Gauss's law involves only the three figures in the column to the left. The Gaussian surfaces are "closed" because each surface separates an "inside" volume from the outside. This permits us to define as an outward unit normal, as well as the vector surface area differential
Gauss's law equates any integral involving the electric field over a closed surface to an integral of the electric charge density enclosed by that closed surface.
In contrast to the situation with one-dimensional integrals encountered in a first-year calculus course, it is not possible to specify the limit of integration using two indexes at the top and bottom of the integral sign. Henceforth we shall use a superscript on the integral sign, with the understanding that the reader will understand the details from the context of the discussion. In this case, GS refers to a Gaussian surface, and EV denotes the volume enclosed by the same Gaussian surface. The Greek lower-case tau is sometimes used to denote a volume element, which can also be expressed in Cartesian, cylindrical, and spherical coordinates, respectively as:
The use of a tilde (or prime) on variables of integration is not mandatory, but sometimes useful to remind ourselves that these variables are arbitrary and do not appear when the final result is obtained. In our case, we will use a bit of intuition to deduce the electric field at some point in space, using surface and volume integrals where the variables of integration might be
Fortunately, the charge distributions discussed here will be so symmetrically simple that the integration may be performed virtually without calculus.
The field point can be either inside or outside the charged sphere of radius The Gaussian sphere is denoted as GS.
Shown to the right is a uniformly charged sphere of radius and a concentric Gaussian sphere of radius . The surface of this Gaussian sphere identifies the location where we want to calculate the electric field: We seek to find , where is called the field point. To be useful in calculating the field, field point must lie on the Gaussian surface. The four red arrows on the Gaussian sphere indicate the direction of the outward unit normal.
Since the Gaussian sphere has a radius of we shall replace by the symbol and use the fact that the electric is field is uniform and always parallel to the outward unit normal to write where is constant and is an element of area. Since the surface area of a circle is we have:
, or the charge enclosed by Gaussian surface, is obtained by integrating charge density over larger sphere of radius . But the charge density is non-zero only within the smaller sphere of radius . In other words, the function is discontinuous for a uniformly charged sphere of radius :
where is the volume of a sphere. Once these steps have been digested by the reader, they can be combined into a simple two step calculation based on Gauss's law for the electric field outside a charged sphere:
If the field point is inside the charged surface, the enclosed charge includes only the portion of the charge inside the Gaussian sphere. First we calculate the uniform charge density inside the sphere:
Now the enclosed charge includes only the volume inside the Gaussian sphere of radius . Since the sphere is uniformly charged, is a constant that can be taken out integral sign:
Shown here is the case for , i.e., where the field point is outside the source charge.
In cylindrical coordinates a symmetry exists if the field variables do not depend on or . This requires that the system extend to infinity in both directions on the z-axis. In the case of a charged cylinder, we may use a Gaussian Surface that is a cylinder of finite length, . Such a cylinder has three distinct surfaces: A hollow cylinder with a curved surface of length , and two flat circular discs at each end of the same radius. But if the field is in the radial direction only, there is no flux through the two ends.
The charge enclosed by a Gaussian surface of length depends on whether the surface is inside or outside the charged sphere. The area of the cylinder's curved surface is the product of circumference and length:
Here the charge is occupies the plane at . The Gaussian surface is a box of length in the x direction and occupies and area A in the yz plane.
This so-called "slab geometry" is extremely important for two reasons. First, it models both the parallel plate capacitor, as well as the field near a flat metal surface. Perhaps more significant is that it leads directly to a superior form of Maxwell's equations that uses partial derivatives instead of integrals.
Here we make three assumptions:
The charge density depends only on the x variable:
The electric field depends also depends only on x.
And, the electric field points only in the x-direction:
The charged object is shown as a thin sheet that extends between . First we apply Gauss's law for a field point outside the charged sheet. The GS is the six-sided box with area and length as shown in the figure. But since the electric field is in the x-direction, our surface integral involves only the two faces of area Since these faces have opposite outward unit normals, the sum of the fluxes becomes a difference in electric fields:
Defining the infinite plane: The disk has an "infinite" radius when viewed from a distance z along the axis if r>>z. If the disc is charged and z>>r, it can be viewed in approximation as a point charge.
where is the surface charge density associated with the sheet, and represents the field to the right of the sheet. The ambiguity of this equation needs to be understood. In the previous examples we could resolve such ambiguities with a symmetry argument. It can be easily argued that an isolated charge with spherical symmetry would generate a field that points towards or away from the center. But an isolated infinite plane is difficult to imagine, as discussed in the figure to the right. This ambiguous formula has two important applications. The most obvious is the symmetric case where equal and opposite field exist on both sides of an isolated plane of surface charge:
If you ignore the fact that matter is made of atoms, it is possible to imagine a point so close to a curved surface that the boundary is effectively "flat" that a charged surface can be modeled as in infinite plane of uniform charge.
