Current Loop in Magnetic Field

[justadream]

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TBR says that if you pushing a current loop into a magnetic field (created by small magnets), you will feel a repulsive force (going against the Force applied to push the loop into the field).

How about when you are bringing the current loop out of the magnetic field? Would you feel a repulsive force?

 

[Cawolf]

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[mehc012]

You would feel a repulsive force as you moved in, no force when you aren't moving, and an attractive force as you pulled the loop away (a force attempting to keep the loop in the field).

This is per Lenz's law that states that induced currents (and their associated B fields) form to resist the change in flux.

I am confused...how is the force in the same direction as the velocity?

 

[Cawolf]

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[mehc012]

@mehc012 I might have written it oddly, but I meant it as the opposite of velocity. You are pulling the loop towards you and the B field is attractive, so the loop resists your pull (opposite of velocity).

My attractive/repulsive terms are written from the point of view of the existing B field.

I am fine with it generating an opposing/attracting magnetic field, but not so much with there being a force exerted which is in any way parallel to the velocity. It was my understanding that the magnetic field, velocity, and force were always perpendicular to each other.

 

[Cawolf]

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[mehc012]

It is my understanding that we are discussing a B field which is pointing vertically while moving a loop into it that is parallel to the ground (perpendicular to field).

Or as you mention, there would be no force.

At this point, I am confused as to which force we are even discussing.

I am proposing that the force exerted ON the moving charge BY the magnetic field must be perpendicular to both the velocity of the charge and the direction of the magnetic field. This is the force which causes the current. There should be no component of force ON the loop which is parallel to the velocity.

Now, if we are talking magnetic FIELD generated, that is a different story. Then, when entering the field, the current will flow in the loop in such a direction as to generate a magnetic field in opposition to the one it is moving into, and when moving out of the field, the current will flow in such a direction as to generate a magnetic field in the same direction as the one it is leaving.

None of this is parallel to the velocity of the loop, however.

 

[Cawolf]

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[mehc012]

Oh I see - I think I am now confused as to what I was answering.

Is a "current loop" a loop with a current running through it or a loop capable of conducting current?

If it is carrying current, then as @mehc012 said, there would be a torque exerted to twist the loop.

Apologies for my confusion.

No, that's not what I am saying. I am saying that if you move a conducting loop (one with no preexisting current or voltage in it), the 'force' generated by the magnetic field is what causes the induced current. It does not actually cause a net force on the loop itself (and if so, certainly not in the direction of the velocity). The magnetic field cannot cause a force in the direction of the velocity.

 

[justadream]

@Cawolf
@mehc012

I believe the "current loop" is a loop that initially has no current but gets a current once it enters the magnetic field (or as Cawolf put it "loop capable of conducting current")

I second mehc012's question (if I understand her question correctly). Where is this force that you (Cawolf) are speaking of? Cawolf, are you saying that initially when the loop enters the field, it feels a repulsive force (that would push the loop to the RIGHT in this picture). I believe that's what TBR is saying as well but I'm not sure how exactly this "repulsive force" is generated since such a force would be PARALLEL to the VELOCITY.

Here is the scenario:

 

[mehc012]

@Cawolf
@mehc012

I believe the "current loop" is a loop that initially has no current but gets a current once it enters the magnetic field (or as Cawolf put it "loop capable of conducting current")

I second mehc012's question (if I understand her question correctly). Where is this force that you (Cawolf) are speaking of? Cawolf, are you that initially when the loop enters the field, it feels a repulsive force (that would push the loop to the RIGHT in this picture). I believe that's what TBR is saying as well but I'm not sure how exactly this "repulsive force" is generated since such a force would be PARALLEL to the VELOCITY.

Here is the scenario:

The only thing I can think of in terms of a force parallel to the velocity of the loop is very difficult to explain without pictures...

I suppose that, when you initially move the loop in, the magnetic field acts on the charges inside the loop to generate a current. Thus, the charges begin to move within the conducting material of the loop itself.
Now that there is current, you have a new set of 'moving charges within a magnetic field' to consider: the charges moving in a circle around the loop.

I'll picture a square 'loop' because it's easier. Entering the field you have 2 parallel sides which are also parallel to the original velocity. Any force generated on the current in those sides will be equal in magnitude to each other, yet opposite in direction. Basically, they cancel out.

The other two sides are perpendicular to the velocity of the overall loop...and one is within the magnetic field while the other is still external. The magnetic field will enact a force upon the current moving within the leading edge of that loop...and this force will be perpendicular to the current, and thereby parallel to the velocity of the overall loop.

I know that's a dense word salad, but try drawing a pic, you'll see.

That is the only explanation I can come up with off the top of my head for a force parallel to the velocity of the overall loop - and it would be in opposite directions when the loop enters and leaves the field.

 

[justadream]

@mehc012

Your explanation is actually pretty decent. I hope you agree with the direction of my currents (counterclockwise when going in, clockwise when leaving)

Here's a diagram that I hope reflects what you are thinking

 

[mehc012]

@mehc012

Your explanation is actually pretty decent. I hope you agree with the direction of my currents (counterclockwise when going in, clockwise when leaving)

Here's a diagram that I hope reflects what you are thinking

Assuming that your velocity is the same as before, yes, that seems right to me...however, that is only step one of my word salad!
Also, I find the X vs 0 drawing for a magnetic field in the z plane to be a LOT easier to draw/visualize, which might make the next part easier!

 

[justadream]

@mehc012

What is the other part of your "word salad"? (lol such an interesting phrase) The sides of the square loop that are parallel are irrelevant (as you mentioned).

Maybe you are referring to the fact that I didn't draw the direction of the magnetic force? Yeah you need to use right-hand-rule with my diagram for that.

@Cawolf please enlighten us.

 

[mehc012]

@mehc012

What is the other part of your "word salad"? (lol such an interesting phrase) The sides of the square loop that are parallel are irrelevant (as you mentioned).

Maybe you are referring to the fact that I didn't draw the direction of the magnetic force? Yeah you need to use right-hand-rule with my diagram for that.

@Cawolf please enlighten us.

Oh, yeah I guess that's all that is missing.
Anyway, sorry it took so long to get to this point, I just didn't figure they were going to be talking in so many layers of 'this happens, and then this happens in response, and then this happens in response'. Oh, MCAT...