Magnetic Fields & Torque

[NA19]

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Hyperlearning MCAT Science Workbook 2011 Edition Physics Passage 35
Question #5

Can someone help me with this question? I'm having trouble visualizing where the normal would be before and after the adjustment?

 

[sps27]

Please type in the question not the whole passage, but some blurbs etc., to make sense of the question. Some folks like myself don't have TPRH SW, but we would still like to learn from your question, nonetheless.

 

[NA19]

Okay, so the passage mentions this loop of wire carrying a current in a uniform magnetic field B (which is pointing to the right). Then it mentions that torque on the loop is equal to IABsintheta, where theta is the angle between the magnetic field and a vector normal to the plane of the loop.

Question 5 asks:

If the loop were rotated so that the magnetic field were perpendicular to the plane of the wire, what would then be the torque on the loop?

The answer is 0 because:

"If the field is perpendicular to the plane of the wire, then it is parallel to the vector normal to the plane of the wire. When visualizing normal vectors, it is helpful to imagine a stubby little arrow pointing out of the plane. When the loop rotates, the little arrow rotates with it. If the field and normal are parallel, the angle theta is either 0 or 180. Either way sintheta = 0, so the torque is zero."

I don't really understand how the arrow is parallel to the normal and I'm just having trouble visualizing this whole thing.

 

[ashtonjam]

In that picture, the loop is set up so that its area vector is pointing directly out of the screen at your face. The angle between the area vector and the magnetic field lines is what determines torque, hence the IA x B cross product.

So if you rotate the loop in that picture to the left or right so that its area vector is pointing in the plane of the page to the left/right, the angle will be 0 or 180 degrees. Since sin0 and sin180 are both 0, the torque is 0.

If the plane of the loop is your flat desk, and if you balanced a pen so it pointed straight upwards, that pen would be the area vector.

 

[NA19]

Ohmygosh! Thanks!