Poundal

Spherical coordinates (r, θ, φ) as commonly used in physics: radial distance r, polar angle θ (theta), and azimuthal angle φ (phi). The symbol ρ (rho) is often used instead of r.

NOTE: This page uses common physics notation for spherical coordinates, in which $\theta$ is the angle between the z axis and the radius vector connecting the origin to the point in question, while $\varphi$ is the angle between the projection of the radius vector onto the x-y plane and the x axis. Several other definitions are in use, and so care must be taken in comparing different sources.

Cylindrical coordinate system

Vector fields

Vectors are defined in cylindrical coordinates by (r, θ, z), where

• r is the length of the vector projected onto the xy-plane,
• θ is the angle between the projection of the vector onto the xy-plane (i.e. r) and the positive x-axis (0 ≤ θ < 2π),
• z is the regular z-coordinate.

(r, θ, z) is given in cartesian coordinates by:

$\left[\begin{array}{c}r\\ \theta \\ z\end{array}\right]=\left[\begin{array}{c}\sqrt{x^2+y^2}\\ \arctan (y/x)\\ z\end{array}\right],0\le \theta <2\pi ,$

or inversely by:

$\left[\begin{array}{c}x\\ y\\ z\end{array}\right]=\left[\begin{array}{c}r\cos \theta \\ r\sin \theta \\ z\end{array}\right].$

Any vector field can be written in terms of the unit vectors as:

$𝐀=A_x\widehat{𝐱}+A_y\widehat{𝐲}+A_z\widehat{𝐳}=A_r\widehat{𝐫}+A_{\theta }\widehat{𝜽}+A_z\widehat{𝐳}$

The cylindrical unit vectors are related to the cartesian unit vectors by:

$\left[\begin{array}{c}\widehat{𝐫}\\ \widehat{𝜽}\\ \widehat{𝐳}\end{array}\right]=\left[\begin{array}{ccc}\cos \theta & \sin \theta & 0\\ -\sin \theta & \cos \theta & 0\\ 0& 0& 1\end{array}\right]\left[\begin{array}{c}\widehat{𝐱}\\ \widehat{𝐲}\\ \widehat{𝐳}\end{array}\right]$

• Note: the matrix is an orthogonal matrix, that is, its inverse is simply its transpose.

Time derivative of a vector field

To find out how the vector field A changes in time we calculate the time derivatives. For this purpose we use Newton's notation for the time derivative ($\dot{𝐀}$). In cartesian coordinates this is simply:

$\dot{𝐀}={\dot{A}}_x\widehat{𝐱}+{\dot{A}}_y\widehat{𝐲}+{\dot{A}}_z\widehat{𝐳}$

However, in cylindrical coordinates this becomes:

$\dot{𝐀}={\dot{A}}_r\widehat{𝒓}+A_r\dot{\widehat{𝒓}}+{\dot{A}}_{\theta }\widehat{𝜽}+A_{\theta }\dot{\widehat{𝜽}}+{\dot{A}}_z\widehat{𝒛}+A_z\dot{\widehat{𝒛}}$

We need the time derivatives of the unit vectors. They are given by:

$\begin{array}{cc}\dot{\widehat{𝐫}}& =\dot{\theta }\widehat{𝜽}\\ \dot{\widehat{𝜽}}& =-\dot{\theta }\widehat{𝐫}\\ \dot{\widehat{𝐳}}& =0\end{array}$

So the time derivative simplifies to:

$\dot{𝐀}=\widehat{𝒓}({\dot{A}}_r-A_{\theta }\dot{\theta })+\widehat{𝜽}({\dot{A}}_{\theta }+A_r\dot{\theta })+\widehat{𝐳}{\dot{A}}_z$

Second time derivative of a vector field

The second time derivative is of interest in physics, as it is found in equations of motion for classical mechanical systems. The second time derivative of a vector field in cylindrical coordinates is given by:

$\ddot{𝐀}=\widehat{𝐫}({\ddot{A}}_r-A_{\theta }\ddot{\theta }-2{\dot{A}}_{\theta }\dot{\theta }-A_r{\dot{\theta }}^2)+\widehat{𝜽}({\ddot{A}}_{\theta }+A_r\ddot{\theta }+2{\dot{A}}_r\dot{\theta }-A_{\theta }{\dot{\theta }}^2)+\widehat{𝐳}{\ddot{A}}_z$

To understand this expression, we substitute A = P, where p is the vector (r, θ, z).

This means that $𝐀=𝐏=r\widehat{𝐫}+z\widehat{𝐳}$.

