Gauss's law

Gauss's law in its integral form is particularly useful when, by symmetry reasons, a closed surface (GS) can be found along which the electric field is uniform. The electric flux is then a simple product of the surface area and the strength of the electric field, and is proportional to the total charge enclosed by the surface. Here, the electric field outside (r > R) and inside (r < R) of a charged sphere is being calculated (see Wikiversity).

In physics (specifically electromagnetism), Gauss's law, also known as Gauss's flux theorem (or sometimes Gauss's theorem), is one of Maxwell's equations. It is an application of the divergence theorem, and it relates the distribution of electric charge to the resulting electric field.

Definition

In its integral form, it states that the flux of the electric field out of an arbitrary closed surface is proportional to the electric charge enclosed by the surface, irrespective of how that charge is distributed. Even though the law alone is insufficient to determine the electric field across a surface enclosing any charge distribution, this may be possible in cases where symmetry mandates uniformity of the field. Where no such symmetry exists, Gauss's law can be used in its differential form, which states that the divergence of the electric field is proportional to the local density of charge.

The law was first formulated by Joseph-Louis Lagrange in 1773, followed by Carl Friedrich Gauss in 1835, both in the context of the attraction of ellipsoids. It is one of Maxwell's equations, which forms the basis of classical electrodynamics. Gauss's law can be used to derive Coulomb's law, and vice versa. Template:Electromagnetism

Qualitative description

In words, Gauss's law states:

The net electric flux through any hypothetical closed surface is equal to Template:Math times the net electric charge enclosed within that closed surface. The closed surface is also referred to as Gaussian surface.

Gauss's law has a close mathematical similarity with a number of laws in other areas of physics, such as Gauss's law for magnetism and 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 Coulomb's law, and Gauss's law for gravity is essentially equivalent to Newton's law of gravity, both of which are inverse-square laws.

The law can be expressed mathematically using vector calculus in integral form and differential form; both are equivalent since they are related by the divergence theorem, also called Gauss's theorem. Each of these forms in turn can also be expressed two ways: In terms of a relation between the electric field Template:Math and the total electric charge, or in terms of the electric displacement field Template:Math and the free electric charge.

Equation involving the Template:Math field

Gauss's law can be stated using either the electric field Template:Math or the electric displacement field Template:Math. This section shows some of the forms with Template:Math; the form with Template:Math is below, as are other forms with Template:Math.

Integral form

Electric flux through an arbitrary surface is proportional to the total charge enclosed by the surface.

No charge is enclosed by the sphere. Electric flux through its surface is zero.

Gauss's law may be expressed as:

$${\mathrm{\Phi }}_E=\frac{Q}{{\epsilon }_0}$$

where Template:Math is the electric flux through a closed surface Template:Mvar enclosing any volume Template:Mvar, Template:Mvar is the total charge enclosed within Template:Mvar, and Template:Math is the electric constant. The electric flux Template:Math is defined as a surface integral of the electric field:

Template:Oiint

where Template:Math is the electric field, Template:Math is a vector representing an infinitesimal element of area of the surface,Template:Refn and Template:Math represents the dot product of two vectors.

In a curved spacetime, the flux of an electromagnetic field through a closed surface is expressed as

Template:Oiint

where $c$ is the speed of light; $F^{\kappa 0}$ denotes the time components of the electromagnetic tensor; $g$ is the determinant of metric tensor; $\mathrm{d}{S}_{\kappa }=\mathrm{d}{S}^{ij}=\mathrm{d}{x}^i\mathrm{d}{x}^j$ is an orthonormal element of the two-dimensional surface surrounding the charge $Q$; indices $i,j,\kappa =1,2,3$ and do not match each other.

Since the flux is defined as an integral of the electric field, this expression of Gauss's law is called the integral form.

A tiny Gauss's box whose sides are perpendicular to a conductor's surface is used to find the local surface charge once the electric potential and the electric field are calculated by solving Laplace's equation. The electric field is perpendicular, locally, to the equipotential surface of the conductor, and zero inside; its flux πa²·E, by Gauss's law equals πa²·σ/ε₀. Thus, Template:Nowrap.

