Getting incorrect results applying Ampere's law












8














Let’s say we have a current wire with a current $I$ flowing. We know there is a field of $B=frac{mu_0I}{2pi r}$ by using Ampère's law, and a simple integration path which goes circularly around the wire. Now if we take the path of integration as so the surface spans doesn’t intercept the wire we trivially get a $B=0$ which is obviously incorrect.



I see that I have essentially treated it as if there is no current even present. But a similar argument is used in other situations without fault.



Take for example a conducting cylinder with a hollow, cylindrical shaped space inside. By the same argument there is no field inside.



To further illustrate my point, the derivation of the B field inside of a solenoid requires you to intercept the currents. You can’t simply do the loop inside of the air gap.



This, at least to me, seems like the same thing, and I can’t justify why one is incorrect and the other is incorrect. Please point out why I am stupid.










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    8














    Let’s say we have a current wire with a current $I$ flowing. We know there is a field of $B=frac{mu_0I}{2pi r}$ by using Ampère's law, and a simple integration path which goes circularly around the wire. Now if we take the path of integration as so the surface spans doesn’t intercept the wire we trivially get a $B=0$ which is obviously incorrect.



    I see that I have essentially treated it as if there is no current even present. But a similar argument is used in other situations without fault.



    Take for example a conducting cylinder with a hollow, cylindrical shaped space inside. By the same argument there is no field inside.



    To further illustrate my point, the derivation of the B field inside of a solenoid requires you to intercept the currents. You can’t simply do the loop inside of the air gap.



    This, at least to me, seems like the same thing, and I can’t justify why one is incorrect and the other is incorrect. Please point out why I am stupid.










    share|cite|improve this question



























      8












      8








      8


      2





      Let’s say we have a current wire with a current $I$ flowing. We know there is a field of $B=frac{mu_0I}{2pi r}$ by using Ampère's law, and a simple integration path which goes circularly around the wire. Now if we take the path of integration as so the surface spans doesn’t intercept the wire we trivially get a $B=0$ which is obviously incorrect.



      I see that I have essentially treated it as if there is no current even present. But a similar argument is used in other situations without fault.



      Take for example a conducting cylinder with a hollow, cylindrical shaped space inside. By the same argument there is no field inside.



      To further illustrate my point, the derivation of the B field inside of a solenoid requires you to intercept the currents. You can’t simply do the loop inside of the air gap.



      This, at least to me, seems like the same thing, and I can’t justify why one is incorrect and the other is incorrect. Please point out why I am stupid.










      share|cite|improve this question















      Let’s say we have a current wire with a current $I$ flowing. We know there is a field of $B=frac{mu_0I}{2pi r}$ by using Ampère's law, and a simple integration path which goes circularly around the wire. Now if we take the path of integration as so the surface spans doesn’t intercept the wire we trivially get a $B=0$ which is obviously incorrect.



      I see that I have essentially treated it as if there is no current even present. But a similar argument is used in other situations without fault.



      Take for example a conducting cylinder with a hollow, cylindrical shaped space inside. By the same argument there is no field inside.



      To further illustrate my point, the derivation of the B field inside of a solenoid requires you to intercept the currents. You can’t simply do the loop inside of the air gap.



      This, at least to me, seems like the same thing, and I can’t justify why one is incorrect and the other is incorrect. Please point out why I am stupid.







      electromagnetism






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      edited yesterday









      knzhou

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      Jake RoseJake Rose

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          You aren't stupid, you are just learning things the same way we all did: the school of hard knocks. Ampere's law says that the integral around that closed path is zero, not that the field is zero at every point. What the law tells us is that the field is sometimes "positive" and sometimes "negative" on that path, and when we add up contributions from everywhere on the path, we get zero.






          share|cite|improve this answer





















          • What about if you take a circularly symmetric path in the solenoid?
            – Jake Rose
            yesterday






