# Magnetic fields question

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#1
Im struggling to visualise what the coils look like and how they cut the field lines since in the diagram it looks like a rectangle.
Last edited by Htx_x346; 3 months ago
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3 months ago
#2
(Original post by Htx_x346)
Im struggling to visualise what the coils look like and how they cut the field lines since in the diagram it looks like a rectangle.
It’s an insulating wire, so force has to be 0N.

GGG
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#3
It’s an insulating wire, so force has to be 0N.

GGG
Google: A current carrying wire, whether it is insulated or is not insulated, does produce a magnetic field.

Btw how would you imagine the coil anyway x
1
3 months ago
#4
(Original post by Htx_x346)
Google: A current carrying wire, whether it is insulated or is not insulated, does produce a magnetic field.

Btw how would you imagine the coil anyway x
A force doesn’t act on it, it isn’t conducting.

It’s a single loop of wire, no turns on the coil. If we imagine it be conducting, then we use the dominos F = BIL sintheta, where theta is the angle between the wire and the field lines. That angle is 30 degrees.

GGG
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#5
A force doesn’t act on it, it isn’t conducting.

It’s a single loop of wire, no turns on the coil. If we imagine it be conducting, then we use the dominos F = BIL sintheta, where theta is the angle between the wire and the field lines. That angle is 30 degrees.

GGG
Wait It says there’s a current flowing through it
Last edited by Htx_x346; 3 months ago
0
3 months ago
#6
A force doesn’t act on it, it isn’t conducting.

It’s a single loop of wire, no turns on the coil. If we imagine it be conducting, then we use the dominos F = BIL sintheta, where theta is the angle between the wire and the field lines. That angle is 30 degrees.

GGG
The mark scheme agrees, though I don't believe that the Q is answerable (or based in reality.) See below, though I might be wrong.

(Original post by Htx_x346)
Wait It says there’s a current flowing through it
Tbf I think the question is a write-off. It's already a "trick question" in that the net force is zero, but on sub-sections, the net force isn't zero. It is physically impossible to provide an answer to this (unless you know that the loop circumference is 1 cm, and thus you can say for certain that the force experienced by a 1 cm section is indeed zero, given that that's the entire loop! ) I have to add that there is a current on the loop- we're not just dealing with some loop without current dumped into a static magnetic field.

Hell, even if you knew the circumference of the wire, you would either need to be given all sorts of angles/geometry and be savvy with your vectors/integrals to get a numerical value for some localized section of wire, or just go about calculating an "average magnitude" at best. For example, the torque on a magnetic dipole is given by Now of the loop (I forgot the power), at least for a given current, which would allow you to estimate some magnitude of "torque per unit length" or some nonsensical quantity like that, and then rudimentarily guess an average value for "force magnitude" or something or other. But that's a hell of a long way around it.

The thing is that while the net force on the loop is zero, the force on each localised section of the loop isn't zero. There is going to be a torque, implying localized net force-per-unit-length. The loop is conducting and the loop does react to the magnetic field. Unless that loop is coated in some sort of magical impermeable superconductor, which nothing in this question indicates (nor do I believe most A-Level syllabi to my knowledge actually cover, at least w.r.t the permeability) then the loop will try to orientate to minimize its magnetic potential energy, indicating that there should be some forces acting (see: sections of loop, even if net cancels to zero.)

In any case, I don't believe this question is answerable, at least not in a physically consistent way. Just because you coat a loop of wire in an insulator, it doesn't mean it won't react to external magnetic fields. Even if you coat a loop of wire in a material of very low permeability, ot doesn't mean that it won't react to external magnetic fields- it will just react less. Someone else will have to confirm what I've said, but if I were doing this at A-Level, even without any other experience, I would have gotten hit off by the "1.0 cm section" bit to begin with.

On another note, if they reworded it, like "What is the net force experienced by the loop," or perhaps "What is the average net force on a given section," then the answer would be zero (since the net force on the loop is, by definition, zero. Just locally though, the magnitude isn't necessarily zero.
Last edited by Callicious; 3 months ago
1
3 months ago
#7
All you have here is basically 2 straight wires side by side.

In one the current goes one way, and in the other the current goes the other way.
(The 2 are double backed on each other.)

The force on the one is exactly equal and opposite to the force on the other, because they are in the same field and have the same current but in opposite directions.
The resultant force is zero for this simple reason. The angle is of no significance.
It's a bit of a trick question. The diagram is a bit misleading. Personally I would not have allowed it on an exam paper in that form.
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