magnetics: ampere law

[autumn cipher]

I have trouble understanding the sign in this problem. I got mu0i/4 but how do we know whether it's positive/negative? Ans key ke acc the ans is correct but has a negative sign.

[storm hinge]

Thank you for your response @autumn cipher.

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Note for OP

+solved @user1 @user2... to close the thread when your doubt is solved. Mention the users who helped you solve the doubt. This will be added to their stats.

[autumn cipher]

i completed the amperian loop like this

and obv since the field is perpendicular to those red lines, b.dl for those lines will be 0

which means b.dl from a to b is mu0/2pi times the angle which is pi/2 rad

[autumn cipher]

can someone plss help

[blazing swan]

as u can see in image the M.F. lines are in a ciruclr pattern which goes from B pont to A point but question asks line integral from A to B so there will be a negative sign

[autumn cipher]

ohhh okay

thanks a lot @blazing swan

i get it

[mellow sparrow]

wait dont close

Why can't we do this

initially i got the same ans as u with sign but then this striked me

approximation ??

@blazing swan

[autumn cipher]

the problem is probably cuz u cant take dx as rdtheta?

[mellow sparrow]

dx= rdtheta seems a bit wrong

the ans does match tho

[autumn cipher]

yeah like the perpendicular u draw on it will be rdtheta.. i guess an additional cos theta component will be there

[blazing swan]

yes using dx=rdtheta is wrong here its a straight line not curved or anything

[mellow sparrow]

hmm

[autumn cipher]

+solved @blazing swan