ r²

Q₂ should be recognised as the size of the point charge and this should be changed should be changed into Coulombs (nC = 1 x 10^-9 C).
r should be recognised as the distance from the point charge and this should be changed into metres (mm = 1 x 10^-3 m).
These numbers and the constants should be substituted into the final equation to give an electric field strength of -1.08 x 10⁵ C (3 significant figures).