          • 2




            Ahhh, the field is perpendicular, so B (in that direction) really does = 0..
            – Jake Rose
            yesterday










          • @JakeRose In that case $vec{B}$ is perpendicular to $dvec{l}$, so the integral is 0 (without the field being 0)
            – Poon Levi
            yesterday



















          8














          Your argument is incorrect. $oint vec Bcdot dvec ell$ is $0$ when no current is enclosed but this does not imply $B=0$: you cannot use $oint vec Bcdot dvec ell= Btimes 2pi r =mu_0 I_{encl}$ since $vec B$ is not constant on the loop defined by the contour: in other words, $ointvec Bcdot dvec ell$ is not $Btimes 2pi r$ unless the contour is one where $vec Bcdot dvec ell$ is constant.






          share|cite|improve this answer























          • What if you take a circularly symmetric loop in the solenoid?
            – Jake Rose
            yesterday










          • Ahhh, the field is perpendicular, so B (in that direction) really does = 0
            – Jake Rose
            yesterday






          • 2




            @JakeRose Yes... Ampere's law is always true, but it's now always useful in recovering $vec B$: only in some specialized symmetric situations can one do this (see physics.stackexchange.com/q/318183/36194).
            – ZeroTheHero
            yesterday



















          5














          You basically have to evaluate a line integral $oint_{rm C} vec B cdot dvec l$ over a closed loop which in general is difficult to do.



          Here are two examples to show that even if there is a magnetic field present the line integral is zero.



          enter image description here



          In the left hand case $$oint_{rm C} vec B cdot dvec l = int_{rm WX} vec B cdot dvec l+int_{rm XY} vec B cdot dvec l+int_{rm YZ} vec B cdot dvec l+int_{rm ZX} vec B cdot dvec l = B,2R+0+(-B,2R)+0 =0$$



          The right hand case is a little trickier.



          $$oint_{rm C} vec B cdot dvec l = int_{0}^{2pi} B,Rdtheta ,sin theta = 0$$



          and if one looks at a quadrant between $theta =0$ and $theta = frac pi 2$ the line integral is $displaystyle int_{0}^{frac{pi}{2}} B,Rdtheta ,sin theta = +BR$



          Then going round in $frac pi 2$ steps the integral around the closed loop is $+BR+BR+(-BR)+(-BR) =0$ as before.






          share|cite|improve this answer





















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            3 Answers
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            20














            You aren't stupid, you are just learning things the same way we all did: the school of hard knocks. Ampere's law says that the integral around that closed path is zero, not that the field is zero at every point. What the law tells us is that the field is sometimes "positive" and sometimes "negative" on that path, and when we add up contributions from everywhere on the path, we get zero.






            share|cite|improve this answer





















            • What about if you take a circularly symmetric path in the solenoid?
              – Jake Rose
              yesterday






            • 2




              Ahhh, the field is perpendicular, so B (in that direction) really does = 0..
              – Jake Rose
              yesterday










            • @JakeRose In that case $vec{B}$ is perpendicular to $dvec{l}$, so the integral is 0 (without the field being 0)
              – Poon Levi
              yesterday
















            20














            You aren't stupid, you are just learning things the same way we all did: the school of hard knocks. Ampere's law says that the integral around that closed path is zero, not that the field is zero at every point. What the law tells us is that the field is sometimes "positive" and sometimes "negative" on that path, and when we add up contributions from everywhere on the path, we get zero.






            share|cite|improve this answer





















            • What about if you take a circularly symmetric path in the solenoid?
              – Jake Rose
              yesterday






            • 2




              Ahhh, the field is perpendicular, so B (in that direction) really does = 0..
              – Jake Rose
              yesterday










            • @JakeRose In that case $vec{B}$ is perpendicular to $dvec{l}$, so the integral is 0 (without the field being 0)
              – Poon Levi
              yesterday














