Integral in cylindrical coordinates with only one differential and written basis?

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Let's say I have this integral in cylindrical coordinates:
$$int_0^pi 2zrhoê_rho dphi$$



How is this calculated? I need some help in understanding how to interpret this integral.



  1. Why is there a $ê_rho$ after $rho$, but not a $ê_z$ after $z$? Does the $ê_rho$ "hold" all of the $2zrho$ as the "coefficient"?

  2. Can it then be written as a vector then, ie. $int_0^pi (2zrho,0,0)dphi$?

  3. Why is there only a $dphi$? Since there's no $phi$ in the function (vector?), can the $2zp$ be treated as "constants" and put outside the integral? Would the answer to the integral then be $2pi zrho$ or $2pi zrho ê_rho$
    ?

Kind of blurry questions, but the answers to them might help me understand more about coordinate basis and integrals.







share|cite|improve this question
























    up vote
    1
    down vote

    favorite












    Let's say I have this integral in cylindrical coordinates:
    $$int_0^pi 2zrhoê_rho dphi$$



    How is this calculated? I need some help in understanding how to interpret this integral.



    1. Why is there a $ê_rho$ after $rho$, but not a $ê_z$ after $z$? Does the $ê_rho$ "hold" all of the $2zrho$ as the "coefficient"?

    2. Can it then be written as a vector then, ie. $int_0^pi (2zrho,0,0)dphi$?

    3. Why is there only a $dphi$? Since there's no $phi$ in the function (vector?), can the $2zp$ be treated as "constants" and put outside the integral? Would the answer to the integral then be $2pi zrho$ or $2pi zrho ê_rho$
      ?

    Kind of blurry questions, but the answers to them might help me understand more about coordinate basis and integrals.







    share|cite|improve this question






















      up vote
      1
      down vote

      favorite









      up vote
      1
      down vote

      favorite











      Let's say I have this integral in cylindrical coordinates:
      $$int_0^pi 2zrhoê_rho dphi$$



      How is this calculated? I need some help in understanding how to interpret this integral.



      1. Why is there a $ê_rho$ after $rho$, but not a $ê_z$ after $z$? Does the $ê_rho$ "hold" all of the $2zrho$ as the "coefficient"?

      2. Can it then be written as a vector then, ie. $int_0^pi (2zrho,0,0)dphi$?

      3. Why is there only a $dphi$? Since there's no $phi$ in the function (vector?), can the $2zp$ be treated as "constants" and put outside the integral? Would the answer to the integral then be $2pi zrho$ or $2pi zrho ê_rho$
        ?

      Kind of blurry questions, but the answers to them might help me understand more about coordinate basis and integrals.







      share|cite|improve this question












      Let's say I have this integral in cylindrical coordinates:
      $$int_0^pi 2zrhoê_rho dphi$$



      How is this calculated? I need some help in understanding how to interpret this integral.



      1. Why is there a $ê_rho$ after $rho$, but not a $ê_z$ after $z$? Does the $ê_rho$ "hold" all of the $2zrho$ as the "coefficient"?

      2. Can it then be written as a vector then, ie. $int_0^pi (2zrho,0,0)dphi$?

      3. Why is there only a $dphi$? Since there's no $phi$ in the function (vector?), can the $2zp$ be treated as "constants" and put outside the integral? Would the answer to the integral then be $2pi zrho$ or $2pi zrho ê_rho$
        ?

      Kind of blurry questions, but the answers to them might help me understand more about coordinate basis and integrals.









      share|cite|improve this question











      share|cite|improve this question




      share|cite|improve this question










      asked Aug 28 at 16:52









      Defindun

      296




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          1 Answer
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          The scalar factor $2zrho$ is independent of $phi$ and can be taken outside the integral, but the vector $hate_rho$ is actually a function of $phi$; at each point, it's the unit vector pointing in the $rho$ direction at that point, so in terms of its $xyz$ components it's given by
          $$
          hate_rho =
          beginpmatrix
          cosphi \
          sinphi \
          0
          endpmatrix
          .
          $$
          So what you have is the integral of a vector-valued function, and you can integrate it component-wise after writing $hate_rho$ as above.






