A circular loop of wire, of radius r, carries current I. It is placed in a magnetic field whose straight lines seem to diverge from a point a distance d below the loop on its axis. (That is, the field makes an angle A with the loop at all points in the figure, where tan A = r/d.)

Determine the force on the loop. Suppose that z-axis is vertical and points upward, x-axis is parallel to the r and points to the right.

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So, I got an answer.  And my feedback said that it doens't depend on the z direction.  I can't understand this.  The x directions should cancel out, and every time I try the cross product, I find that there should be at least a net force in the vertical direction (mine is pointing down).  I'm not sure what I did wrong...

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Just put down that the force was interrupted by quantum particles. Which as we all know can do anything anytime for abosultely no reason at all.

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It's an online system.  There's only one acceptable answer, and I'm pretty sure that on is not it.

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i'm not entirely sure because i haven't done physics in a while and it's hard for me without a diagram, but i think it depends on the direction of the current. i don't really understand what form your answer is supposed to be in, and i don't know what you mean by "depend on the z direction"

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Think of a cone.  The slanted sides of the cone is the direction of the magnetic field.  The circle base of the cone is the current.  The z directon is basically up and down, going through the center of the circle and the tip of the cone.  The x direction is on a parallel plane with the base.

Just looking at the direction of the current with the field, it doesn't make sense to NOT have a force in the up and down direction, which is what the computer system is telling me...

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ok good, i'm with you so far. now when you determine the force on the loop, is that supposed to be a numerical value?

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No, I have to determine in terms of variables, and I'm suppose to split it into unit vectors.  This would mean writing the magnitude of the force vertically+ the magnitude of the force horizontally.

If you look at the picture though, the horizontal force should cancel out, because of the loop, and the force is always pointing outside.  So that leaves just the vertical.  But when I gave an answer, my feedback was that my final answer shouldn't even depend on the vertical direction.

But that just doesn't make sense to me.  I mean, I've had glitches before in the homework system... but I only get four more shots and I get deducted a few points each time I get it wrong, lol.

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ok i see. i don't know how to do vector cross products but conceptually it seems to me that there would be a net force in the z direction. however, the direction of the current and direction of the magnetic field determine whether it is up or down.

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Yeah, but from the system, if it was a matter of sign, then it would have told me to check my signs instead of saying it's not dependent on z at all.

But like you said, it makes more sense to have movement in that direction.  Lol... I mean, the cross product, in short, will give you a vector that's perpendicular to both things in question...  I dunno -__-'  Lol, I'm going to ask the teaching assistant later today, so hopefully that will clear it up :(.

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ok i see. i don't know how to do vector cross products but conceptually it seems to me that there would be a net force in the z direction. however, the direction of the current and direction of the magnetic field determine whether it is up or down.

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Turns out... that even though they called it the z-axis, and we typically use z in class, on the online work they use k instead.  So my answer was perfect other than the fact that I wrote z instead of k -__-'.