Understanding Torque in a Magnetic Field with Loop

[annamal]

Ok, so you found the net force is zero. But we don't care about the net force, we care about the net torque. So find the torque on an element and then integrate.

What do you mean by that
$\vec r \times \vec F = (rcos\theta, rsin\theta, 0) \times (0, 0, 0)$ = 0?

 

[haruspex]

What do you mean by that
$\vec r \times \vec F = (rcos\theta, rsin\theta, 0) \times (0, 0, 0)$ = 0?

No, $\vec r=\vec r(\theta)$, so you have to find the torque on an element of the loop and then integrate.
$\tau=\int \vec r\times\vec F.d\theta$.

 

[annamal]

No, $\vec r=\vec r(\theta)$, so you have to find the torque on an element of the loop and then integrate.
$\tau=\int \vec r\times\vec F.d\theta$.

Ok, I get it.