Flux Through a Circle - 3 - Corrected Version

Consider a circle of unit radius centered at the origin of the x y xy -plane. There exists an infinitely long current-carrying wire along the straight line:

x 5 2 = y 3 4 = z 9 \frac{x-5}{2} = \frac{y-3}{4} = \frac{z}{9}

The magnitude of the current flowing through this wire is:

I = 2 π μ o I = \frac{2\pi}{\mu_o}

Here μ o \mu_o is the permeability of free space. Compute the magnitude of the magnetic flux through the circle.

Also Try:

Flux Through a Circle

Flux Through a Circle - 2

Note: The problem is a corrected version of an earlier question. Thanks to Hosam Hajjir and Steven Chase for their inputs.


The answer is 0.14941518577.

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1 solution

Karan Chatrath
Aug 16, 2019

Consider the line:

x 5 2 = y 3 4 = z 9 = p \frac{x-5}{2}=\frac{y-3}{4}=\frac{z}{9} = p

When p = 0 p=0 and p = 1 p=1 , two points passing through the line are obtained which are ( 5 , 3 , 0 ) (5,3,0) and ( 7 , 7 , 9 ) (7,7,9) . From here, a unit vector in the direction of the line can be obtained. That unit vector is:

L ^ = 2 i ^ + 4 j ^ + 9 k ^ 101 \hat{L} = \frac{2\hat{i} + 4\hat{j} +9\hat{k}}{\sqrt{101}}

An elementary length of the wire is:

d L = ( d p ) L ^ d\vec{L} = (dp) \hat{L}

The position vector of any point on the line can be parameterized as:

r l = ( 2 p 101 + 5 ) i ^ + ( 4 p 101 + 3 ) j ^ + ( 9 p 101 ) k ^ \vec{r}_l = \left(\frac{2p}{\sqrt{101}}+5\right)\hat{i}+\left(\frac{4p}{\sqrt{101}}+3\right)\hat{j}+\left(\frac{9p}{\sqrt{101}}\right)\hat{k}

An arbitrary point on the unit circle is defined as:

r c = x i ^ + y j ^ \vec{r}_c = x\hat{i}+y\hat{j}

r = r c r l \vec{r} = \vec{r}_c - \vec{r}_l

Using Biot Savart Law:

d B = μ o I 4 π ( d L × r ^ r 2 ) d\vec{B} = \frac{\mu_oI}{4\pi} \left(\frac{d\vec{L} \times \hat{r}}{\mid \vec{r} \mid^2}\right)

The elementary area vector of the unit circle is:

d A = ( d x ) ( d y ) k ^ d\vec{A} = (dx) (dy)\hat{k}

The magnetic flux is:

d ϕ = d B . d A d\phi = d\vec{B}.d\vec{A}

And after simplifying, we get:

ϕ = 1 1 1 x 2 1 x 2 101 ( y 101 2 x 101 + 7 101 ) ( ( 2 p x + 5 ) 2 + ( 4 p y + 3 ) 2 + 81 p 2 ) 3 / 2 ( d y ) ( d x ) ( d p ) \phi = \int_{-\infty}^{\infty} \int_{-1}^{1} \int_{-\sqrt{1-x^2}}^{\sqrt{1-x^2}} \frac{\sqrt{101}\,\left(\frac{y}{101}-\frac{2\,x}{101}+\frac{7}{101}\right)}{{\left({\left(2\,p-x+5\right)}^2+{\left(4\,p-y+3\right)}^2+81\,p^2\right)}^{3/2}}(dy) (dx) (dp)

Numerically integrating the above monster gives the result of 0.14941518577 T m 2 \boxed{0.14941518577}Tm^2

Thanks for posting. Is Part 4 coming soon?

Steven Chase - 1 year, 9 months ago

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Working on a variant where the wire and circle are both located arbitrarily in 3D space. Will post it as soon as I can

Karan Chatrath - 1 year, 9 months ago

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Cool, looking forward to it

Steven Chase - 1 year, 9 months ago

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