Line of action of force in torque experienced by a current loop

[logearav]

Homework Statement

Please see my attachment

Homework Equations

In the diagram, F1 and F2 are equal and opposite also F3 and F4 also equal and opposite.
Its been mentioned that F1 and F2 have same line of action of force and the resultant effect on the loop is zero. But F2 and F4 have different line of action and hence constitute a couple.

The Attempt at a Solution

I have drawn lines of action for F1 and F2, F2 and F4 in dotted line in red colours. I can't find any difference between F1 and F2 , F2 and F4. Both are straight lines. So in what way the lines of action of F2 and F4 are different and how do they constitute a couple?
2) What is the rule that i has to apply to find the direction of force, in this case?
I mean why all the forces are directed outwards to the loop?
Thanks in advance revered members

 

[jbsteele]

. A:F1 and F2 are equal and opposite, so their line of action is the same. F3 and F4 also have equal and opposite force, but their line of action is different. If you draw a vector from the point where F3 is applied to the point where F4 is applied, then you have a couple force, which tends to rotate the loop. The direction of rotation is determined by the right-hand rule.