- Gauss's law
physics, Gauss's law, also known as Gauss's flux theorem, is a law relating the distribution of electric chargeto the resulting electric field. It is one of the four Maxwell's equations, which form the basis of classical electrodynamics, and is also closely related to Coulomb's law. The law was formulated by Carl Friedrich Gaussin 1835, but was not published until 1867. Fact|date=April 2008
Gauss's law has two forms, an "integral form" and a "differential form". They are related by the
divergence theorem, also called "Gauss's theorem". Each of these forms can also be expressed two ways: In terms of a relation between the electric fieldE and the total electric charge, or in terms of the electric displacement fieldD and the "free" electric charge. (The former are given in sections 1 and 2, the latter in Section 3.)
Gauss's law has a close mathematical similarity with a number of laws in other areas of physics. See, for example,
Gauss's law for magnetismand Gauss's law for gravity. In fact, any "inverse-square law" can be formulated in a way similar to Gauss's law: For example, Gauss's law itself is essentially equivalent to the inverse-square Coulomb's law, and Gauss's law for gravity is essentially equivalent to the inverse-square Newton's law of gravity. See the article Divergence theoremfor more detail.
Gauss's law can be used to demonstrate that there is no electric field inside a
Faraday cagewith no electric charges. Gauss's law is something of an electrical analogue of Ampère's law, which deals with magnetism. Both equations were later integrated into Maxwell's equations.
In its integral form (in SI units), the law states that, for any volume "V" in space, with surface "S", the following equation holds::where
* "E,S", called the "
electric fluxthrough "S", is defined by , where is the electric field, and is a differential area on the surface with an outward facing surface normaldefining its direction. (See surface integralfor more details.) The surface "S" is the surface bounding the volume "V".
* is the total
electric chargein the volume "V", including both free chargeand bound charge(bound charge arises in the context of dielectricmaterials; see below).
* is the
electric constant, a fundamental physical constant.
Applying the integral form
If the electric field is known everywhere, Gauss's law makes it quite easy, in principle, to find the distribution of electric charge: The charge in any given region can be deduced by integrating the electric field to find the flux.
However, much more often, it is the reverse problem that needs to be solved: The electric charge distribution is known, and the electric field needs to be computed. This is much more difficult, since if you know the total flux through a given surface, that gives almost no information about the electric field, which (for all you know) could go in and out of the surface in arbitrarily complicated patterns.
An exception is if there is some "symmetry" in the situation, which mandates that the electric field passes through the surface in a uniform way. Then, if the total flux is known, the field itself can be deduced at every point. Common examples of symmetries which lend themselves to Gauss's law include cylindrical symmetry, planar symmetry, and spherical symmetry. See the article
Gaussian surfacefor examples where these symmetries are exploited to compute electric fields.
In differential form, Gauss's law states:
*E is the
* is the total electric charge density (in units of C/m³), including both free and bound charge (see below).
* is the
electric constant, a fundamental constant of nature.
This is mathematically equivalent to the integral form, because of the
Gauss's law in terms of free charge
Note on free charge versus bound charge
The electric charge that arises in the simplest textbook situations would be classified as "free charge"—for example, the charge which is transferred in
static electricity, or the charge on a capacitorplate. In contrast, "bound charge" arises only in the context of dielectric(polarizable) materials. (All materials are polarizable to some extent.) When such materials are placed in an external electric field, the electrons remain bound to their respective atoms, but shift a microscopic distance in response to the field, so that they're more on one side of the atom than the other. All these microscopic displacements add up to give a macroscopic net charge distribution, and this constitutes the "bound charge".
Although microscopically, all charge is fundamentally the same, there are often practical reasons for wanting to treat bound charge differently from free charge. The result is that the more "fundamental" Gauss's law, in terms of E, is sometimes put into the equivalent form below, which is in terms of D and the free charge only. For a detailed definition of free charge and bound charge, and the proof that the two formulations are equivalent, see the "proof" section below.