            20












            20








            20






            You aren't stupid, you are just learning things the same way we all did: the school of hard knocks. Ampere's law says that the integral around that closed path is zero, not that the field is zero at every point. What the law tells us is that the field is sometimes "positive" and sometimes "negative" on that path, and when we add up contributions from everywhere on the path, we get zero.






            share|cite|improve this answer












            You aren't stupid, you are just learning things the same way we all did: the school of hard knocks. Ampere's law says that the integral around that closed path is zero, not that the field is zero at every point. What the law tells us is that the field is sometimes "positive" and sometimes "negative" on that path, and when we add up contributions from everywhere on the path, we get zero.







            share|cite|improve this answer












            share|cite|improve this answer



            share|cite|improve this answer










            answered 2 days ago









            garypgaryp

            16.7k12963




            16.7k12963












            • What about if you take a circularly symmetric path in the solenoid?
              – Jake Rose
              yesterday






            • 2




              Ahhh, the field is perpendicular, so B (in that direction) really does = 0..
              – Jake Rose
              yesterday










            • @JakeRose In that case $vec{B}$ is perpendicular to $dvec{l}$, so the integral is 0 (without the field being 0)
              – Poon Levi
              yesterday


















            • What about if you take a circularly symmetric path in the solenoid?
              – Jake Rose
              yesterday






            • 2




              Ahhh, the field is perpendicular, so B (in that direction) really does = 0..
              – Jake Rose
              yesterday










            • @JakeRose In that case $vec{B}$ is perpendicular to $dvec{l}$, so the integral is 0 (without the field being 0)
              – Poon Levi
              yesterday
















            What about if you take a circularly symmetric path in the solenoid?
            – Jake Rose
            yesterday




            What about if you take a circularly symmetric path in the solenoid?
            – Jake Rose
            yesterday




            2




            2




            Ahhh, the field is perpendicular, so B (in that direction) really does = 0..
            – Jake Rose
            yesterday




            Ahhh, the field is perpendicular, so B (in that direction) really does = 0..
            – Jake Rose
            yesterday












            @JakeRose In that case $vec{B}$ is perpendicular to $dvec{l}$, so the integral is 0 (without the field being 0)
            – Poon Levi
            yesterday




            @JakeRose In that case $vec{B}$ is perpendicular to $dvec{l}$, so the integral is 0 (without the field being 0)
            – Poon Levi
            yesterday











            8














            Your argument is incorrect. $oint vec Bcdot dvec ell$ is $0$ when no current is enclosed but this does not imply $B=0$: you cannot use $oint vec Bcdot dvec ell= Btimes 2pi r =mu_0 I_{encl}$ since $vec B$ is not constant on the loop defined by the contour: in other words, $ointvec Bcdot dvec ell$ is not $Btimes 2pi r$ unless the contour is one where $vec Bcdot dvec ell$ is constant.






            share|cite|improve this answer























            • What if you take a circularly symmetric loop in the solenoid?
              – Jake Rose
              yesterday










            • Ahhh, the field is perpendicular, so B (in that direction) really does = 0
              – Jake Rose
              yesterday






            • 2




              @JakeRose Yes... Ampere's law is always true, but it's now always useful in recovering $vec B$: only in some specialized symmetric situations can one do this (see physics.stackexchange.com/q/318183/36194).
              – ZeroTheHero
              yesterday
















            8














            Your argument is incorrect. $oint vec Bcdot dvec ell$ is $0$ when no current is enclosed but this does not imply $B=0$: you cannot use $oint vec Bcdot dvec ell= Btimes 2pi r =mu_0 I_{encl}$ since $vec B$ is not constant on the loop defined by the contour: in other words, $ointvec Bcdot dvec ell$ is not $Btimes 2pi r$ unless the contour is one where $vec Bcdot dvec ell$ is constant.






            share|cite|improve this answer























            • What if you take a circularly symmetric loop in the solenoid?
              – Jake Rose
              yesterday










            • Ahhh, the field is perpendicular, so B (in that direction) really does = 0
              – Jake Rose
              yesterday






