I'm having trouble converting a vector from the Cartesian coordinate system to the cylindrical coordinate system (second year vector calculus)
Represent the vector $\mathbf A(x,y,z) = z\ \hat i – 2x\ \hat j + y\ \hat k $ in cylindrical coordinates by writing it in the form
$$\mathbf A(\rho, \phi, z) = A_1(\rho, \phi, z){\hat \rho} + A_2(\rho, \phi, z){\hat \phi} + A_3(\rho, \phi, z){\hat z}$$
I know that
$$x=\rho\cos(\phi) \ \ , \ \ y = \rho\sin(\phi) \ \ , \ \ z = z$$
and
$$\rho=\sqrt{x^2+y^2} \ \ , \ \ \phi = \arctan\Big(\frac{y}{x}\Big) \ \ , \ \ z = z$$
So I put
$$\mathbf A(\rho,\phi,z) = z\ \hat i – 2\rho\cos(\phi) \ \hat j + \rho\sin(\phi) \ \hat k$$
I've tried computing the tangent vectors to the curve:
$$\vec \rho = \Big(\frac{\partial A_x}{\partial \rho}, \frac{\partial A_y}{\partial \rho} , \frac{\partial A_z}{\partial \rho} \Big) = (0,-2\cos(\phi),\sin(\phi))$$
$$\vec \phi = \Big(\frac{\partial A_x}{\partial \phi}, \frac{\partial A_y}{\partial \phi} , \frac{\partial A_z}{\partial \phi} \Big) = (0, 2\rho\sin(\phi), \rho\cos(\phi))$$
$$\vec z = \Big(\frac{\partial A_x}{\partial z}, \frac{\partial A_y}{\partial z} , \frac{\partial A_z}{\partial z} \Big) = (1,0,0)$$
Scale factors:
$$h_\rho = |\vec \rho| = \sqrt{4\cos^2(\phi) + \sin^2(\phi)}$$
$$h_\phi = |\vec \phi| = \sqrt{4\rho^2\sin^2(\phi) + \rho^2\cos^2(\phi)}$$
$$h_z = |\vec z| = 1$$
I also know that
$$\hat \rho = \frac{1}{h_\rho}\vec \rho \ \ , \ \ \hat \phi = \frac{1}{h_\phi}\vec \phi \ \ , \ \ \hat z = \frac{1}{h_z}\vec z$$
But from here I'm not sure how to proceed to the correct answer of $$\mathbf A(\rho,\phi,z) = (z\cos(\phi)-\rho\sin(2\phi))\ \hat \rho – (z\sin(\phi)+2\rho\cos^2(\phi))\ \hat \phi + \rho\sin(\phi)\ \hat z$$
Any help would be greatly appreciated, cheers!
Best Answer
Try a different way: find the expression of the cartesian unitary vectors in cylindrical unitary vectors. We have to invert this:
$$\left\{ \begin{matrix} \hat\rho=\cos\phi\hat i+\sin\phi\hat j\\ \hat\phi=-\sin\phi\hat i+\cos\phi\hat j\\ \hat k=\hat k\\ \end{matrix} \right.$$
From this:
$$ \begin{matrix} \sin\phi\hat\rho=\sin\phi\cos\phi\hat i+\sin^2\phi\hat j & \cos\phi\hat\rho=\cos^2\phi\hat i+\cos\phi\sin\phi\hat j\\ \cos\phi\hat\phi=-\cos\phi\sin\phi\hat i+\cos^2\phi\hat j & -\sin\phi\hat\phi=\sin^2\phi\hat i-\sin\phi\cos\phi\hat j\\ \end{matrix} $$
we end with
$$\left\{ \begin{matrix} \hat i=\cos\phi\hat\rho-\sin\phi\hat\phi\\ \hat j=\sin\phi\hat\rho+\cos\phi\hat\phi\\ \hat k=\hat k\\ \end{matrix} \right.$$
Now simply substitute:
$\mathbf A(x,y,z) = z\ \hat i - 2x\ \hat j + y\ \hat k=$
$=z(\cos\phi\hat\rho-\sin\phi\hat\phi)-2\rho\cos\phi(\sin\phi\hat\rho+\cos\phi\hat\phi)+\rho\sin\phi\hat k$
Rearrange and we are done.
$$\mathbf A(\rho,\phi,z) =(z\cos\phi-2\rho\sin(2\phi))\hat\rho-(z\sin\phi+2\rho\cos^2\phi)\hat\phi+\rho\sin\phi\hat k$$