Cylindrical coordinate system

File:Coord system CY 1.svg

A cylindrical coordinate system with origin Template:Mvar, polar axis Template:Mvar, and longitudinal axis Template:Mvar. The dot is the point with radial distance Template:Math, angular coordinate Template:Math, and height Template:Math. The reference plane contains the purple section.

A cylindrical coordinate system is a three-dimensional coordinate system that specifies point positions around a main axis (a chosen directed line) and an auxiliary axis (a reference ray). The three cylindrical coordinates are: the point perpendicular distance Template:Math from the main axis; the point signed distance z along the main axis from a chosen origin; and the plane angle Template:Math of the point projection on a reference plane (passing through the origin and perpendicular to the main axis)

The main axis is variously called the cylindrical or longitudinal axis. The auxiliary axis is called the polar axis, which lies in the reference plane, starting at the origin, and pointing in the reference direction. Other directions perpendicular to the longitudinal axis are called radial lines.

The distance from the axis may be called the radial distance or radius, while the angular coordinate is sometimes referred to as the angular position or as the azimuth. The radius and the azimuth are together called the polar coordinates, as they correspond to a two-dimensional polar coordinate system in the plane through the point, parallel to the reference plane. The third coordinate may be called the height or altitude (if the reference plane is considered horizontal), longitudinal position, or axial position.

Cylindrical coordinates are useful in connection with objects and phenomena that have some rotational symmetry about the longitudinal axis, such as water flow in a straight pipe with round cross-section, heat distribution in a metal cylinder, electromagnetic fields produced by an electric current in a long, straight wire, accretion disks in astronomy, and so on.

They are sometimes called cylindrical polar coordinates or polar cylindrical coordinates, and are sometimes used to specify the position of stars in a galaxy (galactocentric cylindrical polar coordinates).

Definition

The three coordinates (Template:Mvar, Template:Mvar, Template:Mvar) of a point Template:Mvar are defined as:

• The radial distance Template:Mvar is the Euclidean distance from the Template:Mvar-axis to the point Template:Mvar.
• The azimuth Template:Mvar is the angle between the reference direction on the chosen plane and the line from the origin to the projection of Template:Mvar on the plane.
• The axial coordinate or height Template:Mvar is the signed distance from the chosen plane to the point Template:Mvar.

Unique cylindrical coordinates

As in polar coordinates, the same point with cylindrical coordinates Template:Math has infinitely many equivalent coordinates, namely Template:Math and Template:Math where Template:Mvar is any integer. Moreover, if the radius Template:Mvar is zero, the azimuth is arbitrary.

In situations where someone wants a unique set of coordinates for each point, one may restrict the radius to be non-negative (Template:Math) and the azimuth Template:Mvar to lie in a specific interval spanning 360°, such as Template:Math or Template:Math.

Conventions

The notation for cylindrical coordinates is not uniform. The ISO standard 31-11 recommends Template:Math, where Template:Mvar is the radial coordinate, Template:Mvar the azimuth, and Template:Mvar the height. However, the radius is also often denoted Template:Mvar or Template:Mvar, the azimuth by Template:Mvar or Template:Mvar, and the third coordinate by Template:Mvar or (if the cylindrical axis is considered horizontal) Template:Mvar, or any context-specific letter.

File:Cylindrical coordinate surfaces.png

The coordinate surfaces of the cylindrical coordinates Template:Math. The red cylinder shows the points with Template:Math, the blue plane shows the points with Template:Math, and the yellow half-plane shows the points with Template:Math. The Template:Mvar-axis is vertical and the Template:Mvar-axis is highlighted in green. The three surfaces intersect at the point Template:Mvar with those coordinates (shown as a black sphere); the Cartesian coordinates of Template:Mvar are roughly (1.0, −1.732, 1.0).

File:Cylindrical coordinate surfaces.gif

Cylindrical coordinate surfaces. The three orthogonal components, Template:Mvar (green), Template:Mvar (red), and Template:Mvar (blue), each increasing at a constant rate. The point is at the intersection between the three colored surfaces.

