G.Y
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#1
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What happens after half a turn to make the rectangular coil stop spinning? Do the upward and downward forces just balance each other?
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Amanzhol
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Imagine how the current flows around the coil. Considering that the magnets remain in their fixed position, when the coil has turned half a turn the current is now flowing in opposite directions in relation to the magnets, even though the direction of the current doesn't change in relation to the coil itself. This causes the forces to change and makes the coil want to turn the other way, but when it does and has gone back to its original orientation after half a turn, the current is once again flowing in opposite directions. So it becomes an endless cycle of the coil making opposing half-turns and therefore it doesn't spin properly.
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Razz13
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A split ring commutator has a gap down the middle to allow the coil to overcome the half turn where, without the split ring, the magnets would oppose the flow of current and the coil would be forced to spin the other way, meaning the coil doesn't spin successfully and just half spins back and forth. The ring's gap means the magnets cannot exert an opposing force on the coil once is has spun halfway, so the coil is able to spin all the way around and then the magnets simply reinforce the magnets motion and successfully spin it one way continuously.
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G.Y
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(Original post by Razz13)
A split ring commutator has a gap down the middle to allow the coil to overcome the half turn where, without the split ring, the magnets would oppose the flow of current and the coil would be forced to spin the other way, meaning the coil doesn't spin successfully and just half spins back and forth. The ring's gap means the magnets cannot exert an opposing force on the coil once is has spun halfway, so the coil is able to spin all the way around and then the magnets simply reinforce the magnets motion and successfully spin it one way continuously.
I thought that the coil became stationary in the vertical position when no resultant force acts rather than spinning back and forth?
So the force of the couple when it is horizontal is actually enough to get it to spin a full 180 degrees? And then from this position it spins back the other way?
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Razz13
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(Original post by G.Y)
I thought that the coil became stationary in the vertical position when no resultant force acts rather than spinning back and forth?
So the force of the couple when it is horizontal is actually enough to get it to spin a full 180 degrees? And then from this position it spins back the other way?
With the split ring, no forces act on the coil when it is vertical but it's momentum allows it to spin the full way around. Without the ring, the magnets would oppose its momentum every time the coil reached the vertical position and so the ring spins back and forth and doesn't spin successfully.
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G.Y
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(Original post by Razz13)
With the split ring, no forces act on the coil when it is vertical but it's momentum allows it to spin the full way around. Without the ring, the magnets would oppose its momentum every time the coil reached the vertical position and so the ring spins back and forth and doesn't spin successfully.
https://www.alevelphysicsonline.com/aqa-magnetic-fields

Can you watch the 'electric motors' video from about 2.20 mins in

Because he's saying WITHOUT a split ring no forces act in the vertical position
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Razz13
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(Original post by G.Y)
https://www.alevelphysicsonline.com/aqa-magnetic-fields

Can you watch the 'electric motors' video from about 2.20 mins in

Because he's saying WITHOUT a split ring no forces act in the vertical position
I don't have access to that. What is the question exactly?
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Eimmanuel
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(Original post by G.Y)
https://www.alevelphysicsonline.com/aqa-magnetic-fields

Can you watch the 'electric motors' video from about 2.20 mins in

Because he's saying WITHOUT a split ring no forces act in the vertical position
You may want to take a snapshot of the picture of the setup of the motor that describes your question.

Your (original) question "What happens after half a turn to make the rectangular coil stop spinning? Do the upward and downward forces just balance each other? " is quite ambiguous without the setup that you are referring to.
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