            • 2




              @JakeRose Yes... Ampere's law is always true, but it's now always useful in recovering $vec B$: only in some specialized symmetric situations can one do this (see physics.stackexchange.com/q/318183/36194).
              – ZeroTheHero
              yesterday














            8












            8








            8






            Your argument is incorrect. $oint vec Bcdot dvec ell$ is $0$ when no current is enclosed but this does not imply $B=0$: you cannot use $oint vec Bcdot dvec ell= Btimes 2pi r =mu_0 I_{encl}$ since $vec B$ is not constant on the loop defined by the contour: in other words, $ointvec Bcdot dvec ell$ is not $Btimes 2pi r$ unless the contour is one where $vec Bcdot dvec ell$ is constant.






            share|cite|improve this answer














            Your argument is incorrect. $oint vec Bcdot dvec ell$ is $0$ when no current is enclosed but this does not imply $B=0$: you cannot use $oint vec Bcdot dvec ell= Btimes 2pi r =mu_0 I_{encl}$ since $vec B$ is not constant on the loop defined by the contour: in other words, $ointvec Bcdot dvec ell$ is not $Btimes 2pi r$ unless the contour is one where $vec Bcdot dvec ell$ is constant.







            share|cite|improve this answer














            share|cite|improve this answer



            share|cite|improve this answer








            edited yesterday

























            answered 2 days ago









            ZeroTheHeroZeroTheHero

            19k52957




            19k52957












            • What if you take a circularly symmetric loop in the solenoid?
              – Jake Rose
              yesterday










            • Ahhh, the field is perpendicular, so B (in that direction) really does = 0
              – Jake Rose
              yesterday






            • 2




              @JakeRose Yes... Ampere's law is always true, but it's now always useful in recovering $vec B$: only in some specialized symmetric situations can one do this (see physics.stackexchange.com/q/318183/36194).
              – ZeroTheHero
              yesterday


















            • What if you take a circularly symmetric loop in the solenoid?
              – Jake Rose
              yesterday










            • Ahhh, the field is perpendicular, so B (in that direction) really does = 0
              – Jake Rose
              yesterday






            • 2




              @JakeRose Yes... Ampere's law is always true, but it's now always useful in recovering $vec B$: only in some specialized symmetric situations can one do this (see physics.stackexchange.com/q/318183/36194).
              – ZeroTheHero
              yesterday
















            What if you take a circularly symmetric loop in the solenoid?
            – Jake Rose
            yesterday




            What if you take a circularly symmetric loop in the solenoid?
            – Jake Rose
            yesterday












            Ahhh, the field is perpendicular, so B (in that direction) really does = 0
            – Jake Rose
            yesterday




            Ahhh, the field is perpendicular, so B (in that direction) really does = 0
            – Jake Rose
            yesterday




            2




            2




            @JakeRose Yes... Ampere's law is always true, but it's now always useful in recovering $vec B$: only in some specialized symmetric situations can one do this (see physics.stackexchange.com/q/318183/36194).
            – ZeroTheHero
            yesterday




            @JakeRose Yes... Ampere's law is always true, but it's now always useful in recovering $vec B$: only in some specialized symmetric situations can one do this (see physics.stackexchange.com/q/318183/36194).
            – ZeroTheHero
            yesterday











            5














            You basically have to evaluate a line integral $oint_{rm C} vec B cdot dvec l$ over a closed loop which in general is difficult to do.



