The electrostatic force between two charges q1 and q2 is given by
![F=k_e \frac{q_1 q_2}{r^2}](https://tex.z-dn.net/?f=F%3Dk_e%20%20%5Cfrac%7Bq_1%20q_2%7D%7Br%5E2%7D%20)
where
![k_e = 8.99 \cdot 10^9 N m^2 C^{-2}](https://tex.z-dn.net/?f=k_e%20%3D%208.99%20%5Ccdot%2010%5E9%20N%20m%5E2%20C%5E%7B-2%7D)
is the Coulomb's constant
![r=3.00 \cdot 10^{-6} m](https://tex.z-dn.net/?f=r%3D3.00%20%5Ccdot%2010%5E%7B-6%7D%20m)
is the distance between the two charges.
In our problem, the two charges are two electrons, so their charges are equal and equal to
![q_1=q_2=q=-1.6 \cdot 10^{-19}C](https://tex.z-dn.net/?f=q_1%3Dq_2%3Dq%3D-1.6%20%5Ccdot%2010%5E%7B-19%7DC)
By substituting these values, we find the intensity of the force between the two electrons:
![F=(8.99 \cdot 10^9 N m^2 C^{-2}) \frac{(-1.6 \cdot 10^{-19}C)(-1.6 \cdot 10^{-19}C)}{(3.00 \cdot 10^{-6} m)^2}=2.6 \cdot 10^{-17}N](https://tex.z-dn.net/?f=F%3D%288.99%20%5Ccdot%2010%5E9%20N%20m%5E2%20C%5E%7B-2%7D%29%20%20%5Cfrac%7B%28-1.6%20%5Ccdot%2010%5E%7B-19%7DC%29%28-1.6%20%5Ccdot%2010%5E%7B-19%7DC%29%7D%7B%283.00%20%5Ccdot%2010%5E%7B-6%7D%20m%29%5E2%7D%3D2.6%20%5Ccdot%2010%5E%7B-17%7DN%20)
This is the magnitude of the force each electron exerts to the other one. The direction is given by the sign of the charges: since the two electrons have same charge, they repel each other, so the force exerted by electron 1 is toward electron 2 and viceversa.