In problems involving conductors set at known potentials, the potential away from them is obtained by solving Laplace's equation, either analytically or numerically. The electric field is then calculated as the potential's negative gradient. Gauss's law makes it possible to find the distribution of electric charge: The charge in any given region of the conductor can be deduced by integrating the electric field to find the flux through a small box whose sides are perpendicular to the conductor's surface and by noting that the electric field is perpendicular to the surface, and zero inside the conductor.

The reverse problem, when the electric charge distribution is known and the electric field must be computed, is much more difficult. The total flux through a given surface gives little information about the electric field, and can go in and out of the surface in arbitrarily complicated patterns.

An exception is if there is some symmetry in the problem, 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 surface for examples where these symmetries are exploited to compute electric fields.

Differential form

By the divergence theorem, Gauss's law can alternatively be written in the differential form: $$\mathrm{\nabla }\cdot 𝐄=\frac{\rho }{{\epsilon }_0}$$

where Template:Math is the divergence of the electric field, Template:Math is the vacuum permittivity and Template:Mvar is the total volume charge density (charge per unit volume).

Equivalence of integral and differential forms

The integral and differential forms are mathematically equivalent, by the divergence theorem. Here is the argument more specifically.

Equation involving the Template:Math field

Free, bound, and total 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 capacitor plate. 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 Template:Math (above), is sometimes put into the equivalent form below, which is in terms of Template:Math and the free charge only.

Integral form

This formulation of Gauss's law states the total charge form:

$${\mathrm{\Phi }}_D=Q_{\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{e}}$$

where Template:Math is the [[electric displacement field|Template:Math-field]] flux through a surface Template:Mvar which encloses a volume Template:Mvar, and Template:Math is the free charge contained in Template:Mvar. The flux Template:Math is defined analogously to the flux Template:Math of the electric field Template:Math through Template:Mvar:

Template:Oiint

Differential form

The differential form of Gauss's law, involving free charge only, states: $$\mathrm{\nabla }\cdot 𝐃={\rho }_{\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{e}}$$

where Template:Math is the divergence of the electric displacement field, and Template:Math is the free electric charge density.

Equation for linear materials

In homogeneous, isotropic, nondispersive, linear materials, there is a simple relationship between Template:Math and Template:Math:

$$𝐃=\epsilon 𝐄$$

where Template:Mvar is the permittivity of the material. For the case of vacuum (aka free space), Template:Math. Under these circumstances, Gauss's law modifies to

$${\mathrm{\Phi }}_E=\frac{Q_{\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{e}}}{\epsilon }$$

for the integral form, and

$$\mathrm{\nabla }\cdot 𝐄=\frac{{\rho }_{\mathrm{f}\mathrm{r}\mathrm{e}\mathrm{e}}}{\epsilon }$$

for the differential form.

Relation to Coulomb's law

Deriving Gauss's law from Coulomb's law

Template:Citation needed Strictly speaking, Gauss's law cannot be derived from Coulomb's law alone, since Coulomb's law gives the electric field due to an individual, electrostatic point charge only. However, Gauss's law can be proven from Coulomb's law if it is assumed, in addition, that the electric field obeys the superposition principle. The superposition principle states that the resulting field is the vector sum of fields generated by each particle (or the integral, if the charges are distributed smoothly in space).

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

Strictly speaking, Coulomb's law cannot be derived from Gauss's law alone, since Gauss's law does not give any information regarding the curl of Template:Math (see Helmholtz decomposition and 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 charge is 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).

See also

• Method of image charges
• Uniqueness theorem for Poisson's equation
• List of examples of Stigler's law

References

• Template:Cite book Digital version
• Template:Cite book David J. Griffiths (6th ed.)

External links

• Template:Commons category-inline
• MIT Video Lecture Series (30 x 50 minute lectures)- Electricity and Magnetism Taught by Professor Walter Lewin.
• section on Gauss's law in an online textbook Template:Webarchive
• MISN-0-132 Gauss's Law for Spherical Symmetry (PDF file) by Peter Signell for Project PHYSNET.
• MISN-0-133 Gauss's Law Applied to Cylindrical and Planar Charge Distributions (PDF file) by Peter Signell for Project PHYSNET.