# Potential functions

Can someone please explain what a potential is? Example. velocity potential in ideal flows, acoustic potential (gradient of which gives the particle velocity in a sound wave). Whenever I see potential functions complex analysis is applied to compute integrals, complex functions for conformal mapping. I vaguely understand that potential functions are independent of path and in complex plane the integration around a contour is independent of path as well (the integration depends only on the end points, hence in a closed curve say region $Z$, if function, $f$ is analytic then integration of $f$ over $Z$ is 0). Can you please explain on this?

• can you give some reference to such functions? It might be helpful – Santosh Linkha Dec 22 '10 at 6:09
• Consider a scalar function, phi $\phi$, in fluids the velocity(a vector quantity) of flow is given by grad($\phi$). Such functions are called as the scalar potential. – vijay Dec 22 '10 at 6:21
• You may want to be a little bit more specific in your question as the explanation for 'what is a potential?' is considerably different from the explanation to 'what is the velocity potential in ideal flows?'. By the way, for the last question, Stream function is a good starter. – Robert Smith Dec 22 '10 at 6:28

Potential is a special case of a more general construction in differential geometry. Let's start abstractly and we'll get to the potentials again at the end.

## Differential forms

The framework of differential forms provides a basis for integration on arbitrary manifold. Differential $p$-forms are totally antisymmetric covariant $p$-tensors. What's special about them is that you can define exterior derivative taking $p$-form to $(p+1)$-form. Now, $0$-forms are just functions and $1$-forms are like usual vectors (if you have a metric on your manifold, as is the case e.g. in Euclidean space, you can freely move between forms (also called covectors) and vectors).

Now, exterior derivative on functions gives us a 1-form. Let's say ${\rm d}f = \alpha$. The important point about the operator $\rm d$ is that it is nilpotent ${\rm d}^2 = 0$. So this means that ${\rm d} \alpha = {\rm d}^2 f = 0$. Important point is that this relation can be reversed (at least on topologically trivial manifolds that don't have any holes in them, and so on -- see Poincaré lemma for more information. But as always, Euclidean space is fine): if you have some form $\alpha$ such that ${\rm d} \alpha = 0$ (we say that the form is closed) then there will exist another form $\beta$ such that $\alpha = {\rm d} \beta$ (we say that the form is exact). So this is you potential in a general setting.

Now to understand why potentials are useful in the first place we have to talk a bit about integration. It is possible to integrate $n$-forms on $n$-dimensional manifolds (the reason for this is that they have the similar transformation properties to Jacobian of usual integration substitution). So if you have some $p$-form $\alpha$ you can integrate it over some $p$-dimensional subset $U$ of the manifold and this is denoted by $\int_U \alpha$. The punchline is that if $\alpha$ has a potential $\alpha = {\rm d} \beta$ we can use Stokes' theorem that tells us

$$\int_U \alpha = \int_U {\rm d} \beta = \int_{\partial U} \beta$$.

where $\partial U$ is a boundary of the given subset. So that we can transform some integrals into others which can often simplify calculations.

## Physics

To connect again with your questions: potentials arise from closed forms. The closedness conditions can take various guises in standard vector formalism. The usual one is for conservative forces $\nabla \times {\mathbf F} = 0$. This can be translated to the language of the differential forms as the condition on $\mathbf F$ being closed and so we know that there must be another form, say $\phi$ such that ${\mathbf F} = \nabla \phi$ (notice the identity $\nabla \times \nabla \phi = 0$ -- this is our good old ${\rm d}^2 = 0$ in action again). Because of the Stoke's theorem we know that the usefulness of the concept of conservative forces stems from the fact that their integral over a closed path doesn't depend on the path (this is a trivial consequence of $U$ having no boundary in that case).

Another famous closed form is magnetic induction ${\mathbf B}$ because there are no monopoles (yet): $\nabla \cdot {\mathbf B} = 0$. This gives us

$${\mathbf B} = \nabla \times {\mathbf A}$$

where $\mathbf A$ is a vector potential. Again by using Stokes' theorem we can find that flow of $\mathbf B$ through any closed surface is zero.

Note: it might seems strange that we integrate vector $\mathbf B$ over a surface which is two-dimensional. This is not how we defined the integration for forms. But $\mathbf B$ is actually a two-form (you can see this from its relation to $\mathbf A$ which is a genuine one-form) and one is exploiting that in three-dimensional space one can identify these with one-forms. This is actually the usual identification of antisymmetric $3\times 3$ matrix with a pseudovector.

## Complex analysis

The complex analysis is very similar (although slightly harder) setting. The complex plane can be regarded as a two-dimensional real manifold so that there are two linearly independent one-forms: holomorphic and antiholomorphic forms ${\rm d}z$ and ${\rm d} \bar z$ where $z = x + iy$ and $\bar z = x - iy$. It can be shown that holomorphic functions satisfy $\partial_{\bar z} f = 0$ and that any holomorphic form $h = f {\rm d}z$ is closed so this gives the Cauchy formula $$\oint_{\gamma} f {\rm d}z = 0$$ as a special case of Stokes' theorem.

Alternatively, one can exploit the full power of the complex formalism by using both holomorphic and antiholomorphic functions to encode any information that can be found in a plane which is usually described by functions ${\mathbb R}^2 \to {\mathbb R}^2$ as functions ${\mathbb C} \to {\mathbb C}$. It is again possible to translate all the language of differential forms and potentials into the complex setting.

• +1 insightful and correct. Hopefully the level of the mathematics is consistent with the level understood by the OP. – Sklivvz Dec 23 '10 at 11:38
• @Sklivvz: Thanks. Though I'd like to mention that I am open to elaborating on and simplifying parts of my answer if OP so wishes. – Marek Dec 23 '10 at 12:56

Your question needs improvement, but I'll try to help with an answer. You're not really asking one thing specifically, so I'll try clarify a few points.

• The word "potential" doesn't necessarily mean anything it needs to be part of an expression.
• Holomorphic functions, which you usually study in complex analysis, and conformal mapping is a common way of find the static electric potential for complicated geometries.
• The contour integration of a vector field that's the gradient of a scalar function (called the potential) does not depend on the shape of the curve, only on its end points.
• Countour integration of a scalar analytic function over a closed curve on the complex plane also does not depend on the shape of the curve.
• The velocity vector in fluid dynamics is a vector field. Sometimes, it is convenient to define this vector field as the gradient of a scalar field, which might be called the velocity potential (I'm not sure).
• Acoustic waves are waves in the vector field of a fluid, so the same as the above applies.

If you wanted a more in depth explanation on one of these, please specify what you want to know.