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Motional EMF

  1. Apr 21, 2015 #1
    • Member warned about lack of template
    consider the graph below, the bar of length ζ moving to the right with speed v in a uniform magnetic field B going out the page.
    2ed001a8-0915-4e5d-a0c0-cbb8fc0d8ca8.gif


    i don't understand why does the textbook say that there is a change in the magnetic fluxΦ? i think the area doesn't change, although it's moving. the original saying is that: During a time interval dt, the area of the loop increases by a differential rectangular strip of area dA=ldt this change in area causes a differential change in flux given by dΦ=B⋅dA .............
     
  2. jcsd
  3. Apr 21, 2015 #2

    TSny

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    Only the blue bar is moving. The resistor is at rest. A is the area between the resistor and the bar.
     
    Last edited: Apr 21, 2015
  4. Apr 22, 2015 #3
    right, so..... what does that mean?
     
  5. Apr 22, 2015 #4
    is it true that the moving object in motional emf has to be a conductor?
     
  6. Apr 22, 2015 #5
    and the emf created is from the seperation of charges, therefore creating a potiential difference?
     
  7. Apr 22, 2015 #6
    i feel sorry to ask those stupid questions. i don't have a teacher currently teaching me AP Physics.
     
  8. Apr 22, 2015 #7

    TSny

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    As shown in the figure below, as the rod travels to the right the shaded area A increases. So, the amount of magnetic flux through the circuit also increases.
     

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  9. Apr 22, 2015 #8
    interesting!!! let me think hard about it, and thank you for your help:smile:
     
  10. Apr 22, 2015 #9

    rude man

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    You can also look at his from a different perspective. Starting with the Lorentz force F = qv x B you can determine that the emf across the moving bar is Blv. This view is "flux cutting" of a B field (uniform or not) by a bar of length l moving with veloc ity v.

    In some cases, using Faraday's law, as you did with emf = -dφ/dt, in moving media does not work. So the "Blv" law is preferred whenever media (like your bar) is moving.
    Yes, but not a perfect conductor. In fact, anything other than a perfect insulator.
     
    Last edited: Apr 22, 2015
  11. Apr 22, 2015 #10
    right, i found out that there are many ways to analyse motional emf, for example, relativity, parallel capacitor, potential difference or magnetic flux, etc
     
  12. Apr 22, 2015 #11
    but the results is the same,this Blv expression occured in a lot of the results, am i right?
     
  13. Apr 22, 2015 #12

    rude man

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    Right.
    Truth to tell, I'm not sure what to make of your comment. Bit of a jumble, seems like.
    I restricted my comment to the generation and determination of emf by magnetic means.
     
  14. Apr 27, 2015 #13
    thanks for the reply, i found it hard to understand the magnetic field and therefore don't like to use magnetic means, too.
     
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