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Homework Help: Influence of magnetic fields

  1. Nov 19, 2011 #1
    I'm just trying to get my head around how magnetic fields attract and repel eachother,

    Say you have two loops of current a distance d apart, the current through the top loop is counter clock wise when looked at from above.

    If the current through the top loop is slowly decreased, a current through the bottom loop will be induced to stabilise the system, the induced current will also be counter clockwise right? To maintain the original field. So the fields due to both will be in a similar direct and attract eachother?

    So if the current is increased in the top loop, the induced current in the bottom loop will flow in the opposite direction, causing the loops to repel?

    Is that correct or have a gotten the repel and attract the wrong way around?

    So like, if you apply a current through a spring, it would compress since the from each loop is in the same direction.

    Is that the right way to see it?

  2. jcsd
  3. Nov 19, 2011 #2

    Simon Bridge

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    The induced field opposes the cause.
    http://www.physics.upenn.edu/courses/gladney/phys151/lectures/lecture_mar_17_2003.shtml [Broken]
    Last edited by a moderator: May 5, 2017
  4. Nov 19, 2011 #3
    Yea, that's what I wrote, but i'm asking about the repulsion and attraction of the wires due to the field.

    Each current produces a field, in the first case they are both in the same direction, because the current is reduced in the top wire, reducing the field, resulting in a current through the bottom wire to oppose the change. So the direction of the current in the bottom wire is the same as the is the reduction of the field, to oppose that would be to induce a field in the same direction. So that means the two wires would be attracted?

    In the second case the currents flow in opposite directions, as do the fields. For similar reasoning to above, the above current increases, increasing the field, so to decrease the field the bottom wire must oppose the change. So this means the wires repel each other? Opposing fields repel?

    Like in the case of the spring, each loop of the spring would generate the same field, so they would all attract and compress the spring, rather then cause the expansion of the spring.
  5. Nov 19, 2011 #4

    Simon Bridge

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    Oh I see what you are saying now.
    What you are describing is how maglev works.
  6. Nov 20, 2011 #5
    Magnetic levitation has more to do with magnetisation, diamagnetics.

    My question is,

    Do two B fields attract or repel when they are in the same direction, In more detail as explained above.

    Its pretty trivial, I'm pretty sure what I have is correct especially when considering two bar magnets. I just wanted to double check.
    Last edited: Nov 20, 2011
  7. Nov 20, 2011 #6

    Simon Bridge

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    Opposite poles attract.
    The coils attract and repel each other pretty much as described.
    But you asked about the relationship between this and the direction of the B vector.

    Have a look at the field between two wires carrying a current. If the field between them has the same direction, do they attract?
  8. Nov 20, 2011 #7
    No I asked simply if the two wires attract each other or repel.

    In case 1 they attract, case 2 the repel.

    You just restated my question in your last post....

    Did you even read the question?
  9. Nov 20, 2011 #8

    Simon Bridge

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    This is the question I was answering.
    I have answered your previous questions.

    Please note: it is not my habit to spoon-feed people. Instead, you are supposed to think some more and come to a deeper understanding. If you want regurgitated answers, fine, but I get paid a fee to do that. This way you will learn more and the lessons will stay with you.

    I intended you to realize that the two wires in my example are straight, while yours are in a loop. Have another go - you know how two straight wires attract and repel each other, and you know how the B field works around the wire. So - you can deduce the effect that the direction of the field has on the attraction/repulsion.

    That help?
  10. Nov 20, 2011 #9
    Honestly dude,

    You clearly have no idea, you have provided me enough evidence that you have no idea about what I have asked.

    I did not require "spoon feeding" I asked a simple, trivial, high school question which I had already provided adequate explanation and though into that required a YES or NO answer. Not a link found off google, not obvious statements and not incorrect assumptions. Two arbitrary wires in free space with arbitrary currents do not describe magnetic levitation. That is due to the opposing magnetisation produced by diamagnetic materials. That is what you deal with in first year physics.

    Please stop wasting my time with your non sense, all you have done is prevented me from being answered by someone who actually knows.

    I noticed that you only joined the forums recently which suggests to me that your just post count hungry.

