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Questions about Induction

  1. Jun 16, 2007 #1
    Hi there, Im new to here, and I bring some questions here too, hope can be solved here~
    (1)if one loop that essentially has a current by power supply, meanwhile it is experienced a induced current too, then what's the resultant current? add them or what?
    (2)as you know, Inductance L = somesomething/I, does it depend on I, well, obviously yes if we judge from the formular, but one text book said it doesn't depend on I, so just wanna ensure this.
    (3)an extra question...hehe...be patient plz...
    as we know, one function of inductor is that it can give a protection while the current suddenly changed, why?
    Cheers guys!
  2. jcsd
  3. Jun 16, 2007 #2


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    Hi bolei and welcome to PF.

    (1) Do you know the direction of the induced current?

    (2) The inductance is a function of the physical characteristics, much like how capacitance is function of the geometrical and physical characteristics of the capacitor. That "somesomething/I"; people will misinterpret when you say I, as this is the symbol for current, in this case I take it you meant to say length of the solenoid?

    (3) I believe when implemented to do this function, they are called chokes. You should read up on Lenz's law. This is partially related to (1).
    Last edited: Jun 16, 2007
  4. Jun 16, 2007 #3
    sorry for the misinterpretation ranger, for question1, let's say they are running to different direction, for question 2,L = NA/I where N is the number of turns, A is the magnetic flux and I is the current.
    Thx again!
    Last edited: Jun 16, 2007
  5. Jun 16, 2007 #4


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    [tex]L = \frac{NA}{i} =\frac{\lambda}{i}[/tex] here is called the self-inductance. Induced voltage across a N-turn coil is by Faraday's law:
    [tex]v=\frac{d (NA)}{dt} = N\frac{dA}{dt}=N\frac{d}{dt}(\mathcal{P} N i) = N^2\mathcal{P} \frac{di}{dt}=L\frac{di}{dt}[/tex]
    here [tex]\mathcal{P}[/tex] is the permeance of space occupied by the flux which just describes the magnetic properties of this space. in some text it is also called the "reluctance of a path for magnetic flux". This concept is usefull in Magnetic circuits (analogous with the electric circuit where there is V=IR) from above you can see that
  6. Jun 16, 2007 #5


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    (1) I don't like the way you are assuming a different current direction. This shows that you have not quite grasped the concepts of how an inductor really works; I have provided a link that may help you. However, generally speaking, if we have a component and we have opposite directional currents flowing through it, they will subtract. But with regards to your inductor question, read this:

    (2) http://hyperphysics.phy-astr.gsu.edu/hbase/electric/indsol.html#c1
    Its purely geometric and physical.
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