This formulation of Gauss's law states that, for any volume "V" in space, with surface "S", the following equation holds::where
* is defined by , where is the
electric displacement field, and the integration is a surface integral.
* is the free electric charge in the volume "V", not including
bound charge(see below).
The differential form of Gauss's law, involving free charge only, states::
*D is the
electric displacement field(in units of C/m²), and * is the "free" electric charge density (in units of C/m³), not including the bound charges in a material.
The differential form and integral form are mathematically equivalent. The proof primarily involves the
Proof of equivalence
In linear materials
In homogeneous, isotropic, nondispersive,
linear materials, there is a nice, simple relationship between E and D::where is the " permittivity" of the material. Under these circumstances, there is yet another pair of equivalent formulations of Gauss's law:::
Relation to Coulomb's law
Deriving Gauss's law from Coulomb's law
Gauss's law can be derived from
Coulomb's law, which states that the electric field due to a stationary point chargeis:
:where:er is the radial
unit vector,:"r" is the radius, |r|,: is the electric constant,:"q" is the charge of the particle, which is assumed to be located at the origin.
Using the expression from Coulomb's law, we get the total field at r by using an integral to sum the field at r due to the infinitesimal charge at each other point s in space, to give
where is the charge density. If we take the divergence of both sides of this equation with respect to r, and use the known theorem [See, for example, cite book | author=Griffiths, David J. | title=Introduction to Electrodynamics (3rd ed.) | publisher=Prentice Hall | year=1998 | id=ISBN 0-13-805326-X | page=50]
:where δ(s) is the
Dirac delta function, the result is
Using the "sifting property" of the Dirac delta function, we arrive at
which is the differential form of Gauss's law, as desired.
Note that since Coulomb's law only applies to "stationary" charges, there is no reason to expect Gauss's law to hold for moving charges "based on this derivation alone". In fact, Gauss's law "does" hold for moving charges, and in this respect Gauss's law is more general than Coulomb's law.
Deriving Coulomb's law from Gauss's law
Coulomb's lawcannot be derived from Gauss's law alone, since Gauss's law does not give any information regarding the curl of E (see Helmholtz decompositionand Faraday's law). However, Coulomb's law "can" be proven from Gauss's law if it is assumed, in addition, that the electric field from a point chargeis spherically-symmetric (this assumption, like Coulomb's law itself, is exactly true if the charge is stationary, and approximately true if the charge is in motion).
Taking "S" in the integral form of Gauss's law to be a spherical surface of radius "r", centered at the point charge "Q", we have: By the assumption of spherical symmetry, the integrand is a constant which can be taken out of the integral. The result is: where is a
unit vectorpointing radially away from the charge. Again by spherical symmetry, E points in the radial direction, and so we get: which is essentially equivalent to Coulomb's law.
inverse-square lawdependence of the electric field in Coulomb's lawfollows from Gauss's law.
Carl Friedrich Gauss
Method of image charges
Jackson, John David (1999). Classical Electrodynamics, 3rd ed., New York: Wiley. ISBN 0-471-30932-X.
* [http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-and-MagnetismSpring2002/VideoAndCaptions/ MIT Video Lecture Series (30 x 50 minute lectures)- Electricity and Magnetism] Taught by Professor
* [http://www.lightandmatter.com/html_books/0sn/ch10/ch10.html#Section10.6 section on Gauss's law in an online textbook]
* [http://184.108.40.206/home/modules/pdf_modules/m132.pdf MISN-0-132 "Gauss's Law for Spherical Symmetry"] (PDF file) by Peter Signell for [http://www.physnet.org Project PHYSNET] .
* [http://220.127.116.11/home/modules/pdf_modules/m133.pdf MISN-0-133 "Gauss's Law Applied to Cylindrical and Planar Charge Distributions] (PDF file) by Peter Signell for Project PHYSNET.
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