Understanding Torque on a Current Loop in a Uniform Magnetic Field

[vg19]

Hey. I've read in my text that the netforce acting on a current loop in a uniform magnetic field is zero. But, in the next section, it mentions how torque is exterted on a loop placed in a uniform magnetic field. How can this be if the netforce must be zero? (I think I am making a really simple error here)

 

[Physics Monkey]

Hi vg19,

The net force in a uniform field will be zero, but the torque doesn't have to be. Imagine, for example, a square current loop in the yz plane with the magnetic field along the z axis. The current in the loop flows counter-clockwise when viewed from the positive x axis. In other words, the current in the bottom part of the wire is in the positive y direction, and the current in the top part of the wire is in the negative y direction. Hopefully you can visual my scenario despite the poor description. The force on the sides of the wire are zero because the current there is parallel to the field. The force on the top of the wire is in the negative x direction while the force on the bottom of the wire is in the positive x direction. These forces have equal magnitude and point in opposite directions as they must. Now what about the torques? So this is my question to you, do the torques exerted by these forces add up or cancel out?

 

[vg19]

Ahhh the torques WILL add up but the netforce will be zero since one will be pointing up and one down.

I totally get it!

Thanks a lot!

 

[Physics Monkey]

Glad I could help.