          share|cite|improve this answer




















          • Okay, I can see that connection. But why would I want to change ê_rho to cartesian coordinates, since the integral is for cylindrical coordinates and the differential is of phi? Is this because ê_rho is dependent on phi as the "position vector circulates around in the cylinder"? So the primitive function would then be 2zρ(sin(phi), -cos(phi), 0)? I thought ê_rho was used to describe a vector instead of the usual (2zρ, 0, 0). Could the integral be written like that as well? How would I then see that I need to convert the ê_rho to your above vector and then integrate component-wise.
            – Defindun
            Aug 28 at 18:25











          • @Defindun: When you integrate, you are “adding up infinitely many infinitesimal contributions”. In this case, you have a bunch of vectors to add up, and if you want to get the grand total in the $x$-direction, you add up all the $x$-components of the contributing vectors. And the same for $y$ and $z$. This makes sense, since the $x$, $y$ and $z$ directions don't change – they are the same at all points. But you can't directly add up all the components in the $rho$ direction, because there is no global $rho$ direction – it changes from point to point.
            – Hans Lundmark
            Aug 28 at 19:46











          • Okay, thank you! So that is because this is a vector-valued function in cylindrical coordinates, which can be written as ∫ (2zρ, 0, 0) dϕ as well? So, since its vector-valued, we have to use coordinates that remains the same, ie. are global? So we convert the "vector-direction" which depends on ϕ (since that's the differential) and can put 2zρ outside as "constants"? My question then is for regular triple integrals for example, where we integrate over ρ, ϕ, and z as they are, so to speak? What are the unit vectors for these? It is after all cylindrical coordinates as well.
            – Defindun
            Aug 29 at 5:45











          • @Defindun: Notation such as $(a,b,c)$ is shorthand and has to be interpreted in context. If what you mean is $a hate_x + b hate_y + c hate_z$, then you can integrate component-wise, since what you are doing is $int (a hate_x + b hate_y + c hate_z = hate_x int a + hate_y int b + hate_z int c$, pulling the constant vectors outside of the integral. But if you mean $a hate_rho + b hate_phi + c hate_z$, then you can't integrate component-wise, since the vectors $hate_rho$ and $hate_phi$ aren't constant.
            – Hans Lundmark
            Aug 29 at 6:08










          • @Defindun: But regarding your question about triple integrals, I don't understand what you mean. It's just standard multivariable calculus, where you integrate a scalar-valued function. A change of variables converts the original region in $xyz$-space to a new region in $rthetaphi$-space, but it's still just an ordinary triple integral no matter what the coordinates are called.
            – Hans Lundmark
            Aug 29 at 6:10










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          1 Answer
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          active

          oldest

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          1 Answer
          1






          active

          oldest

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          active

          oldest

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          active

          oldest

          votes








          up vote
          1
          down vote



          accepted










          The scalar factor $2zrho$ is independent of $phi$ and can be taken outside the integral, but the vector $hate_rho$ is actually a function of $phi$; at each point, it's the unit vector pointing in the $rho$ direction at that point, so in terms of its $xyz$ components it's given by
          $$
          hate_rho =
          beginpmatrix
          cosphi \
          sinphi \
          0
          endpmatrix
          .
          $$
          So what you have is the integral of a vector-valued function, and you can integrate it component-wise after writing $hate_rho$ as above.






          share|cite|improve this answer




















          • Okay, I can see that connection. But why would I want to change ê_rho to cartesian coordinates, since the integral is for cylindrical coordinates and the differential is of phi? Is this because ê_rho is dependent on phi as the "position vector circulates around in the cylinder"? So the primitive function would then be 2zρ(sin(phi), -cos(phi), 0)? I thought ê_rho was used to describe a vector instead of the usual (2zρ, 0, 0). Could the integral be written like that as well? How would I then see that I need to convert the ê_rho to your above vector and then integrate component-wise.
            – Defindun
            Aug 28 at 18:25











          • @Defindun: When you integrate, you are “adding up infinitely many infinitesimal contributions”. In this case, you have a bunch of vectors to add up, and if you want to get the grand total in the $x$-direction, you add up all the $x$-components of the contributing vectors. And the same for $y$ and $z$. This makes sense, since the $x$, $y$ and $z$ directions don't change – they are the same at all points. But you can't directly add up all the components in the $rho$ direction, because there is no global $rho$ direction – it changes from point to point.
            – Hans Lundmark
            Aug 28 at 19:46











