Rectangular loop in magnetic field

Thread Starter


Joined Apr 19, 2016

I don't understand why the loop rotates.
Here's my probelem:
F is the torque twisting the loop. According to right hand rule, the two Fs always point in the direction shown above. Thus when they have twisted the loop so that its plane is perpendicular to B, the two Fs still point that direction. I don't understand why the loop continues to rotate (in clockwise direction).


Joined Nov 29, 2011
Maybe the current in the loop is reversed when the loop face is parallel with the magnet faces - like a motor. Without the current reversal, it would act like a D'Arsonval meter movement.


Joined Feb 21, 2014
You did not show the commutator which will switch the current to the next loop(not shown ). Real motors have multiple loops and commutator segments.

Thread Starter


Joined Apr 19, 2016
All of textbooks I've read don't mention the reverse of current. I found this text helpful:
However, it still doesn't explain the point about rotation. It says the mechanical energy from rotation is used to power another device. It also explains the reverse of torque when θ is negative, but it doesn't say anything about the reverse of current. So it means the loop can't rotate more than half of rotation. Then why the set up can be used to power another devices?
I think the key here is only the current switching like you said. If it's true, why don't textbooks show it to readers to clear the misunderstanding?
Here's a video simulation:


Joined Sep 9, 2010
Just speculation - I suppose the authors believe it's important for the student to grasp the forces involved and how they can produce mechanical shaft work. Understanding the actual operation of a motor is not the goal. Introducing the switching might satisfy the curiosity of students like you that need to see the full picture, but would hopelessly confuse less astute students. It would dilute the main points. My 2¢.