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Homework Help: Lenz's Law & Magnetics

  1. Oct 22, 2009 #1
    1. The problem statement, all variables and given/known data

    http://dl.getdropbox.com/u/119186/Picture%20lenz.png [Broken]

    2. Relevant equations

    Don't need any!

    3. The attempt at a solution

    I know that the first one is A. Loop 1 induces a magnetic field into the Loop2, then using right hand rule you get that it has to be clockwise.

    For b-d I'm not sure.

    I think B is continuous, why wouldn't it be? There's no resistor or anything to stop the current and if they don't move away from eachother it wouldn't decrease...

    C should be to left, ya? They repel...

    D should be to right... again because they will repel eachother because in between they'll both have a magnetic field pointing out of page. Similar mag field will repel...
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Oct 22, 2009 #2
    (b) The induced current in loop 2 is created by a >>>>>CHANGE<<<<< In the magnetic flux through loop 2. Is the magnetic flux through loop 2 changing?

    (c and d) If there is a current in loop 1, and a current in loop 2, then how is the force between the two loops related to the direction and magnitude of the current? I'm sure your textbook has addressed the magnetic force between two parallel current-carrying wires, how does this force depend on the direction of the current, and how does it depend on the distance between the wires?

    If you noticed, I just gave you more questions to answer, even though you were initially looking for an answer to a question to you already had. I'm sick like that. (Actually I'm probably not supposed to simply answer your question, and I think if you think of the answers to my questions, you will be able to answer yours.) Best of luck to you, respond if you have any further questions, I hope my question stacking has helped.
  4. Oct 22, 2009 #3
    So for (b), current is created by flux, and flux eventually goes away for loop 2 I'm guessing? If that's the case the current would be brief.

    For C and D I'm still confused. Even if I used the right hand rule I'm not sure where to point. I mean, there's no velocity for either so I can't use V x B or LxB right? So I can use the right hand rule to show that they both have magnetic fields going into and out of the page.

    For Loop one, magnetic field goes into page inside, out of page outside. Same for Loop 2 since it will have current going CW...they both go CW.

    But I don't know what that tells me about the magnetic force acting on them.

    I guess I'm still confused about lenz's law. Will loop 2 eventually have no current in it? I can't find any good explanations in simple terms of lenz law...
  5. Oct 22, 2009 #4
    Lenz's law is kinda hard to picture, even to me after getting an A in intro E&M. The way I deal with it is the following: Think of an arrow pointing in the direction of the magnetic flux through a loop. Using the right hand rule (please use your right hand, It makes me feel anxious and sad when I see a kid maneuvering his or her left hand during an E&M test, I just want to get their attention but I can't), grab the arrow with your thumb pointing in the direction of the arrow. NOW, if the flux is increasing, turn your hand around, but if the flux is decreasing, leave your hand as it is. The direction your fingers are curling is the direction in which the current is flowing.

    I don't know what textbook you are using, but my intro book says that Lenz's law states "The direction of any magnetic induction effect is such as to oppose the cause of the effect." (I'm using Sears and Zemansky's University Physics, 12th ed.). This means that if an initial magnetic field is INCREASING through a loop, an electric field goes through the loop such that the magnetic field created by this current opposes the original magnetic field. This suggests that the original magnetic field is killed down by the induced magnetic field. PM me or post a reply if you have any further questions.
    Last edited: Oct 22, 2009
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