Two fixed charges of magnitude +Q and +3Q repel each other with a force F - AQA - A-Level Physics - Question 14 - 2020 - Paper 2

When a charge of -2Q is added to each of the original charges, we need to analyze the forces between all three charges: +Q, +3Q, and -2Q. The charges +Q and +3Q will still repel each other with a force F. However, both +Q and +3Q will attract the charge -2Q.

To find the resultant force due to these interactions:

1. The force between +Q and -2Q is given by Coulomb's law: $F_{Q,-2Q} = k \frac{|Q \cdot (-2Q)|}{r^2} = k \frac{2Q^2}{r^2}$

2. The force between +3Q and -2Q is: $F_{3Q,-2Q} = k \frac{|3Q \cdot (-2Q)|}{r^2} = k \frac{6Q^2}{r^2}$

Both forces act in the same direction towards the -2Q charge and will result in a net attractive force towards -2Q.