Why the magnetic field doesn't have to describe a circle?

[kuruman]

You misinterpreted what you were asked to do. Here is a re-statement of the problem as seen in post #29

An infinite wire carries current $I$. Calculate the integral $\oint \vec B\cdot d\vec l$ around a square Amperian loop of side $a=R\sqrt{2}$ oriented so that the wire goes through the center of the square and is perpendicular to its plane.

Maybe this will make it clearer what you need to do.

 

[JD_PM]

Oh, I see. So the drawing'd be something like this:

By symmetry, the magnitude of $B$ is constant around the Amperian square loop. Then, taking just one of the square's edges I calculate $B$ having the field point in the square's center:

$$\oint \vec B \cdot d\vec l = B \oint dl = B R\sqrt{2} = \mu_0~I$$

Note that both $B$ and $dl$ are parallel; we can drop the dot product.

Solving for B:

$$B = \frac{\mu_0~I}{R\sqrt{2}}$$

This is consistent with Ampere's law; for one edge:

$$\oint \vec B \cdot d\vec l= \frac{\mu_0~I}{R\sqrt{2}}R\sqrt{2}=\mu_0~I_{enc.}$$

For the entire square:

$$\oint \vec B \cdot d\vec l= 4\frac{\mu_0~I}{R\sqrt{2}}R\sqrt{2}=4\mu_0~I_{enc.}$$

Am I right? If not, please let me know and I'll keep trying.

 

[Orodruin]

By symmetry, the magnitude of $B$ is constant around the Amperian square loop.

No it isn't.

 

[kuruman]

Am I right? If not, please let me know and I'll keep trying.

You are not right. Look at the drawing below that shows the situation. The magnetic field has constant magnitude on the dashed circle but not on the square because the distance to the center of the square varies as you move along the side of the square. You need to find an expression for vector $\vec B$ at an arbitrary point on the side shown, find $d\vec l$ (for this side it is $dx~\hat x$), find the dot product $\vec B \cdot d\vec l$ (note that $\vec B$ and $d \vec l$ are not parallel) and add all such dot products continuously (i.e. integrate) from one corner of the square to the other. Then and only then you multiply the result by 4. Note: The wire carrying current into the screen and the circle of radius $R$ enclosing the square are not shown for clarity.

 

[JD_PM]

You are not right. Look at the drawing below that shows the situation. The magnetic field has constant magnitude on the dashed circle but not on the square because the distance to the center of the square varies as you move along the side of the square. You need to find an expression for vector $\vec B$ at an arbitrary point on the side shown, find $d\vec l$ (for this side it is $dx~\hat x$), find the dot product $\vec B \cdot d\vec l$ (note that $\vec B$ and $d \vec l$ are not parallel) and add all such dot products continuously (i.e. integrate) from one corner of the square to the other. Then and only then you multiply the result by 4. Note: The wire carrying current into the screen and the circle of radius $R$ enclosing the square are not shown for clarity.

View attachment 240643

Thank you, enlightening explanation and drawing (may you tell what app did you use to make it up? :) )

You need to find an expression for vector $\vec B$ at an arbitrary point on the side shown

My bad, the magnetic field isn't constant; I'm unsure whether the following is correct:

I'd say we are interested in the component of the magnetic field lying on the x direction (it must be perpendicular to the current) So our magnetic field depends on $\alpha$:

$$\oint \vec B \cdot d\vec l= \oint \frac{\mu_0~I}{2 \pi r} cos \alpha dx$$

Once here I feel like the idea is good; it is just about figuring out how $r$ depends on $x$ so that I can solve the integral.

Am I going right?

 

[robphy]

This might help:
https://www.glowscript.org/#/user/m...older/matterandinteractions/program/13-fields (matterandinteractions/program/13-fields)
Select Magnetic.
Measurement type: Ampere's Law.

Draw a square-like loop... (release the mouse to complete the loop).
Drag a current-carrying wire into the center of the loop.
The circulation $\oint \vec B\cdot d\vec l$ (essentially, the sum of tangential-components of $\vec B$ times the perimeter) is shown.

Note how $\vec B\cdot \Delta\vec l$ varies along the Amperian loop.
The circulation is the total around the loop.
As you reposition the wire [the loop can't be repositioned in this software],
$\vec B\cdot \Delta\vec l$ varies,
but the circulation is unchanged as long as the current is enclosed by the loop.

You should play with different configurations to get a feel for the calculations.

 

[kuruman]

Thank you, enlightening explanation and drawing (may you tell what app did you use to make it up? :) )

I used PowerPoint and saved the figure as a .PNG file.

Once here I feel like the idea is good; it is just about figuring out how rr depends on xx so that I can solve the integral.

Am I going right?

You're doing fine. Now you have to make sure that there is only one variable under the integral sign. For the time being you have three, $\alpha$, $x$ and $r$. Do the trig.

 

[JD_PM]

Do the trig.

I've been thinking in using the law of cosines to express $r$:

$$r^2 = \gamma^2 + (z + \frac{R\sqrt{2}}{2})^2 - 2\gamma(z + \frac{R\sqrt{2}}{2})cos\beta$$

But this over-complicates things...

I've been trying to play with $\alpha$, $x$ and $r$ so that I get an expression in function of just one of these, but got nothing of interest...

May you give me a hint?

 

[kuruman]

Forget the upper triangle. It's not needed and I am not sure what $z$ is all about and why it is there. You need to calculate the $\vec B \cdot d\vec l$ along the horizontal side of the bottom right triangle. Label the angles of the bottom triangle, they have to be related to $\alpha$ and $\beta$. Also, don't worry about the law of cosines; you have a right triangle so the Pythagorean theorem works just as well to relate the square of one side to the squares of the other two.

 

[JD_PM]

Forget the upper triangle. It's not needed and I am not sure what $z$ is all about and why it is there. You need to calculate the $\vec B \cdot d\vec l$ along the horizontal side of the bottom right triangle. Label the angles of the bottom triangle, they have to be related to $\alpha$ and $\beta$. Also, don't worry about the law of cosines; you have a right triangle so the Pythagorean theorem works just as well to relate the square of one side to the squares of the other two.

OK Let's see now.

I was dealing with the line integral:

$$\oint \vec B \cdot d\vec l= \oint \frac{\mu_0~I}{2 \pi r} cos (\alpha) dx $$

I had to do the trig:

We just need to focus on an infinitesimal path of the line integral (just on one of the four sides), so doing the trig just with one of these triangles suffices:

$$\tan (\alpha) = \frac{x}{s}$$

$$x = s \tan (\alpha)$$

$$dx = s \sec^2 (\alpha) d\alpha$$

We also know that:

$$r = s \sec (\alpha)$$

We end up with:

$$\frac{\mu_0~I}{2 \pi}\int_{-\pi/4}^{\pi/4}d\alpha = \frac{\mu_0~I}{4}$$

But this is the line integral of just one side of the square amperian loop. If we multiply per 4 we indeed get:

$$\oint \vec B \cdot d\vec l = \mu_0~I$$

How do you see it now?

Sorry for the late reply.

 

[kuruman]

Looks good. Don't worry about the late reply. I'm glad you didn't give up and saw it through the end. I hope you understand line integrals a tad better now.

 

[JD_PM]

Thank you for you patience hahaha

I am glad of being part of PF community.