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A coil of wire is placed along the axis of a solenoid?

  1. Aug 17, 2013 #1
    http://img42.imageshack.us/img42/6336/14vu.png [Broken]

    plz explain your answer! also if u know of similar problems or where this problem is taken from do tell me coz i have an admission exam coming up soon

    plz explain the following:

    1. what is the flux like?

    3. how the coil spins?

    z. how is the rotation about x or y differ from z?
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Aug 17, 2013 #2


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    What do you know about the laws of induction? What do you get when you apply them to this question?
    (BTW - did you see the notice about No Homework of Course Questions to be posted here? There is a whole forum dedicated to this sort of thing)
  4. Aug 17, 2013 #3
    that's not homework! that's a question from a sample test of an admission exam that i will take soon! sorry i'm new here i'm not familiar with the forum rules, did i post in the wrong forum?

    my trial at the solution, this looks similar to the dynamo, when the "thing that rotates, sorry can't find the word here English is not my first language" is perpendicular to the flux, as it is when it's rotating around the z axis, emf will be zero because the nagle between the perpendicular on the "thing that rotates" and the flux is zero, and emf = NBAW sin theta, sin 0 = zero emf = zero, is that it? btw i already know the answer but this is my explanation that fits the answer!
  5. Aug 17, 2013 #4


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    The question to ask yourself is "does the magnetic flux change when you make the movement or change in each part of the question".
  6. Aug 17, 2013 #5


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    Even though it is not literally "homework", it is a "homework-like" problem, that is, you are trying to find the solution to a specific exercise. Such exercises belong in one of our "Homework and Coursework Questions" forums. I've moved this one for you. The next time you post a similar question, take note of the rules that are posted at the top of this forum (the one I've moved you to).
  7. Aug 17, 2013 #6
    exactly i asked myself that, two problems though i don't know how high the coil above the solenoid, so i'm not sure what the flux direction will be, assume it's downwards to upwards, problem two is that i'm not really sure if that movement will cause change in flux, i'm not even sure how it's rotated, does it rotate like a dynamo, or does it "spin" around the z axis in a circular fashion, these kinds of questions are unclear but if they gave enough information, i will solve it with ease coz i understand induction, so plz break it down for me, thx
  8. Aug 17, 2013 #7
    Don't think about dynamos, they are more complex than this problem, so it makes no sense whatsoever to talk about them when you just need to solve this problem.

    You need to understand what magnetic field the solenoid generates to solve this problem. Do you understand what it looks like?
  9. Aug 17, 2013 #8
    yes i have an idea of what it looks like, it's complicated! btw guyz just exp[lain the solution and save me all this suspense xD
  10. Aug 17, 2013 #9
    "just explain the solution" is not how this forum works. We can help you solve a problem, but we need that you make an effort on your own.

    Is there any symmetry in the field of the solenoid? How can you move the solenoid, using that symmetry, so that its field does not change?
  11. Aug 17, 2013 #10
    i don't know, plz explain :( believe me i spent days thinking about this i can't imagine it, i need clear description!
  12. Aug 17, 2013 #11
  13. Aug 17, 2013 #12
    ok so the flux is directed from downwards to upward, the coil is spinning along the Z axis which is in the same direction as the flux, what does this mean then? is there a change in flux?
  14. Aug 17, 2013 #13
    What would change in the magnetic field if the solenoid were rotated about the z axis?
  15. Aug 17, 2013 #14
    i'm sorry but this is no way to help me, i have an exam coming i'm studying this is taking too much time, if i know that then i know the answer, is this how u really handle questions here or are u lazy to type? plus the coil rotates not the solenoid!!!! so far u haven't explained what the flux look like, how the coil spins or rotates, i repeat the problem is not my understanding of induction, the problem is to understand the problem, plz somebody help me! i'm low on time!
  16. Aug 17, 2013 #15
    You cannot prepare for your exam by being given answers to sample question. That will get your nowhere. You can only get prepared if you study the subject and learn how you can reason and apply your knowledge. What we do here is help you with reasoning.

    You say that you understand induction. That is fine. Then you should have no problem answering whether rotating the coil (or the solenoid, which is the same thing) about the z-axis would change the magnetic field. Do you understand that the magnetic field is radially symmetric with respect to the axis of the solenoid?
  17. Aug 17, 2013 #16
    radially?!!!!! u do understand that i don't speak English for a first language don't you? unless u gonna give me explanation in simple english or rephrase the problem for me in a simple way to make it easy plz don't bother, and plz don't feel offended but i have no time for this
  18. Aug 17, 2013 #17
    i apologize about yesterday, i was studying and i kept getting the notifications which interrupted me and it seemed like u didn't want to help me! let's give your method a try, but so far u haven't commented on my attempts at the solutions! so again the flux points upwards, both rotation about x, y and z will be perpendicular to the flux, your comment?!

    btw if u explain to me what i need to know, i will be able to solve any problem like this, it's not just the solution to this particular problem, which is useless really in itself! but the skills that i will learn from solving it is what matters!
    Last edited: Aug 17, 2013
  19. Aug 18, 2013 #18


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    I do not understand what's going on here. You have been given a question yet you, apparently, have not been taught the basics of the topic. The field around a solenoid is very well known. The laws of induction are usually taught early on. Anything that changes the flux (or, as it is often stated, 'when lines of force are cut') will induce an emf. Those situations in the questions are not very difficult to interpret. Were you given any examples on your course of where induction can take place?
    If I were you, I would complain, if not.
  20. Aug 18, 2013 #19
    My comment is that rotations about x and y will indeed be perpendicular to the flux. But the rotation about z is not. So will it change the magnetic field sensed by the coil?
  21. Aug 18, 2013 #20


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    Rotating the coil about the axis of the solenoid doesn't change the area enclosing the magnetic flux so you would not expect an induced emf. The other cases seem to imply either the area changes or the flux changes - hence an induced emf.
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