Cylindrical coordinate system

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 from the main axis; the point signed distance ''z'' along the main axis from a chosen origin; and the plane angle 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 (, , ) of a point are defined as:
* The ''radial distance'' is the Euclidean distance from the -axis to the point .
* The ''azimuth'' is the angle between the reference direction on the chosen plane and the line from the origin to the projection of on the plane.
* The ''axial coordinate'' or ''height'' is the signed distance from the chosen plane to the point .

Unique cylindrical coordinates

As in polar coordinates, the same point with cylindrical coordinates has infinitely many equivalent coordinates, namely and where is any integer. Moreover, if the radius 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 () and the azimuth to lie in a specific interval spanning 360°, such as or .

Conventions

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

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 -plane (with equation ), and the cylindrical axis is the Cartesian -axis. Then the -coordinate is the same in both systems, and the correspondence between cylindrical and Cartesian are the same as for polar coordinates, namely
$\begin x &= \rho \cos \varphi \\ y &= \rho \sin \varphi \\ z &= z \end$
in one direction, and
$\begin \rho &= \sqrt \\ \varphi &= \begin \text & \text x = 0 \text y = 0\\ \arcsin\left(\frac\right) & \text x \geq 0 \\ -\arcsin\left(\frac\right) + \pi & \mbox x < 0 \text y \ge 0\\ -\arcsin\left(\frac\right) - \pi & \mbox x < 0 \text y < 0 \end \end$
in the other. The arcsine function is the inverse of the sine function, and is assumed to return an angle in the range = . These formulas yield an azimuth in the range .

By using the arctangent function that returns also an angle in the range = , one may also compute $\varphi$ without computing $\rho$ first
$\begin \varphi &= \begin \text & \text x = 0 \text y = 0\\ \frac\pi2\frac y & \text x = 0 \text y \ne 0\\ \arctan\left(\frac\right) & \mbox x > 0 \\ \arctan\left(\frac\right)+\pi & \mbox x < 0 \text y \ge 0\\ \arctan\left(\frac\right)-\pi & \mbox x < 0 \text y < 0 \end \end$
For other formulas, see the article Polar coordinate system.

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

Spherical coordinates

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

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
$\mathrm\boldsymbol = \mathrm\rho\,\boldsymbol + \rho\,\mathrm\varphi\,\boldsymbol + \mathrmz\,\boldsymbol.$

The volume element is
$\mathrmV = \rho\,\mathrm\rho\,\mathrm\varphi\,\mathrmz.$

The surface element in a surface of constant radius (a vertical cylinder) is
$\mathrmS_\rho = \rho\,\mathrm\varphi\,\mathrmz.$

The surface element in a surface of constant azimuth (a vertical half-plane) is
$\mathrmS_\varphi = \mathrm\rho\,\mathrmz.$

The surface element in a surface of constant height (a horizontal plane) is
$\mathrmS_z = \rho\,\mathrm\rho\,\mathrm\varphi.$

The del operator in this system leads to the following expressions for gradient, divergence, curl and Laplacian:
\begin
\nabla f &= \frac\boldsymbol + \frac\frac\boldsymbol + \frac\boldsymbol \\ px
\nabla \cdot \boldsymbol &= \frac\frac\left(\rho A_\rho\right) + \frac \frac + \frac \\ px
\nabla \times \boldsymbol &= \left(\frac\frac - \frac\right)\boldsymbol +
\left(\frac - \frac\right)\boldsymbol +
\frac\left(\frac\left(\rho A_\varphi\right) - \frac\right) \boldsymbol \\ px
\nabla^2 f &= \frac \frac \left(\rho \frac\right) +
\frac \frac + \frac
\end

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
$\boldsymbol = \rho\,\boldsymbol + z\,\boldsymbol.$
The velocity of the particle is the time derivative of its position,
$\boldsymbol = \frac = \dot\,\boldsymbol + \rho\,\dot\varphi\,\hat + \dot\,\hat,$
where the term $\rho \dot\varphi\hat\varphi$ comes from the Poisson formula $\frac = \dot\varphi\hat z\times \hat\rho$. Its acceleration is
$\boldsymbol = \frac = \left( \ddot - \rho\,\dot\varphi^2 \right)\boldsymbol + \left( 2\dot\,\dot\varphi + \rho\,\ddot\varphi \right) \hat + \ddot\,\hat$

See also

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

References

Further reading

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External links

*
MathWorld description of cylindrical coordinates
Animations illustrating cylindrical coordinates by Frank Wattenberg

{{Orthogonal coordinate systems

Three-dimensional coordinate systems
Orthogonal coordinate systems

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