# Gauss's law

In physics, **Gauss's law**, also known as **Gauss's flux theorem**, 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^{[1]} formulated by Joseph-Louis Lagrange in 1773,^{[2]} followed by Carl Friedrich Gauss in 1813,^{[3]} both in the context of the attraction of ellipsoids. It is one of Maxwell's four equations, which form the basis of classical electrodynamics.^{[note 1]} Gauss's law can be used to derive Coulomb's law,^{[4]} and vice versa.

## Contents

## Qualitative descriptionEdit

In words, Gauss's law states that

*The net electric flux through any hypothetical closed surface is equal to * *times the net electric charge within that closed surface*.^{[5]}

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.^{[6]}

## Equation involving the E fieldEdit

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 formEdit

Gauss's law may be expressed as:^{[6]}

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,^{[note 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*.

#### Applying the integral formEdit

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 formEdit

By the divergence theorem, Gauss's law can alternatively be written in the *differential form*:

where ∇ · **E** is the divergence of the electric field, ε_{0} is the electric constant, and ρ is the total electric charge density (charge per unit volume).

### Equivalence of integral and differential formsEdit

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

Outline of proof The integral form of Gauss' law is: for any closed surface S containing charge Q. By the divergence theorem, this equation is equivalent to:

for any volume V containing charge Q. By the relation between charge and charge density, this equation is equivalent to:

for any volume V. In order for this equation to be

*simultaneously true*for*every*possible volume V, it is necessary (and sufficient) for the integrands to be equal everywhere. Therefore, this equation is equivalent to:Thus the integral and differential forms are equivalent.

## Equation involving the D fieldEdit

### Free, bound, and total chargeEdit

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 formEdit

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 formEdit

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 statementsEdit

Proof that the formulations of Gauss's law in terms of free charge are equivalent to the formulations involving total charge. In this proof, we will show that the equation is equivalent to the equation

Note that we are only dealing with the differential forms, not the integral forms, but that is sufficient since the differential and integral forms are equivalent in each case, by the divergence theorem.

We introduce the polarization density

**P**, which has the following relation to**E**and**D**:and the following relation to the bound charge:

Now, consider the three equations:

The key insight is that the sum of the first two equations is the third equation. This completes the proof: The first equation is true by definition, and therefore the second equation is true if and only if the third equation is true. So the second and third equations are equivalent, which is what we wanted to prove.

## Equation for linear materialsEdit

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.

## InterpretationsEdit

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### In terms of fields of forceEdit

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 form 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 filed. 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 lawEdit

### Deriving Gauss's law from Coulomb's lawEdit

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).

Outline of proof Coulomb's law states that the electric field due to a stationary point charge is: where

**e**_{r}is the radial unit vector,- r is the radius, |
**r**|, *ε*_{0}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 givewhere ρ is the charge density. If we take the divergence of both sides of this equation with respect to

**r**, and use the known theorem^{[8]}where

*δ*(**r**) is the Dirac delta function, the result isUsing the "sifting property" of the Dirac delta function, we arrive at

which is the differential form of Gauss' 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 lawEdit

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 Taking S in the integral form of Gauss' 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

**r̂**is a unit vector pointing radially away from the charge. Again by spherical symmetry,**E**points in the radial direction, and so we getwhich is essentially equivalent to Coulomb's law. Thus the inverse-square law dependence of the electric field in Coulomb's law follows from Gauss' law.

## See alsoEdit

## NotesEdit

**^**The other three of Maxwell's equations are: Gauss' law for magnetism, Faraday's law of induction, and "Ampère"'s law with Maxwell's correction**^**More specifically, the infinitesimal area is thought of as planar and with area d*A*. The vector d**A**is normal to this area element and has magnitude d*A*.^{[7]}

## CitationsEdit

**^**Duhem, Pierre.*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.**^**Lagrange, Joseph-Louis (1773). "Sur l'attraction des sphéroïdes elliptiques".*Mémoires de l'Académie de Berlin*(in French): 125.**^**Gauss, Carl Friedrich.*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.**^**Halliday, David; Resnick, Robert (1970).*Fundamentals of Physics*. John Wiley & Sons. pp. 452–453.**^**Serway, Raymond A. (1996).*Physics for Scientists and Engineers with Modern Physics*(4th ed.). p. 687.- ^
^{a}^{b}Grant, I. S.; Phillips, W. R. (2008).*Electromagnetism*. Manchester Physics (2nd ed.). John Wiley & Sons. ISBN 978-0-471-92712-9. **^**Matthews, Paul (1998).*Vector Calculus*. Springer. ISBN 3-540-76180-2.**^**See, for example, Griffiths, David J. (2013).*Introduction to Electrodynamics*(4th ed.). Prentice Hall. p. 50.

## ReferencesEdit

- Jackson, John David (1998).
*Classical Electrodynamics*(3rd ed.). New York: Wiley. ISBN 0-471-30932-X. David J. Griffiths (6th ed.)

## External linksEdit

- 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
- 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.