# Understanding Torque in a Magnetic Field with Loop

• annamal
In summary, the conversation discusses the confusion about the direction of magnetic force represented by the black arrows and the angle between the normal to the loop and the magnetic field. It is clarified that the normal to the loop is vertical and the field is at a 45-degree angle below the horizontal. The conversation also mentions the use of electronic devices with flat screens instead of pages.
haruspex said:
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?

annamal said:
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##.

haruspex said:
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.

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