Coloumb's Law

[runic marten]

I'm assuming that your initial force calculations are correct

I'm also assuming that the value of d is given in the question? But you can do it roughly without calculating as well since the distance d is the same

Ok forget everything

You know that the force between these charges is going to vary because of 2 variables, the distance and the multiple of the charges

Forget the + and - in the charge for now

We will apply that as sign convention in determining the direction of the force

Now, force is directly proportional to the multiple of the charges meaning if the charges are bigger, force is more and inversely proportional to the distance meaning that more distance between charges, less the force

Based on that, your initial force calculations seem incorrect

F12 has to be greater than F13 even at just a glance

The correct answer should be Q1

Unless I'm completely wrong and I should really stop yapping 😭

[runic marten]

Because Q1 × Q2 > Q1 × Q3

Also because the denominator according to coloumb's law would be d^2 for F12 but (2d)^2 for F13

So you apply the same concept for F23 as well