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What is the difference?flux and charge

  1. Nov 10, 2011 #1
    i can't seem to wrap my head around flux theory.

    what is the difference between electric flux and charge?
    both have same units(coulombs) and symbol q or Q
    but gauss' law uses flux density (D) and not charge

    also if phi(F) is magnetic flux then what is component like 'charge' in magnetism?
  2. jcsd
  3. Nov 10, 2011 #2
    I doubt anyone here will be able to explain it better than it has already been explained in numerous resources. Google the concepts, and if there is something particular regarding the concepts that is a bit hazy, then come back to clarify a minute detail.
  4. Nov 10, 2011 #3
    Electric Flux is the flow of charge through a certain area per time. It is a quantity that describes the electric field strength. The stronger an electric field, the more charge per unit area.
  5. Nov 10, 2011 #4

    jim hardy

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    Gauss was one heavy dude.
    sometimes i resort to the ridiculous to get my thinking straight,,, reducto ad absurdum.....

    i think, as previous poster said, the equations do not make it intuitively obvious whether they are describing total flux or flux density. so try this simple thought then go back to your text and see if we missed some subtlety..,

    imagine a closed gaussian surface surrounding a skunk.
    if that gaussian surface has an area of just one square meter
    there'll be an intense field of lepew emanating from it.

    if one moves his gaussian surface further out, for the obvious reason, and repeats his closed surface integral, well, it'll be a more comfortable measurement to make even if more tedious.
    If the new gaussian surface has an area of 1,000 sq meters the field strength there is attenuated a thousandfold. The same lepew is spread over a thousand times the area.
    But the charge enclosed is the same - one skunk..

    one skunk per sq meter is too intense, one per acre is tolerable.

    i hope i did not mis-understand your question.
  6. Nov 10, 2011 #5
    Another way to think of this (it turns out to be the same inverse square law) is gravity.
  7. Nov 10, 2011 #6

    jim hardy

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    indeed if E is volts/meter

    think of point source and inverse square law
  8. Nov 10, 2011 #7
    If you think of electric field as radially spreading field lines(usual notation), the electric field is greater where field lines are denser, ergo electric field is in direct proportion of the density of the field lines(in a plane perpendicular to the field lines).

    Integrating that density over the entire surface(that field lines are penetrating) gives the number of field lines penetrating the surface, and that is exactly what the expression for electric flux gives.

    This is what flux represents. Good ANALOGY is flow of water. If you ask yourself, how many particles are flowing through some random chosen closed surface, you will get the flux through that surface.

    But this is analogy, and this should only HELP you get your intuition, but do not consider ELECTRIC flux as flow of particles.
  9. Nov 10, 2011 #8


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    If you have a QuickTime player, this animation explains it very well:


    [QUOTE]This video clip was obtained from the "Core Concepts in Physics" CD-ROM produced by Saunders College Publishing. They are to be used for Educational Purposes only; do not copy.[/QUOTE]

    Last edited by a moderator: Apr 26, 2017
  10. Nov 11, 2011 #9
    Flux is the sum total of all the influences on your wallet while charge is the contents therein.
  11. Nov 11, 2011 #10
    Electric flux and charge are totally different. Electric charge is physical charge, electric flux is field lines generated by charges.

    Electric charge unit is coulomb, Flux is more a force kind of thing that the unit is V/m. As in volt per meter. It is a force. Usually we talk about flux as field lines where you see pictures of curve lines from a +ve charge to a -ve charge.

    In math point of view, Flux lines are vector fields where every point in space has it's own vector with amplitude and direction as a function.
  12. Nov 11, 2011 #11
    Flux is not a flow of charges. Flux is generated by charges. It is a force field. Look at any book for Calculus III multi-variables under the topic of Vector Field and you'll see example of electric field and explanation of static electric field is a conservative field that is path independent.
    Last edited: Nov 11, 2011
  13. Nov 11, 2011 #12
    What you are referring to is only a specific condition where there is only a point charge and the fields are radial fields that generated at the point of charge and radiate out evenly in all direction and the intensity diminish as inverse square of distance from the point.

    In real life, we can have distribution of charges, moving charges and they form complicated shape. that is where vector fields come into play. From the distribution of charges, you find the sum of the fields of all charges at any point and get the resultant vector field of that point.

    For point charge:

    [tex]\vec E =\frac {q\hat r}{4\pi \epsilon_0 r^2}\;\hbox {where }\; \hat r \;\hbox { is the radial vector from the center to the point and }\; r\;\hbox { is the distance from the center.}[/tex]

    For a distribution of n charges:

    [tex] \vec E = \sum_{i=1}^n \frac {q\hat r_i}{4\pi \epsilon_0 r_i^2}\;\hbox { where }\;\hat r_i \;\hbox { is the vector from the i^th charge to the point.}[/tex]
  14. Nov 11, 2011 #13
    thanks the skunk theory helped....also thank you for the video...
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