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- Thread starter cupid.callin
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- #2

zorro

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Note that the question mentions 'thin wire' (radius tends to 0)

- #3

cupid.callin

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because according to ampere circuital law ... when you draw a closed loop around the center of wire, just at the center ... i

- #4

zorro

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- #5

cupid.callin

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it doesnt mater .. .if radius tends to 0 then the radius of loop also tends to 0

- #6

zorro

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To add, the loop does not remain a loop when its radius tends to 0. It becomes a point.

- #7

cupid.callin

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but this explanation is not satisfactory

- #8

zorro

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The cross-section of the wire is not defined (which is not possible in reality offcourse, such assumptions for 'thin wires' are valid in problems). So you cannot draw an Amperian loop inside because there is no such 'inside'.

- #9

Phrak

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but this is not the case for a small diameter wire. Implicitly, small diameter

wire means small with respect to the distance between wires.

The object of this lesson is to find the fields surrounding the wires. You should

be able to convince yourself that for the two wires at x = +d, and x= -d that

the field contributions from both wires is zero at x=0. But the problem you are

given is to find all the points in the x-y plane where B=0.

- #10

cupid.callin

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But the problem you are given is to find all the points in the x-y plane where B=0.

No, i am asked to find the locus of point where B=0, in any plane

and also Y axis in in XY plane ... its x=0,z=0 ... which is a line in XY plane

@Abdul

i guess Phrak is right about the definition of thin in the question ... if we talk about wire with radius=0 ... how will there be any current through it? ... if there is place for current to pass then of course there will be a place to draw a gaussian surface

- #11

cupid.callin

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help someone ...

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Phrak

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No, i am asked to find the locus of point where B=0, in any plane

You're correct. I meant the X-Z plane. The field values are independent of Z.

There are three points in the X-Z plane. Can you name them?

- #13

zorro

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i guess Phrak is right about the definition of thin in the question ...

He might be correct. But I don't interpret the question in that way. The magnetic field is undefined at the centre of the middle wire.

As I said before, if you consider the wire to be thick enough (small diameter as Phrak says), then the net magnetic field should be 0 all along the Y-axis. This is not included in the answer so I think radius ----> 0 is a possible interpretation.

if we talk about wire with radius=0 ... how will there be any current through it? ... if there is place for current to pass then of course there will be a place to draw a gaussian surface

I never said radius=0, I said radius

- #14

cupid.callin

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You're correct. I meant the X-Z plane. The field values are independent of Z.

There are three points in the X-Z plane. Can you name them?

i cant get you

- #15

cupid.callin

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I never said radius=0, I said radiustendsto 0

Even if radius --> 0 ... there will be some place through which current will travel ... it cant travel inside a simple line .... reember i=q/t ... q is charge crossing any surface perpendicular to wire in time t

if radius ---> 0

area --> 0

so how will there be current?

- #16

zorro

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Think about this - The question says infinite wires. Do you have an infinite source of charge? (imaginary!)

- #17

Phrak

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i cant get you

Move your perspective the X-Z plane. The wires are represented as

dots at x=-d, x=0, and x=d.

and inversely proportional to the distance from the wire. Where in the

plane do the fields cancel? That is, find the locus of points B(x,z)=0.

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