            Here are two examples to show that even if there is a magnetic field present the line integral is zero.



            enter image description here



            In the left hand case $$oint_{rm C} vec B cdot dvec l = int_{rm WX} vec B cdot dvec l+int_{rm XY} vec B cdot dvec l+int_{rm YZ} vec B cdot dvec l+int_{rm ZX} vec B cdot dvec l = B,2R+0+(-B,2R)+0 =0$$



            The right hand case is a little trickier.



            $$oint_{rm C} vec B cdot dvec l = int_{0}^{2pi} B,Rdtheta ,sin theta = 0$$



            and if one looks at a quadrant between $theta =0$ and $theta = frac pi 2$ the line integral is $displaystyle int_{0}^{frac{pi}{2}} B,Rdtheta ,sin theta = +BR$



            Then going round in $frac pi 2$ steps the integral around the closed loop is $+BR+BR+(-BR)+(-BR) =0$ as before.






            share|cite|improve this answer


























              5














              You basically have to evaluate a line integral $oint_{rm C} vec B cdot dvec l$ over a closed loop which in general is difficult to do.



              Here are two examples to show that even if there is a magnetic field present the line integral is zero.



              enter image description here



              In the left hand case $$oint_{rm C} vec B cdot dvec l = int_{rm WX} vec B cdot dvec l+int_{rm XY} vec B cdot dvec l+int_{rm YZ} vec B cdot dvec l+int_{rm ZX} vec B cdot dvec l = B,2R+0+(-B,2R)+0 =0$$



              The right hand case is a little trickier.



              $$oint_{rm C} vec B cdot dvec l = int_{0}^{2pi} B,Rdtheta ,sin theta = 0$$



              and if one looks at a quadrant between $theta =0$ and $theta = frac pi 2$ the line integral is $displaystyle int_{0}^{frac{pi}{2}} B,Rdtheta ,sin theta = +BR$



              Then going round in $frac pi 2$ steps the integral around the closed loop is $+BR+BR+(-BR)+(-BR) =0$ as before.






              share|cite|improve this answer
























                5












                5








                5






                You basically have to evaluate a line integral $oint_{rm C} vec B cdot dvec l$ over a closed loop which in general is difficult to do.



                Here are two examples to show that even if there is a magnetic field present the line integral is zero.



                enter image description here



                In the left hand case $$oint_{rm C} vec B cdot dvec l = int_{rm WX} vec B cdot dvec l+int_{rm XY} vec B cdot dvec l+int_{rm YZ} vec B cdot dvec l+int_{rm ZX} vec B cdot dvec l = B,2R+0+(-B,2R)+0 =0$$



                The right hand case is a little trickier.



                $$oint_{rm C} vec B cdot dvec l = int_{0}^{2pi} B,Rdtheta ,sin theta = 0$$



                and if one looks at a quadrant between $theta =0$ and $theta = frac pi 2$ the line integral is $displaystyle int_{0}^{frac{pi}{2}} B,Rdtheta ,sin theta = +BR$



                Then going round in $frac pi 2$ steps the integral around the closed loop is $+BR+BR+(-BR)+(-BR) =0$ as before.






                share|cite|improve this answer












                You basically have to evaluate a line integral $oint_{rm C} vec B cdot dvec l$ over a closed loop which in general is difficult to do.



                Here are two examples to show that even if there is a magnetic field present the line integral is zero.



                enter image description here



                In the left hand case $$oint_{rm C} vec B cdot dvec l = int_{rm WX} vec B cdot dvec l+int_{rm XY} vec B cdot dvec l+int_{rm YZ} vec B cdot dvec l+int_{rm ZX} vec B cdot dvec l = B,2R+0+(-B,2R)+0 =0$$



                The right hand case is a little trickier.



                $$oint_{rm C} vec B cdot dvec l = int_{0}^{2pi} B,Rdtheta ,sin theta = 0$$



                and if one looks at a quadrant between $theta =0$ and $theta = frac pi 2$ the line integral is $displaystyle int_{0}^{frac{pi}{2}} B,Rdtheta ,sin theta = +BR$



                Then going round in $frac pi 2$ steps the integral around the closed loop is $+BR+BR+(-BR)+(-BR) =0$ as before.







                share|cite|improve this answer












                share|cite|improve this answer



                share|cite|improve this answer










                answered yesterday









                FarcherFarcher

                47.7k33796




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