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What is flux

  1. Jul 24, 2014 #1
    Definition/Summary

    Flux sometimes means total flow through a surface (a scalar), and sometimes means flow per unit area (a vector).

    In electromagnetism, flux always means total flow through a surface (a scalar), and is measured in webers (magnetic flux) or volt-metres (electric flux).

    Scalar flux is the amount of a vector field going through a surface: it is the integral (over the surface) of the normal component of the field: [itex]\Phi\ =\ \oint_S\mathbf{E}\cdot d\mathbf{A}[/itex]

    For a closed surface, this equals (Gauss' theorem, or the divergence theorem) the integral (over the interior) of the divergence of the field: [itex]\Phi\ =\ \int\int\int_V \mathbf{\nabla}\cdot\mathbf{E}\,dxdydz[/itex].

    Therefore the scalar flux, through a closed surface, of an electric field is proportional to the enclosed charge (Gauss' law: [itex]\Phi_{E}\ =\ Q_{total}/\varepsilon_0,\ \ \Phi_{D}\ =\ Q_{free}/\varepsilon_0,\ \ \Phi_{P}\ =\ -Q_{bound}/\varepsilon_0[/itex]), and of a magnetic field is zero (Gauss' law for magnetism: [itex]\Phi_{B}\ =\ \Phi_{H}\ =\ \Phi_{M}\ =\ 0[/itex]).

    Equations

    FLUX THROUGH A CLOSED SURFACE, S:

    Gauss' Law:

    [tex]\Phi_\mathbf{E}(S)\ =\ \oint_S\mathbf{E}\cdot d\mathbf{A}\ =\ Q/\varepsilon_0[/tex]

    Gauss' Law for Magnetism:

    [tex]\Phi_\mathbf{B}(S)\ =\ \oint_S\mathbf{B}\cdot d\mathbf{A}\ =\ 0[/tex]

    RATE OF CHANGE OF FLUX THROUGH A CLOSED CURVE, C:

    Ampère-Maxwell Law:

    [tex]\mu_0\varepsilon_0\frac{\partial\Phi_\mathbf{E}(S)}{\partial t}\ =\ \mu_0\varepsilon_0\frac{\partial}{\partial t}\int_S\mathbf{E}\cdot d\mathbf{A}\ =\ \oint_C\mathbf{B}\cdot d\mathbf{\ell}\ -\ \mu_0I[/tex]

    Faraday's law:

    [tex]\frac{\partial\Phi_\mathbf{B}(S)}{\partial t}\ =\ \frac{\partial}{\partial t}\int_S \mathbf{B}\cdot d\mathbf{A}\ =\ -\oint_C\mathbf{E}\cdot d\mathbf{\ell}[/tex]

    E and B are the electric and magnetic fields; a closed surface is the boundary of a volume, and Q is the charge within that volume; in the last two laws, S is any surface whose boundary is the closed curve C; I is the current passing through C or S; the symbol [itex]\oint[/itex] indicates that the integral is over a closed surface or curve

    those are the flux (or integral) versions of the total-charge versions of Maxwell's equations; there are also free-charge versions of Gauss' law and the Ampère-Maxwell law which use D H free charge and free current:

    Gauss' Law:

    [tex]\Phi_\mathbf{D}(S)\ =\ \oint_S\mathbf{D}\cdot d\mathbf{A}\ =\ Q_{free}[/tex]

    Ampère-Maxwell Law:

    [tex]\frac{\partial\Phi_\mathbf{D}(S)}{\partial t}\ =\ \frac{\partial}{\partial t}\int_S\mathbf{D}\cdot d\mathbf{A}\ =\ \oint_C\mathbf{H}\cdot d\mathbf{\ell}\ -\ I_{free}[/tex]

    Extended explanation

    Scalar flux vs vector flux:

    The vector form of flux is the density (per area, not the usual density per volume :wink:) of the scalar form of flux.

    In electromagnetism, it is called the flux density …

    ie, in electromagnetism, flux is flow across a surface, and flux density is the density (per area) of that flow;

    flux in other topics, is the same as flux density in electromagnetism.​

    Flux density in electromagnetism:

    Magnetic flux, [itex]\Phi_m[/itex], is a scalar, measured in webers (or volt-seconds), and is a total amount measured across a surface (ie, you don't have flux at a point).

    Magnetic flux density, [itex]\mathbf{B}[/itex], is a vector, measured in webers per square metre (or teslas), and exists at each point.

    The flux across a surface S is the integral of the magnetic flux density over that surface:
    [itex]\Phi_m\ =\ \int\int_S\ \mathbf{B}\cdot d\mathbf{S}[/itex]​
    (and is zero across a closed surface)

    Magnetic flux density is what physicists more commonly call the magnetic field. :rolleyes:

    It is a density per area, rather than the usual density per volume.

    Similarly, electric flux, [itex]\Phi_e[/itex], is a scalar, measured in volt-metres, and electric flux density (also a density per area), [itex]\mathbf{E}[/itex], is a vector, measured in volts per metre (and is more commonly called the electric field).​

    * This entry is from our old Library feature. If you know who wrote it, please let us know so we can attribute a writer. Thanks!
     
  2. jcsd
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