In concrete situations, and in many mathematical illustrations, a positive angular coordinate is measured counterclockwise as seen from any point with positive height.

Coordinate system conversions

The cylindrical coordinate system is one of many three-dimensional coordinate systems. The following formulae may be used to convert between them.

Cartesian coordinates

For the conversion between cylindrical and Cartesian coordinates, it is convenient to assume that the reference plane of the former is the Cartesian Template:Mvar-plane (with equation Template:Math), and the cylindrical axis is the Cartesian Template:Mvar-axis. Then the Template:Mvar-coordinate is the same in both systems, and the correspondence between cylindrical Template:Math and Cartesian Template:Math are the same as for polar coordinates, namely $$\begin{array}{rl}x& =\rho \cos \phi \\ y& =\rho \sin \phi \\ z& =z\end{array}$$ in one direction, and $$\begin{array}{rl}\rho & =\sqrt{x^2+y^2}\\ \phi & =\{\begin{array}{ll}\text{indeterminate}& \text{if }x=0\text{ and }y=0\\ \arcsin \left(\frac{y}{\rho }\right)& \text{if }x\ge 0\\ -\arcsin \left(\frac{y}{\rho }\right)+\pi & \text{if }x<0\text{ and }y\ge 0\\ -\arcsin \left(\frac{y}{\rho }\right)-\pi & \text{if }x<0\text{ and }y<0\end{array}\end{array}$$ in the other. The arcsine function is the inverse of the sine function, and is assumed to return an angle in the range Template:Math = Template:Math. These formulas yield an azimuth Template:Mvar in the range Template:Math.

By using the arctangent function that returns also an angle in the range Template:Math = Template:Math, one may also compute $\phi$ without computing $\rho$ first $$\begin{array}{rl}\phi & =\{\begin{array}{ll}\text{indeterminate}& \text{if }x=0\text{ and }y=0\\ \frac{\pi }{2}\frac{y}{|y|}& \text{if }x=0\text{ and }y\ne 0\\ \arctan \left(\frac{y}{x}\right)& \text{if }x>0\\ \arctan \left(\frac{y}{x}\right)+\pi & \text{if }x<0\text{ and }y\ge 0\\ \arctan \left(\frac{y}{x}\right)-\pi & \text{if }x<0\text{ and }y<0\end{array}\end{array}$$ For other formulas, see the article Polar coordinate system.

Many modern programming languages provide a function that will compute the correct azimuth Template:Mvar, in the range Template:Math, given x and y, without the need to perform a case analysis as above. For example, this function is called by Template:Mono in the C programming language, and Template:Mono in Common Lisp.

Spherical coordinates

Spherical coordinates (radius Template:Mvar, elevation or inclination Template:Mvar, azimuth Template:Mvar), may be converted to or from cylindrical coordinates, depending on whether Template:Mvar represents elevation or inclination, by the following:

Conversion between spherical and cylindrical coordinates

Conversion to:	Coordinate	Template:Mvar is elevation	Template:Mvar is inclination
Cylindrical	Template:Mvar =	Template:Math	Template:Math
	Template:Mvar =	Template:Mvar
	Template:Mvar =	Template:Math	Template:Math
Spherical	Template:Mvar =	$\sqrt{{\rho }^2+z^2}$
	Template:Mvar =	$\arctan \left(\frac{z}{\rho }\right)$	$\arctan \left(\frac{\rho }{z}\right)$
	Template:Mvar =	Template:Mvar

Line and volume elements

In many problems involving cylindrical polar coordinates, it is useful to know the line and volume elements; these are used in integration to solve problems involving paths and volumes.