After substituting we get:

$\ddot{𝐏}=\widehat{𝐫}(\ddot{r}-r{\dot{\theta }}^2)+\widehat{𝜽}(r\ddot{\theta }+2\dot{r}\dot{\theta })+\widehat{𝐳}\ddot{z}$

In mechanics, the terms of this expression are called:

$\begin{array}{cc}\ddot{r}\widehat{𝐫}& =\text{central outward acceleration}\\ -r{\dot{\theta }}^2\widehat{𝐫}& =\text{centripetal acceleration}\\ r\ddot{\theta }\widehat{𝜽}& =\text{angular acceleration}\\ 2\dot{r}\dot{\theta }\widehat{𝜽}& =\text{Coriolis effect}\\ \ddot{z}\widehat{𝐳}& =\text{z-acceleration}\end{array}$

See also: Centripetal force, Angular acceleration, Coriolis effect.

Spherical coordinate system

Vector fields

Vectors are defined in spherical coordinates by (ρ,θ,φ), where

• ρ is the length of the vector,
• θ is the angle between the positive Z-axis and vector in question (0 ≤ θ ≤ π)
• φ is the angle between the projection of the vector onto the X-Y-plane and the positive X-axis (0 ≤ φ < 2π),

(ρ,θ,φ) is given in cartesian coordinates by:

$\left[\begin{array}{c}\rho \\ \theta \\ \varphi \end{array}\right]=\left[\begin{array}{c}\sqrt{x^2+y^2+z^2}\\ \arccos (z/\rho )\\ \arctan (y/x)\end{array}\right],0\le \theta \le \pi ,0\le \varphi <2\pi ,$

or inversely by:

$\left[\begin{array}{c}x\\ y\\ z\end{array}\right]=\left[\begin{array}{c}\rho \sin \theta \cos \varphi \\ \rho \sin \theta \sin \varphi \\ \rho \cos \theta \end{array}\right].$

Any vector field can be written in terms of the unit vectors as:

$𝐀=A_x\widehat{𝐱}+A_y\widehat{𝐲}+A_z\widehat{𝐳}=A_{\rho }\widehat{𝝆}+A_{\theta }\widehat{𝜽}+A_{\varphi }\widehat{𝝓}$

The spherical unit vectors are related to the cartesian unit vectors by:

$\left[\begin{array}{c}\widehat{𝝆}\\ \widehat{𝜽}\\ \widehat{𝝓}\end{array}\right]=\left[\begin{array}{ccc}\sin \theta \cos \varphi & \sin \theta \sin \varphi & \cos \theta \\ \cos \theta \cos \varphi & \cos \theta \sin \varphi & -\sin \theta \\ -\sin \varphi & \cos \varphi & 0\end{array}\right]\left[\begin{array}{c}\widehat{𝐱}\\ \widehat{𝐲}\\ \widehat{𝐳}\end{array}\right]$

• Note: the matrix is an orthogonal matrix, that is, its inverse is simply its transpose.

Time derivative of a vector field

To find out how the vector field A changes in time we calculate the time derivatives. In cartesian coordinates this is simply:

$\dot{𝐀}={\dot{A}}_x\widehat{𝐱}+{\dot{A}}_y\widehat{𝐲}+{\dot{A}}_z\widehat{𝐳}$

However, in spherical coordinates this becomes:

$\dot{𝐀}={\dot{A}}_{\rho }\widehat{𝝆}+A_{\rho }\dot{\widehat{𝝆}}+{\dot{A}}_{\theta }\widehat{𝜽}+A_{\theta }\dot{\widehat{𝜽}}+{\dot{A}}_{\varphi }\widehat{𝝓}+A_{\varphi }\dot{\widehat{𝝓}}$

We need the time derivatives of the unit vectors. They are given by:

$\begin{array}{cc}\dot{\widehat{𝝆}}& =\dot{\theta }\widehat{𝜽}+\dot{\varphi }\sin \theta \widehat{𝝓}\\ \dot{\widehat{𝜽}}& =-\dot{\theta }\widehat{𝝆}+\dot{\varphi }\cos \theta \widehat{𝝓}\\ \dot{\widehat{𝝓}}& =-\dot{\varphi }\sin \theta \widehat{𝝆}-\dot{\varphi }\cos \theta \widehat{𝜽}\end{array}$

So the time derivative becomes:

$\dot{𝐀}=\widehat{𝝆}({\dot{A}}_{\rho }-A_{\theta }\dot{\theta }-A_{\varphi }\dot{\varphi }\sin \theta )+\widehat{𝜽}({\dot{A}}_{\theta }+A_{\rho }\dot{\theta }-A_{\varphi }\dot{\varphi }\cos \theta )+\widehat{𝝓}({\dot{A}}_{\varphi }+A_{\rho }\dot{\varphi }\sin \theta +A_{\theta }\dot{\varphi }\cos \theta )$

See also

• Del in cylindrical and spherical coordinates for the specification of gradient, divergence, curl, and laplacian in various coordinate systems.