The electric field strength E at a point is the force per unit charge on a small positive test charge placed there: E = F/Q. The unit is N C-1 or V m-1.E is a vector. For a point charge, E = Q/(4πε0r2), and it points away from a positive charge and towards a negative charge.When there are two point charges, the resultant field strength at a point is the vector sum of the field strengths due to each charge. If the two fields act along the same line, add them when they point the same way and subtract them when they point in opposite directions.Between two like charges there is a point where the two fields cancel and the resultant field strength is zero. If the fields are not along the same line, add them using Pythagoras or components.