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# The Flux and Divergence of a Vector

Consider a vector that passes through a specified surface. The vector could be the velocity of a fluid, then the flux of would be the amount of fluid passing through that surface in unit time. The flux, , of a vector through a surface element is defined as
 (1.15)

where is normal to the surface element as shown in Figure 1.10

Hence,
 (1.16)

and is the component of normal to the surface. Note that the largest value of occurs when and if so that in the latter case does not cross the surface at all but is instead tangent to the surface. If the vector is tangent to the surface, then the flux through the surface is obviously zero.

Example 1. .6Consider . This is the MASS FLUX of a fluid or plasma and is the RATE at which fluid crosses the surface . Let us consider the units of in this example.

Consider , the magnetic field, then is the amount of magnetic flux crossing .

The total flux of a vector across a surface (rather than just the surface element) is

 (1.17)

There are two types of surfaces that we consider, namely open surfaces and closed surfaces. A closed surface encloses a volume , as shown in Figure 1.11.

If bounds a finite volume, as in the closed surface case, then points outwards from the volume. In this case the Divergence Theorem of Gauss (Carl Gauss, 1777-1855) gives
 (1.18)

where
 (1.19)

is the divergence of . Hence, represents the outgoing flux per unit volume and we have two important deductions, namely
1. If , a flow is diverging (fluid is flowing out of a volume).

2. If , a flow is converging (fluid is flowing into a volume).

Example 1. .7

1. Consider a fluid of uniform density, , flowing with a velocity given by . What is the mass flux across the square , , ? The situation is shown in Figure 1.12.

Here and so

2. As in the above example but with a density given by . Hence,

Magnetic Flux Tubes are the building blocks of a magnetic field. A magnetic flux tube (for the magnetic field ) or a flow tube (for the mass flux ) is the surface generated by a set of field lines that intersect a simple closed curve, as shown in Figure 1.13.

The magnetic flux crossing is and the total magnetic flux is . Since the flux tube is defined by the magnetic field lines, the walls of the flux tube are parallel to . Hence, if there is no flux created inside the tube, then we may state that

So the magnetic flux is constant along the tube. To prove this, consider a closed surface defined by , and the walls. Hence, the flux through this closed surface is

The last integral is zero since is parallel to the walls. The first integral on the right hand side has a negative sign. This occurs because the flux through into the tube has a direction that is opposite to the direction of the unit normal that is out of the closed surface. Now we make use of Guass's divergence theorem so that

However, the basic equation for magnetic fields, see later, is

Hence, we have

and so
 (1.20)

If the cross-sectional area is small, so that is approximately constant, then the flux is approximately

where is the cross-sectional area and is the field strength. Thus, if increases, the area decreases such that remains constant.

Example 1. .8For the magnetic field given by , the field lines are given by

Therefore,

The flux tube passing through , and has flux

We should check the flux passing through . We need to know where the field lines at have gone to at .

Therefore,

Thus, the flux through equals the flux through .

Next: Curl , , Triple Up: Introduction Previous: The Gradient Operator,
Prof. Alan Hood
2000-11-06