The line element is $$\mathrm{d}\mathit{r}=\mathrm{d}\rho \hat{\mathit{\rho }}+\rho \mathrm{d}\phi \hat{\mathit{\phi }}+\mathrm{d}z\hat{\mathit{z}}.$$

The volume element is $$\mathrm{d}V=\rho \mathrm{d}\rho \mathrm{d}\phi \mathrm{d}z.$$

The surface element in a surface of constant radius Template:Mvar (a vertical cylinder) is $$\mathrm{d}{S}_{\rho }=\rho \mathrm{d}\phi \mathrm{d}z.$$

The surface element in a surface of constant azimuth Template:Mvar (a vertical half-plane) is $$\mathrm{d}{S}_{\phi }=\mathrm{d}\rho \mathrm{d}z.$$

The surface element in a surface of constant height Template:Mvar (a horizontal plane) is $$\mathrm{d}{S}_z=\rho \mathrm{d}\rho \mathrm{d}\phi .$$

The del operator in this system leads to the following expressions for gradient, divergence, curl and Laplacian: $$\begin{array}{rl}\mathrm{\nabla }f& =\frac{\partial f}{\partial \rho }\hat{\mathit{\rho }}+\frac{1}{\rho }\frac{\partial f}{\partial \phi }\hat{\mathit{\phi }}+\frac{\partial f}{\partial z}\hat{\mathit{z}}\\ [8px]\mathrm{\nabla }\cdot \mathit{A}& =\frac{1}{\rho }\frac{\partial }{\partial \rho }\left(\rho A_{\rho }\right)+\frac{1}{\rho }\frac{\partial A_{\phi }}{\partial \phi }+\frac{\partial A_z}{\partial z}\\ [8px]\mathrm{\nabla }\times \mathit{A}& =(\frac{1}{\rho }\frac{\partial A_z}{\partial \phi }-\frac{\partial A_{\phi }}{\partial z})\hat{\mathit{\rho }}+(\frac{\partial A_{\rho }}{\partial z}-\frac{\partial A_z}{\partial \rho })\hat{\mathit{\phi }}+\frac{1}{\rho }(\frac{\partial }{\partial \rho }\left(\rho A_{\phi }\right)-\frac{\partial A_{\rho }}{\partial \phi })\hat{\mathit{z}}\\ [8px]{\mathrm{\nabla }}^{2}f& =\frac{1}{\rho }\frac{\partial }{\partial \rho }\left(\rho \frac{\partial f}{\partial \rho }\right)+\frac{1}{{\rho }^2}\frac{{\partial }^{2}f}{\partial {\phi }^2}+\frac{{\partial }^{2}f}{\partial z^2}\end{array}$$

Cylindrical harmonics

The solutions to the Laplace equation in a system with cylindrical symmetry are called cylindrical harmonics.

Kinematics

In a cylindrical coordinate system, the position of a particle can be written as $$\mathit{r}=\rho \hat{\mathit{\rho }}+z\hat{\mathit{z}}.$$ The velocity of the particle is the time derivative of its position, $$\mathit{v}=\frac{\mathrm{d}\mathit{r}}{\mathrm{d}t}=\dot{\rho }\hat{\mathit{\rho }}+\rho \dot{\phi }\hat{\mathit{\phi }}+\dot{z}\hat{\mathit{z}},$$ where the term $\rho \dot{\phi }\hat{\phi }$ comes from the Poisson formula $\frac{\mathrm{d}\hat{\rho }}{\mathrm{d}t}=\dot{\phi }\hat{z}\times \hat{\rho }$. Its acceleration is $$\mathit{a}=\frac{\mathrm{d}\mathit{v}}{\mathrm{d}t}=(\ddot{\rho }-\rho {\dot{\phi }}^2)\hat{\mathit{\rho }}+(2\dot{\rho }\dot{\phi }+\rho \ddot{\phi })\hat{\mathit{\phi }}+\ddot{z}\hat{\mathit{z}}$$

See also

• List of canonical coordinate transformations
• Vector fields in cylindrical and spherical coordinates
• Del in cylindrical and spherical coordinates

External links

• Template:Springer
• MathWorld description of cylindrical coordinates
• Cylindrical Coordinates Animations illustrating cylindrical coordinates by Frank Wattenberg