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Whats the Concept behind change of electric/magnetic flux ?

  1. Mar 3, 2011 #1
    Whats the Concept behind "change of electric/magnetic flux"?

    What i mean to say is that, i understand the concept of "change of distance", "change of time" and other changes and other time rates....

    but the "time rate of change of electric flux" really confuses me. Does change of flux mean increase or decrease in the number of electric lines passing through some body? or does it mean the "movement" electric lines through it?....

    i mean, if a conductor, say a rod moves thru a UNIFORM magnetic field, there would be transient polarization of the rod. (one end would get slightly positive, the other negative)...So its obvious that there was some instantaneous induced emf in the rod, if so, then the concept of changing magnetic flux as the increase or decrease in the number of lines of magnetic induction (which my teacher provided) fails here...Because faraday's law states

    emf= -N (change of flux)/ (time)

    so my teacher's concept would say that emf should be zero( because there is no increase or decrease in the number of lines of magnetic induction ,since the magnetic field is uniform)...but still we have said that instantaneous emf existed.


    so whats the point here?....if my teacher's concept about the change of magnetic/electric flux is wrong...which is the right concept?



    so if u apply the i
     
  2. jcsd
  3. Mar 4, 2011 #2
    Re: Whats the Concept behind "change of electric/magnetic flux"?

    When you move the rod in a B field. That is the Lorentz force F=qvB .
    The Lorentz force will push the free electrons to one side of the rod creating a hall voltage. If instead you had a square metal frame when the first electron started to move due to the Lorentz force, the E field of the electron would push the e- in front of it and a current would start to flow.
     
  4. Mar 4, 2011 #3
    Re: Whats the Concept behind "change of electric/magnetic flux"?

    i asked for the explanation of the "concept" of change in electric flux?...

    does it mean increase or decrease in number of lines intersecting a body?
     
  5. Mar 4, 2011 #4
    Re: Whats the Concept behind "change of electric/magnetic flux"?

    You could change the field strength or the area .
     
  6. Mar 4, 2011 #5

    HallsofIvy

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    Re: Whats the Concept behind "change of electric/magnetic flux"?

    I think you have the wrong idea about "number of lines". What changes is the only thing that exists at a point- the strength of the magnetic flux. "Number of lines" is purely ficticious- a way of visualizing field strength.
     
  7. Mar 4, 2011 #6

    jtbell

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    Staff: Mentor

    Re: Whats the Concept behind "change of electric/magnetic flux"?

    It can mean either one. If the the flux of the field through a surface increases, then the number of field lines intersecting the surface increases. If the the flux of the field through a surface decreases, then the number of field lines intersecting the surface decreases.
     
  8. Mar 4, 2011 #7
    Re: Whats the Concept behind "change of electric/magnetic flux"?

    so does this mean that if a a body moves thru a uniform magnetic field, there would be no change of flux through it?...if there is no change of flux, then where does that instantaneous emf come from which i explained in my first post?
     
  9. Mar 4, 2011 #8
    Re: Whats the Concept behind "change of electric/magnetic flux"?

    The emf come from the Lorentz force which I explained in post 2.
     
  10. Mar 7, 2011 #9
    Re: Whats the Concept behind "change of electric/magnetic flux"?

    faraday's law is applied to LOOPs.(you can only say electric flux through loop instead of line)
    so you need be aware of what LOOP that you consider(choice).

    if you choice loop IN the rod, then the LOOP's emf really is zero.
    //you explained it right, and another interpretation is -E+E=0

    if you choice another PROPER loop, you also can get the result in post 2.
     
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