When are two equal and opposite forces about an axis not a couple

AI Thread Summary
In a vertical square coil placed in a horizontal magnetic field, the forces on the vertical sides are equal and opposite, but they do not create a couple because they act along the same line and do not produce torque about the vertical axis. The horizontal sides of the coil experience no forces, confirming that only the vertical sides are affected. When the coil is turned slightly, it will experience torque and begin to rotate, indicating that a couple can form when the coil is at an angle to the magnetic field. The discussion highlights the importance of understanding force direction and torque in determining whether a couple exists. Overall, the key takeaway is that equal and opposite forces do not necessarily result in a couple unless they create a rotational effect.
mrcotton
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Homework Statement



A current flows in vertical square coil whose plane is at right angles to a horizontal uniform magnetic field B. The coil can rotate about a vertical axis.
Which is correct
A) The forces on the two vertical sides are equal and opposite.
B) A couple acts on the coil
C) No forces act on the horizontal sides of the coil
D) If the coil is turned through a small angle about the vertical it will remain in position.

Homework Equations


The answer in the mark scheme is A

The Attempt at a Solution


What I can't seem to see is why B is not true also. Why are these two equal and opposite forces about an axis not causing a couple?

Any help greatefully received
 
Last edited:
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A couple implies there's something to couple with.

How would equal and opposite forces create a coupling?
 
hi mrcotton! :smile:
mrcotton said:
A current flows in vertical square coil whose plane is at right angles to a horizontal uniform magnetic field B. The coil can rotate about a vertical axis.
…
Why are these two equal and opposite forces about an axis not causing a couple?

in what direction are these opposite forces? :wink:
 
mrcotton said:
What I can't seem to see is why B is not true also. Why are these two equal and opposite forces about an axis not causing a couple?
Figure out the direction of the forces on the coil. What torque does each force exert about the given axis?
 
Thanks for the responses.
I think my sad old brain has got it.
nocoupleat90degreetothefield-1.jpg


In this picture the left hand is not orientated correctly. If it were it would show the forces given by the red arrows and they just push towards each other. So when at 90 degrees to the field there is no couple.
couple-1.jpg


In my sketch I have tried to indicate the when at an angle between 90 degrees and 0 to the field the two wires experience forces that cause a torque and the wire will turn.

Thanks
Mr C
 
you nailed it! :biggrin:
 
Sad thing is I remember that jarrred with my brain initially but I just couldn't hold my hand in that position long enough for the thought to sink in.
Thanks again
 
Last edited:
Using the "left hand rule" would get me into trouble during exams, as I scrawled with pencil in right hand :wink:.
 
alas in exams my digits only ever perfomed the function of counting.
 

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