Bipolar cylindrical coordinates explained

Bipolar cylindrical coordinates are a three-dimensional orthogonal coordinate system that results from projecting the two-dimensional bipolar coordinate system in theperpendicular

z -direction. The two lines of foci F _{1} and F _{2} of the projected Apollonian circles are generally taken to be defined by x =- a and x =+ a , respectively, (and by y =0 ) in the Cartesian coordinate system.

The term "bipolar" is often used to describe other curves having two singular points (foci), such as ellipses, hyperbolas, and Cassini ovals. However, the term bipolar coordinates is never used to describe coordinates associated with those curves, e.g., elliptic coordinates.

Basic definition

The most common definition of bipolar cylindrical coordinates

(\sigma, \tau, z) isx = a (\sinh \tau)/(\cosh \tau - \cos \sigma) y = a (\sin \sigma)/(\cosh \tau - \cos \sigma) z = z

where the

\sigma coordinate of a point P equals the angle F _{1} P F _{2} and the \tau coordinate equals the natural logarithm of the ratio of the distances d _{1} and d _{2} to the focal lines\tau = ln (d _{1})/(d _{2})

(Recall that the focal lines

F _{1} and F _{2} are located at x =- a and x =+ a , respectively.)

Surfaces of constant

\sigma correspond to cylinders of different radiix ^{2} + \left( y - a \cot \sigma \right) ^{2} = (a ^{2})/(\sin ^{2} \sigma)

that all pass through the focal lines and are not concentric. The surfaces of constant

\tau are non-intersecting cylinders of different radiiy ^{2} + \left( x - a \coth \tau \right) ^{2} = (a ^{2})/(\sinh ^{2} \tau)

that surround the focal lines but again are not concentric. The focal lines and all these cylinders are parallel to the

z -axis (the direction of projection). In the z =0 plane, the centers of the constant-\sigma and constant-\tau cylinders lie on the y and x axes, respectively.

Scale factors

The scale factors for the bipolar coordinates

\sigma and \tau are equalh _{\sigma} = h _{\tau} = (a)/(\cosh \tau - \cos\sigma)

whereas the remaining scale factor

h _{z} =1 . Thus, the infinitesimal volume element equalsdV = (a ^{2})/(\left( \cosh \tau - \cos\sigma \right) ^{2}) d\sigma d\tau dz

and the Laplacian is given by

\nabla ^{2} \Phi = (1)/(a ^{2}) \left( \cosh \tau - \cos\sigma \right) ^{2} \left( (\partial ^{2} \Phi)/(\partial \sigma ^{2}) + (\partial ^{2} \Phi)/(\partial \tau ^{2}) \right) + (\partial ^{2} \Phi)/(\partial z ^{2})

Other differential operators such as

\nabla ⋅ F and \nabla x F can be expressed in the coordinates (\sigma, \tau) by substituting the scale factors into the general formulae found in orthogonal coordinates.

Applications

The classic applications of bipolar coordinates are in solving partial differential equations, e.g., Laplace's equation or the Helmholtz equation, for which bipolar coordinates allow a separation of variables (in 2D). A typical example would be the electric field surrounding two parallel cylindrical conductors.

Bibliography

• Book: Margenau H, Murphy GM . 1956 . The Mathematics of Physics and Chemistry . registration. D. van Nostrand . New York . 187 - 190 . 55010911 .
• Book: . 1961 . Mathematical Handbook for Scientists and Engineers . McGraw-Hill . New York . ASIN B0000CKZX7 . 182 . 59014456.
• Book: Moon P, Spencer DE . 1988 . Conical Coordinates (r, θ, λ) . Field Theory Handbook, Including Coordinate Systems, Differential Equations, and Their Solutions . corrected 2nd ed., 3rd print . Springer-Verlag . New York . 978-0-387-18430-2 . true . unknown.

External links

• MathWorld description of bipolar cylindrical coordinates