          • Okay, thank you! So that is because this is a vector-valued function in cylindrical coordinates, which can be written as ∫ (2zρ, 0, 0) dϕ as well? So, since its vector-valued, we have to use coordinates that remains the same, ie. are global? So we convert the "vector-direction" which depends on ϕ (since that's the differential) and can put 2zρ outside as "constants"? My question then is for regular triple integrals for example, where we integrate over ρ, ϕ, and z as they are, so to speak? What are the unit vectors for these? It is after all cylindrical coordinates as well.
            – Defindun
            Aug 29 at 5:45











          • @Defindun: Notation such as $(a,b,c)$ is shorthand and has to be interpreted in context. If what you mean is $a hate_x + b hate_y + c hate_z$, then you can integrate component-wise, since what you are doing is $int (a hate_x + b hate_y + c hate_z = hate_x int a + hate_y int b + hate_z int c$, pulling the constant vectors outside of the integral. But if you mean $a hate_rho + b hate_phi + c hate_z$, then you can't integrate component-wise, since the vectors $hate_rho$ and $hate_phi$ aren't constant.
            – Hans Lundmark
            Aug 29 at 6:08










          • @Defindun: But regarding your question about triple integrals, I don't understand what you mean. It's just standard multivariable calculus, where you integrate a scalar-valued function. A change of variables converts the original region in $xyz$-space to a new region in $rthetaphi$-space, but it's still just an ordinary triple integral no matter what the coordinates are called.
            – Hans Lundmark
            Aug 29 at 6:10














          up vote
          1
          down vote



          accepted










          The scalar factor $2zrho$ is independent of $phi$ and can be taken outside the integral, but the vector $hate_rho$ is actually a function of $phi$; at each point, it's the unit vector pointing in the $rho$ direction at that point, so in terms of its $xyz$ components it's given by
          $$
          hate_rho =
          beginpmatrix
          cosphi \
          sinphi \
          0
          endpmatrix
          .
          $$
          So what you have is the integral of a vector-valued function, and you can integrate it component-wise after writing $hate_rho$ as above.






          share|cite|improve this answer




















          • Okay, I can see that connection. But why would I want to change ê_rho to cartesian coordinates, since the integral is for cylindrical coordinates and the differential is of phi? Is this because ê_rho is dependent on phi as the "position vector circulates around in the cylinder"? So the primitive function would then be 2zρ(sin(phi), -cos(phi), 0)? I thought ê_rho was used to describe a vector instead of the usual (2zρ, 0, 0). Could the integral be written like that as well? How would I then see that I need to convert the ê_rho to your above vector and then integrate component-wise.
            – Defindun
            Aug 28 at 18:25











          • @Defindun: When you integrate, you are “adding up infinitely many infinitesimal contributions”. In this case, you have a bunch of vectors to add up, and if you want to get the grand total in the $x$-direction, you add up all the $x$-components of the contributing vectors. And the same for $y$ and $z$. This makes sense, since the $x$, $y$ and $z$ directions don't change – they are the same at all points. But you can't directly add up all the components in the $rho$ direction, because there is no global $rho$ direction – it changes from point to point.
            – Hans Lundmark
            Aug 28 at 19:46











          • Okay, thank you! So that is because this is a vector-valued function in cylindrical coordinates, which can be written as ∫ (2zρ, 0, 0) dϕ as well? So, since its vector-valued, we have to use coordinates that remains the same, ie. are global? So we convert the "vector-direction" which depends on ϕ (since that's the differential) and can put 2zρ outside as "constants"? My question then is for regular triple integrals for example, where we integrate over ρ, ϕ, and z as they are, so to speak? What are the unit vectors for these? It is after all cylindrical coordinates as well.
            – Defindun
            Aug 29 at 5:45











