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A small doubt about electric flux

  1. Dec 7, 2014 #1
    Hey there...
    As far as I know, electric flux is a scalar quantity which means that negative values are smaller than zero. This concept really confuses me since most of the exercises I dealt with suggest that negative flux somehow is larger than zero flux

    Example 1
    yf_Figure_22_32.jpg
    the uniform field is directed to the right, which surface has the lower electric flux?
    is it S1 which is negative or S2/S4/S5/S6 ( that's the correct answer according to my book)?

    Example 2
    https://lh6.ggpht.com/f3KBE_uxMhA2zCSgcxfdIu26-n0OOGrOHNwK7lQxsSYGp__YJLIWuP7uSkZUBVLUFpwJQw=s170
    Which surface has the lower electric flux?
    is it S2 which is negative or S1 which is zero ( that's the correct answer according to my book) ?

    So is the problem with me or with the book itself or what exactly?
    Thanks in advance.
     
    Last edited: Dec 7, 2014
  2. jcsd
  3. Dec 7, 2014 #2

    Simon Bridge

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    Welcome to PF;
    The amount of flux passing through a surface is a scalar - not the flux itself.
    The flux may be positive or negative depending on which way it flows through the surface.
     
  4. Dec 7, 2014 #3
    Isn't the flux itself a dot product? which also means it's scalar too. care to elaborate please?
    Thanks for your reply...
     
  5. Dec 7, 2014 #4

    Simon Bridge

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    A dot product of what with what?
    Think of the flux as the amount of flow.
     
  6. Dec 7, 2014 #5
    the electric field and the vector area
     
  7. Dec 7, 2014 #6

    Simon Bridge

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    Backtrack - I think I can see a way through for you.
    This is just like the component of the velocity vector in the x direction of interest is given by a dot product (##\vec v \cdot \hat i##), which will give a scalar, which may be positive or negative; but a negative speed is no slower than a positive speed. Similarly a negative flux is the same amount of flux as a positive flux, it's just headed into the volume rather than out of it ... and that is an arbitrary choice: it is just as good to define positive in and negative out.
     
  8. Dec 7, 2014 #7
    that's what I wanted .. I am really thankful, just to make sure, negative flux>zero flux in both examples? if so then, all the doubt will vanish.. Thanks again!!!
     
  9. Dec 7, 2014 #8

    jtbell

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    I think when your book asks about "higher" or "lower" they are referring to the magnitude (absolute value) of the flux. As a native English speaker, I personally would say "larger" or "smaller", instead, because that implies magnitude. Is this an English-language textbook, or are you trying to translate from some other language? Or maybe the book was written by a non-native English speaker.
     
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