# Gauss's law

# Gauss's law

In physics, **Gauss's law**, also known as **Gauss's flux theorem ^{[19]} **, is a law relating the distribution of electric charge to the resulting electric field.
The surface under consideration may be a closed one enclosing a volume such as a spherical surface.

The law was first^{[3]} formulated by Joseph-Louis Lagrange in 1773,^{[4]} followed by Carl Friedrich Gauss in 1813,^{[5]} both in the context of the attraction of ellipsoids. It is one of Maxwell's four equations, which form the basis of classical electrodynamics.^{[1]} Gauss's law can be used to derive Coulomb's law,^{[6]} and vice versa.

Qualitative description

In words, Gauss's law states that

`*The netelectric fluxthrough any hypotheticalclosed surfaceis equal to**times the netelectric chargewithin that closed surface*.`

^{[7]}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 the inverse-square Coulomb's law, and Gauss's law for gravity is essentially equivalent to the inverse-square Newton's law of gravity.

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 **E** and the total electric charge, or in terms of the electric displacement field **D** and the *free* electric charge.^{[8]}

Equation involving the E field

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

Integral form

Gauss's law may be expressed as:^{[8]}

where Φ*E* is the electric flux through a closed surface S enclosing any volume V, Q is the total charge enclosed within V, and *ε*0 is the electric constant. The electric flux Φ*E* is defined as a surface integral of the electric field:

where **E** is the electric field, d**A** is a vector representing an infinitesimal element of area of the surface,^{[2]} and · represents the dot product of two vectors.

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

An important fact about this fundamental equation often doesn't find a mention in expositions that are not absolutely diligent. The above equation may fail to hold true in case the closed surface S contains a singularity of the electric field, which is physicists' term for a point in space where either a point charge exists and the field strength approaches infinity, or the field's magnitude or direction gets altered discontinuously due to the existence of a surface charge. In 2011, a modification of the above equation, called the Generalized Gauss's Theorem by its original creator, was published in the proceedings of the 2011 Annual Meeting of Electrostatics Society of America.^{[10]} The Generalized Gauss's Theorem allows the closed surface S to pass through singularities of the electric field. A corollary of the Generalized Gauss's Theorem, known as the simplest form of the Generalized Gauss's Theorem, holds true if the surface S is smooth. It states that

where Q is the net charge enclosed within V and Q' is the net charge contained by the closed surface S itself.

Applying the integral form

If the electric field is known everywhere, Gauss's law makes it possible 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.

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*:

where ∇ · **E** is the divergence of the electric field, ε0 is the electric constant, and ρ is the total electric 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.

Outline of proof |
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The integral form of Gauss' law is:simultaneously true for every possible volumeV, it is necessary (and sufficient) for the integrands to be equal everywhere. Therefore, this equation is equivalent to: |

Equation involving the D 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 **E** (above), is sometimes put into the equivalent form below, which is in terms of **D** and the free charge only.

Integral form

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

where Φ*D* is the **D**-field flux through a surface S which encloses a volume V, and *Q*free is the free charge contained in V. The flux Φ*D* is defined analogously to the flux Φ*E* of the electric field **E** through S:

Differential form

The differential form of Gauss's law, involving free charge only, states:

where ∇ · **D** is the divergence of the electric displacement field, and *ρ*free is the free electric charge density.

Equivalence of total and free charge statements

Proof that the formulations of Gauss's law in terms of free charge are equivalent to the formulations involving total charge. |
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In this proof, we will show that the equationP, which has the following relation toEandD: |

Equation for linear materials

In homogeneous, isotropic, nondispersive, linear materials, there is a simple relationship between **E** and **D**:

where ε is the permittivity of the material. For the case of vacuum (aka free space), *ε* = *ε*0. Under these circumstances, Gauss's law modifies to

for the integral form, and

for the differential form.

Interpretations

In terms of fields of force

Gauss's theorem can be interpreted in terms of the lines of force of the field as follows:

The flux through a closed surface is dependent upon both the magnitude and direction of the electric field lines penetrating the surface. In general a positive flux is defined by these lines leaving the surface and negative flux by lines entering this surface. This results in positive charges causing a positive flux and negative charges creating a negative flux. These electric field lines will extend to infinite decreasing in strength by a factor of one over the distance from the source of the charge squared. The larger the number of field lines emanating from a charge the larger the magnitude of the charge is, and the closer together the field lines are the greater the magnitude of the electric field. This has the natural result of the electric field becoming weaker as one moves away from a charged particle, but the surface area also increases so that the net electric field exiting this particle will stay the same. In other words the closed integral of the electric field and the dot product of the derivative of the area will equal the net charge enclosed divided by permittivity of free space.

Relation to Coulomb's law

Deriving Gauss's law from Coulomb's law

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 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 says 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).

**e**_{r}is the radialunit vector,- ris the radius,|
**r**|, *ε*_{0}is theelectric constant,- qis the charge of the particle, which is assumed to be located at theorigin.

Outline of proof |
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Coulomb's lawstates that the electric field due to a stationarypoint chargeis:rby using an integral to sum the field atrdue to the infinitesimal charge at each other pointsin space, to giver, and use the known theorem^{[11]}δ(r)is theDirac delta function, the result is |

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

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 **E** (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).

Outline of proof |
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TakingSin the integral form of Gauss' law to be a spherical surface of radiusr, centered at the point chargeQ, we haver̂is aunit vectorpointing radially away from the charge. Again by spherical symmetry,Epoints in the radial direction, and so we get |

See also

Method of image charges

Uniqueness theorem for Poisson's equation

## References

*A*. The vector d

**A**is normal to this area element and has magnitude d

*A*. Matthews, Paul (1998).

*Vector Calculus*. Springer. ISBN 3-540-76180-2.

*Leçons sur l'électricité et le magnétisme*(in French). vol. 1, ch. 4, p. 22–23. shows that Lagrange has priority over Gauss. Others after Gauss discovered "Gauss' Law", too.

*Mémoires de l'Académie de Berlin*(in French): 125.

*Theoria attractionis corporum sphaeroidicorum ellipticorum homogeneorum methodo nova tractata*(in Latin). (Gauss,

*Werke*, vol. V, p. 1). Gauss mentions Newton's

*Principia*proposition XCI regarding finding the force exerted by a sphere on a point anywhere along an axis passing through the sphere.

*Fundamentals of Physics*. John Wiley & Sons. pp. 452–453.

*Physics for Scientists and Engineers with Modern Physics*(4th ed.). p. 687.

*Electromagnetism*. Manchester Physics (2nd ed.). John Wiley & Sons. ISBN 978-0-471-92712-9.

*Proceedings of the 2011 ESA Annual Meeting on Electrostatics*: C3.

*Introduction to Electrodynamics*(4th ed.). Prentice Hall. p. 50.

*Gauss's Law Applied to Cylindrical and Planar Charge Distributions*