AP Physics C — Net force and net torque on a conductive loop

Wires and loops can experience net forces as a result of changing Magnetic Flux. If those forces are unbalanced, they will have a linear acceleration, and if the Torques are unbalanced, they will have a rotational acceleration.

This is the principle behind electric motors. A coil Rotates in a Magnetic Field from a net torque, and the spinning is harnessed for various applications.

For example:

For sides and , the force is zero because the current and magnetic field are parallel.

For side and , we can use the Right Hand Rule to find the force. has a force going into the page and has a force going out of the page.

We now see that the forces cancel () but the torques do not. There is a net torque in this situation.

If we shift the angle so that the loop is at a angle with :

There will still be a net force of zero. However, the net torque will be smaller.

When the angle between and is :

The net force is still zero since the net force on opposite sides are zero. The net torque is also zero here; using the right hand rule, we can see that there is an outward force on each wire.