A more common application calls for the electric field to vanish on one side of the plate. This can occur on the smooth surface of a solid conducting object, and also between the plates of a parallel plate capacitor. In that case:
The previous examples illustrate how a student can quickly calculate the electric field for simple geometries. A hint as to the profound impact these methods have had on physics can be illustrated by considering how the x-component of the electric field is related to a charge density that only depends on x: Let us select an arbitrary value of x and consider a box shaped Gaussian surface that occupies a large area in the plane, but has a small differential length of along the x axis. Gauss's law is:
Though the result is beyond the scope of most first-year physics courses, the vector methods introduced in this course permit one to derive one of the four Maxwell's equations in differential form:
Other integrals associated with electromagnetism relate to the three figures in the column to the right. The red paths are called "closed" because they do not terminate at a point, but loop around indefinitely. They from the boundaries of the open surfaces.
Here, the effective current includes not only the conventional current carried by moving charge, , but also a term introduced by Maxwell to ensure that these four integral equations are self-consistent:
where is the speed of light in vacuum. The line integral associated with Ampere's law is marked in red and denoted by in the figures to the right. The right hand rule identifies the direction of the differential area element In contrast with the situation in Gauss's law, where the Gaussian surface uniquely defines the volume, the surface integral is valid for any smooth surface that has the Ampere's loop as its boundary (for example the top and bottom surfaces in the column to the right have the same boundary, which is a circle.) Since we are mostly interested in using these integral laws to quickly calculate electromagnetic fields for simple geometries, we will usually integrate over flat surfaces.
Magnetic field due to a straight current-carrying wireEdit
A steady current in a long cylinder of radius creates an azimuthal magnetic field at radius Shown is the case for
The integral integral of around the circle of radius in the figure shown to the right is related to the current, carried by the wire of radius is:
This is valid when the "Ampere loop" lies outside the cylindrical wire. If the current density is uniform inside the wire, we may define the current density as a discontinuous function:
There are a number of equivalent ways to express :
When the field point is inside the wire , we can express Ampere's law using a circle of radius
We can avoid the fallacy of circular reasoning by beginning with the somewhat correct representation of magnetic field due to a solenoid, shown to the left. The circles with dots indicate wires with current coming towards the reader. We see that near the center of this solenoid the magnetic field is relatively (1) strong, (2) horizontal, and (3) uniform with respect to position.
At this point, we postulate that the magnetic field away from the edges of a long, thin solenoid is significant only inside the solenoid, and that the magnetic field is parallel to the axis. Our Ampere's loop is a rectangle with four sides, but contributes to the integral only for a length inside the solenoid. Defining to be the number of turns per unit length, we have:
The figure to the right shows a rectangular toroid, but regardless of the shape, the ampere loop shown establishes a magnetic field that depends on the distance from a vertical axis running though the center:
The Ampere loop if labeled circles a wire carrying a current I. The indicates that it is the boundary of an open surface that get pierced by this current. But this time, we consider two different surfaces with the same
boundary. The surface is the area of the circle defined by , and it is pierced by an actual current shown in the figure. But if the surface is deformed into it instead goes through the capacitor, where a time-varying electric exists, but no current is present. Maxwell concluded that another way to generate a magnetic field was to have a time-varying electric field be present in the surface integral. Hence, he postulated that:
Maxwell reasoned that since charge creates an electric field, and current , it was the electric field between the plates that created the displacement current. Hence, for some unknown constant :
To to show that this constant we use the fact that Gauss's law yields a constant electric field for a parallel plate capacitor:
Hence, "Maxwell's missing term" is:
Having rigorously derived this expression for the ideal parallel plate capacitor, it seems reasonable to speculate that his law holds for non-uniform electric fields.
↑Outline of proof:In two dimensions express two different coordinate systems that differ by a rotation about the z axis: . The proof is not difficult if you restrict yourself to two dimensions and take inner products between the associated unit vectors The only complication is that you need to convince yourself derivatives transform as: . The next step is to prove that transforms into