Question

Two charges of `-1 C` and `2 C` are positioned on a line at points `-2` and `2`, respectively. What is the net force on a charge of `-1 C` at `1`?

Answer 1

`F_3=1.9*10^10N`

Explanation:

Consider the figure. Let the charges `-1C, 2C` and `-1C` be denoted by `q_1,q_2` and `q_3` respectively.
Let the positions at which charges are placed be in the units of meters.

Let `r_13`be the distance between the charges `q_1 and q_3`.
From figure
`r_13=1-(-2)=1+2=3m`

Let `r_23`be the distance between the charges `q_2 and q_3`.
From figure
`r_23=2-1=1m`

Let `F_13` be the force due to charge `q_1` on the charge `q_3`
`F_13=(kq_1q_3)/r_13^2=(9*10^9*(1)(1))/3^2=1*10^9N`
This force is repulsive and is towards charge `q_2`.

Let `F_23` be the force due to charge `q_2` on the charge `q_3`
`F_23=(kq_2q_3)/r_23^2=(9*10^9*(2)(1))/1^2=18*10^9N`
This force is attractive and is towards charge `q_2`.

The total force or net force on charge `q_3` is the sum of above two forces.
Since the above two forces `F_13` and `F_23` are in same direction therefore they can be added directly.
Let `F_3` be the total force on the charge `q_3`.
`implies F_3=F_13+F_23=1*10^9+18*10^9=19*10^9=1.9*10^10N`
`implies F_3=1.9*10^10N`
Since `F_13 and F_23` are toward the charge `q_2` therefore the force `F_3` is also toward the charge `q_2`.