          • @Defindun: Notation such as $(a,b,c)$ is shorthand and has to be interpreted in context. If what you mean is $a hate_x + b hate_y + c hate_z$, then you can integrate component-wise, since what you are doing is $int (a hate_x + b hate_y + c hate_z = hate_x int a + hate_y int b + hate_z int c$, pulling the constant vectors outside of the integral. But if you mean $a hate_rho + b hate_phi + c hate_z$, then you can't integrate component-wise, since the vectors $hate_rho$ and $hate_phi$ aren't constant.
            – Hans Lundmark
            Aug 29 at 6:08










          • @Defindun: But regarding your question about triple integrals, I don't understand what you mean. It's just standard multivariable calculus, where you integrate a scalar-valued function. A change of variables converts the original region in $xyz$-space to a new region in $rthetaphi$-space, but it's still just an ordinary triple integral no matter what the coordinates are called.
            – Hans Lundmark
            Aug 29 at 6:10












          up vote
          1
          down vote



          accepted







          up vote
          1
          down vote



          accepted






          The scalar factor $2zrho$ is independent of $phi$ and can be taken outside the integral, but the vector $hate_rho$ is actually a function of $phi$; at each point, it's the unit vector pointing in the $rho$ direction at that point, so in terms of its $xyz$ components it's given by
          $$
          hate_rho =
          beginpmatrix
          cosphi \
          sinphi \
          0
          endpmatrix
          .
          $$
          So what you have is the integral of a vector-valued function, and you can integrate it component-wise after writing $hate_rho$ as above.






          share|cite|improve this answer












          The scalar factor $2zrho$ is independent of $phi$ and can be taken outside the integral, but the vector $hate_rho$ is actually a function of $phi$; at each point, it's the unit vector pointing in the $rho$ direction at that point, so in terms of its $xyz$ components it's given by
          $$
          hate_rho =
          beginpmatrix
          cosphi \
          sinphi \
          0
          endpmatrix
          .
          $$
          So what you have is the integral of a vector-valued function, and you can integrate it component-wise after writing $hate_rho$ as above.







          share|cite|improve this answer












          share|cite|improve this answer



          share|cite|improve this answer










          answered Aug 28 at 17:09









          Hans Lundmark

          33.5k564109




          33.5k564109











          • Okay, I can see that connection. But why would I want to change ê_rho to cartesian coordinates, since the integral is for cylindrical coordinates and the differential is of phi? Is this because ê_rho is dependent on phi as the "position vector circulates around in the cylinder"? So the primitive function would then be 2zρ(sin(phi), -cos(phi), 0)? I thought ê_rho was used to describe a vector instead of the usual (2zρ, 0, 0). Could the integral be written like that as well? How would I then see that I need to convert the ê_rho to your above vector and then integrate component-wise.
            – Defindun
            Aug 28 at 18:25











          • @Defindun: When you integrate, you are “adding up infinitely many infinitesimal contributions”. In this case, you have a bunch of vectors to add up, and if you want to get the grand total in the $x$-direction, you add up all the $x$-components of the contributing vectors. And the same for $y$ and $z$. This makes sense, since the $x$, $y$ and $z$ directions don't change – they are the same at all points. But you can't directly add up all the components in the $rho$ direction, because there is no global $rho$ direction – it changes from point to point.
            – Hans Lundmark
            Aug 28 at 19:46











          • Okay, thank you! So that is because this is a vector-valued function in cylindrical coordinates, which can be written as ∫ (2zρ, 0, 0) dϕ as well? So, since its vector-valued, we have to use coordinates that remains the same, ie. are global? So we convert the "vector-direction" which depends on ϕ (since that's the differential) and can put 2zρ outside as "constants"? My question then is for regular triple integrals for example, where we integrate over ρ, ϕ, and z as they are, so to speak? What are the unit vectors for these? It is after all cylindrical coordinates as well.
            – Defindun
            Aug 29 at 5:45











          • @Defindun: Notation such as $(a,b,c)$ is shorthand and has to be interpreted in context. If what you mean is $a hate_x + b hate_y + c hate_z$, then you can integrate component-wise, since what you are doing is $int (a hate_x + b hate_y + c hate_z = hate_x int a + hate_y int b + hate_z int c$, pulling the constant vectors outside of the integral. But if you mean $a hate_rho + b hate_phi + c hate_z$, then you can't integrate component-wise, since the vectors $hate_rho$ and $hate_phi$ aren't constant.
            – Hans Lundmark
            Aug 29 at 6:08