    If you would like to know how to think about this problem you simply have to consider the field lines of a bar magnet, compare it with the field lines of the loops in each case and that's all. The whole point of my post was to clarify this was correct. I asked a peer who supplied me with the answer with a simple "Yes" which shows understanding of the physical concepts.

    Please have the decency not to reply as you have been reported.
    Last edited: Nov 20, 2011
  11. Nov 20, 2011 #10

    Simon Bridge

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    Sorry, I seem to have misunderstood your original post when you said:
    ... I kinda got the impression you were talking about loops of wire and not just
    I dunno, could be wrong - hopefully, the mod will be able to provide a reality check here.

    I think you can blacklist people in your profile too.

    meantime: if this is, indeed, such a "trivial, High School question", then how is it you don't know the answer?
    Could it be that this is a sign of a deeper misunderstanding or confusion?

    One of the wonderful things about providing a free service is that you get to give people the answers they need rather than the ones they insist on.
    You want to repeat the question i another thread, I will try not to reply to it :)
    Last edited: Nov 20, 2011
  12. Nov 20, 2011 #11
    The question I asked consisted of loops, Yes!

    I was merely stating it is not an explanation of magnetic levitation. Looped or not!

    It doesn't matter! The whole point of that insignificant sentence was to inform you that it was not an an explanation. Seeing as your accustom to google, google magnetic levitation! There is no need to use these words in an attempt to deform the question.

    The whole point of my asking on this forum was to clarify an old, trivial, high school aspect I have revised. As I stated above! Multiple times!

    I did not insist I knew answers when I did not! I did not ask for irrelevant and already explained information off of google!

    As I stated above, my answer and explanation of the physics were right. I have clearly explained how you can find these solutions yourself as you clearly could not.

    The only service you have provided is annoyance, which I would not pay for. I have read through other posts you have made and have noticed similar ignorance towards questions. Which have lead to many unanswered posts!

    I have not insisted on my answer. I simply stated my logic, which is required by the forums! That being said you did not provide any fraction of the answer I needed anyway! I do not need to repost, as again stated in the previous posts I received the "needed" answer off a peer. Which was simply "Yes", not googled web pages, obvious statements, and certainly not my question regurgitated back to me.

    There was no reason for you to reply to this thread, the answer to each of your statements laid within the content of the thread. You made an attempt to deform my question and showed you had not properly read my posts which again shows a desire for an increased post count.
  13. Nov 20, 2011 #12

    Simon Bridge

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    Unfortunately the answer is not "yes" to all your questions.
    That last question I tried to answer before you blew up was not answerable with yes or no. So I could not answer in the manner you expected. Sorry.

    Recall, the question was:
    In between two wires, the field lines have to oppose in order for the wires to attract.
    If you examine the fields like I suggested you'd see this is the case.
    However - at the poles of an electromagnet, they have to line up. Which you'd already noticed. So the simple answer would have been: "sometimes".

    No responsibility for advise not taken.

    I'm going to wait for the moderator's opinion.
    If the mod thinks I have done wrong, you will have a full apology.
    No worries.
  14. Nov 20, 2011 #13
    Haha I never wanted an apology I just asked for a straight, proper answer!

    It seems like your thinking about magnetisation, like how the magnetisation of diamagnetic opposes causing repulsion.

    Is that why you decided some times?

    For two loops with current in the same direction, each current loop creates a magnetic dipole with the north at the top and the south at the bottom. So the south pole of the top loop is near and attracts the north pole of the bottom loop, and so the two are attracted to each other.
  15. Nov 20, 2011 #14


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    First, if I can just comment: It's easy to misinterpret another poster's question once in a while. Given PF's adamant enforcement about not giving out solutions to questions in the Homework & Coursework area of the forum, it's understandable that members can get overly cautious about not providing too much help. I see some of the discussion in this thread as a simple honest misunderstanding, which isn't helpful when you are the one trying to learn physics but it does happen from time to time.

    Yes. And that's an interesting consequence of magnetic attraction, it had never occurred to me before and I don't recall reading that anywhere either.
  16. Nov 20, 2011 #15
    Thank you very much!
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