          • @Defindun: But regarding your question about triple integrals, I don't understand what you mean. It's just standard multivariable calculus, where you integrate a scalar-valued function. A change of variables converts the original region in $xyz$-space to a new region in $rthetaphi$-space, but it's still just an ordinary triple integral no matter what the coordinates are called.
            – Hans Lundmark
            Aug 29 at 6:10
















          • Okay, I can see that connection. But why would I want to change ê_rho to cartesian coordinates, since the integral is for cylindrical coordinates and the differential is of phi? Is this because ê_rho is dependent on phi as the "position vector circulates around in the cylinder"? So the primitive function would then be 2zρ(sin(phi), -cos(phi), 0)? I thought ê_rho was used to describe a vector instead of the usual (2zρ, 0, 0). Could the integral be written like that as well? How would I then see that I need to convert the ê_rho to your above vector and then integrate component-wise.
            – Defindun
            Aug 28 at 18:25











          • @Defindun: When you integrate, you are “adding up infinitely many infinitesimal contributions”. In this case, you have a bunch of vectors to add up, and if you want to get the grand total in the $x$-direction, you add up all the $x$-components of the contributing vectors. And the same for $y$ and $z$. This makes sense, since the $x$, $y$ and $z$ directions don't change – they are the same at all points. But you can't directly add up all the components in the $rho$ direction, because there is no global $rho$ direction – it changes from point to point.
            – Hans Lundmark
            Aug 28 at 19:46











          • Okay, thank you! So that is because this is a vector-valued function in cylindrical coordinates, which can be written as ∫ (2zρ, 0, 0) dϕ as well? So, since its vector-valued, we have to use coordinates that remains the same, ie. are global? So we convert the "vector-direction" which depends on ϕ (since that's the differential) and can put 2zρ outside as "constants"? My question then is for regular triple integrals for example, where we integrate over ρ, ϕ, and z as they are, so to speak? What are the unit vectors for these? It is after all cylindrical coordinates as well.
            – Defindun
            Aug 29 at 5:45











          • @Defindun: Notation such as $(a,b,c)$ is shorthand and has to be interpreted in context. If what you mean is $a hate_x + b hate_y + c hate_z$, then you can integrate component-wise, since what you are doing is $int (a hate_x + b hate_y + c hate_z = hate_x int a + hate_y int b + hate_z int c$, pulling the constant vectors outside of the integral. But if you mean $a hate_rho + b hate_phi + c hate_z$, then you can't integrate component-wise, since the vectors $hate_rho$ and $hate_phi$ aren't constant.
            – Hans Lundmark
            Aug 29 at 6:08










          • @Defindun: But regarding your question about triple integrals, I don't understand what you mean. It's just standard multivariable calculus, where you integrate a scalar-valued function. A change of variables converts the original region in $xyz$-space to a new region in $rthetaphi$-space, but it's still just an ordinary triple integral no matter what the coordinates are called.
            – Hans Lundmark
            Aug 29 at 6:10















          Okay, I can see that connection. But why would I want to change ê_rho to cartesian coordinates, since the integral is for cylindrical coordinates and the differential is of phi? Is this because ê_rho is dependent on phi as the "position vector circulates around in the cylinder"? So the primitive function would then be 2zρ(sin(phi), -cos(phi), 0)? I thought ê_rho was used to describe a vector instead of the usual (2zρ, 0, 0). Could the integral be written like that as well? How would I then see that I need to convert the ê_rho to your above vector and then integrate component-wise.
          – Defindun
          Aug 28 at 18:25





          Okay, I can see that connection. But why would I want to change ê_rho to cartesian coordinates, since the integral is for cylindrical coordinates and the differential is of phi? Is this because ê_rho is dependent on phi as the "position vector circulates around in the cylinder"? So the primitive function would then be 2zρ(sin(phi), -cos(phi), 0)? I thought ê_rho was used to describe a vector instead of the usual (2zρ, 0, 0). Could the integral be written like that as well? How would I then see that I need to convert the ê_rho to your above vector and then integrate component-wise.
          – Defindun
          Aug 28 at 18:25













          @Defindun: When you integrate, you are “adding up infinitely many infinitesimal contributions”. In this case, you have a bunch of vectors to add up, and if you want to get the grand total in the $x$-direction, you add up all the $x$-components of the contributing vectors. And the same for $y$ and $z$. This makes sense, since the $x$, $y$ and $z$ directions don't change – they are the same at all points. But you can't directly add up all the components in the $rho$ direction, because there is no global $rho$ direction – it changes from point to point.
          – Hans Lundmark
          Aug 28 at 19:46





          @Defindun: When you integrate, you are “adding up infinitely many infinitesimal contributions”. In this case, you have a bunch of vectors to add up, and if you want to get the grand total in the $x$-direction, you add up all the $x$-components of the contributing vectors. And the same for $y$ and $z$. This makes sense, since the $x$, $y$ and $z$ directions don't change – they are the same at all points. But you can't directly add up all the components in the $rho$ direction, because there is no global $rho$ direction – it changes from point to point.
          – Hans Lundmark
          Aug 28 at 19:46













          Okay, thank you! So that is because this is a vector-valued function in cylindrical coordinates, which can be written as ∫ (2zρ, 0, 0) dϕ as well? So, since its vector-valued, we have to use coordinates that remains the same, ie. are global? So we convert the "vector-direction" which depends on ϕ (since that's the differential) and can put 2zρ outside as "constants"? My question then is for regular triple integrals for example, where we integrate over ρ, ϕ, and z as they are, so to speak? What are the unit vectors for these? It is after all cylindrical coordinates as well.
          – Defindun
          Aug 29 at 5:45





          Okay, thank you! So that is because this is a vector-valued function in cylindrical coordinates, which can be written as ∫ (2zρ, 0, 0) dϕ as well? So, since its vector-valued, we have to use coordinates that remains the same, ie. are global? So we convert the "vector-direction" which depends on ϕ (since that's the differential) and can put 2zρ outside as "constants"? My question then is for regular triple integrals for example, where we integrate over ρ, ϕ, and z as they are, so to speak? What are the unit vectors for these? It is after all cylindrical coordinates as well.
          – Defindun
          Aug 29 at 5:45













          @Defindun: Notation such as $(a,b,c)$ is shorthand and has to be interpreted in context. If what you mean is $a hate_x + b hate_y + c hate_z$, then you can integrate component-wise, since what you are doing is $int (a hate_x + b hate_y + c hate_z = hate_x int a + hate_y int b + hate_z int c$, pulling the constant vectors outside of the integral. But if you mean $a hate_rho + b hate_phi + c hate_z$, then you can't integrate component-wise, since the vectors $hate_rho$ and $hate_phi$ aren't constant.
          – Hans Lundmark
          Aug 29 at 6:08




          @Defindun: Notation such as $(a,b,c)$ is shorthand and has to be interpreted in context. If what you mean is $a hate_x + b hate_y + c hate_z$, then you can integrate component-wise, since what you are doing is $int (a hate_x + b hate_y + c hate_z = hate_x int a + hate_y int b + hate_z int c$, pulling the constant vectors outside of the integral. But if you mean $a hate_rho + b hate_phi + c hate_z$, then you can't integrate component-wise, since the vectors $hate_rho$ and $hate_phi$ aren't constant.
          – Hans Lundmark
          Aug 29 at 6:08












          @Defindun: But regarding your question about triple integrals, I don't understand what you mean. It's just standard multivariable calculus, where you integrate a scalar-valued function. A change of variables converts the original region in $xyz$-space to a new region in $rthetaphi$-space, but it's still just an ordinary triple integral no matter what the coordinates are called.
          – Hans Lundmark
          Aug 29 at 6:10




          @Defindun: But regarding your question about triple integrals, I don't understand what you mean. It's just standard multivariable calculus, where you integrate a scalar-valued function. A change of variables converts the original region in $xyz$-space to a new region in $rthetaphi$-space, but it's still just an ordinary triple integral no matter what the coordinates are called.
          – Hans Lundmark
          Aug 29 at 6:10

















           

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