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Homework Help: Magnetism Problems.

  1. Oct 4, 2007 #1
    On these problems all i can figure out is which formula to use..after that i am completely lost..I don't really like physics, the whole pick the right formula for the given problem is too confusing for me...

    Problem 1) I think its Fmag= q*velocity*B*sin(theta) or would it be Fmag= B*I*L(Sin*theta)
    , but whenever i try to plug in whats given i have 2 unknowns Fmag and B...

    Problem 2) The only formula i can think of that would solve this problem would be

    Force/Length= (4(Pi)*10^-7(Ia)(Ib))/2(Pi)d, but once again i have more than one missing variables in the equation.

    Problem 3) Is I think using Faraday's law, but when i apply it, i cant find some of the missing variables once again to complete the formula..

    Problem 4 & 5) I've got the answer already.

    If anyone could help me figure out how to do these problems it would be greatly appreciated. I know it doesnt look like it, but i've spent an hour or so trying to do these problems and looking through the book trying to find a problem similar to this, but no luck

    Thanks in advance.
  2. jcsd
  3. Oct 4, 2007 #2


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    Hmmm... looks like you didn't paste the questions, or forgot to include an attachment which had the questions?
  4. Oct 4, 2007 #3
    oh sorry..i was logged off the first time so it wouldn't let me post.

    Attached Files:

  5. Oct 4, 2007 #4


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    1) For the first problem you need F = qvBsin(theta). What is theta here?

    the particle follows a circular path because the force is always perpendicular to the velocity...

    Use centripetal force here to get what B needs to be so that the particle goes circularly backwards into the slit located d above the first slit....

    2) For the second problem, try to use Ampere's law... what is the field due to one wire... what does the field due to the other wire need to be?

    3) first find the field inside the thin long solenoid... there's a formula to calculate this... then apply faraday's law to get the emf in the outer coil... remember to take into account N the number of turns of the coil.
  6. Oct 8, 2007 #5
    ok, i'm totally lost here..

    1) theta would be 90 degrees? If thats so whats the point of finding that if i'm going to use a different formula to figure out B anyway.

    And centripetal force is what? i dont have that written down in my notes.

    3) whats the formula to calculate the field of the solenoid? You have to put thing into simple terms..Like i said before physics is not one of my stronger subjects..
  7. Oct 8, 2007 #6


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    yes. theta is 90 degrees. So F = qvBsin(90) = qvB.

    you do need this formula. this formula along with the formula for centripetal acceleration will give you B.

    centripetal motion means that the object moves in a circle... and the net force on the object is towards the center of the circle with an acceration of v^2/r.

    So net force = ma
    net force = mv^2/r

    the only force is the force due to the magnetic field.

    qvB = mv^2/r

    now you can solve for B.

    You should have this in your text... look up solenoids. If I just give you the formula you wouldn't understand anything about it... you need to see a discussion of solenoids...

    I understand, but if I make things too simple, then I'll just be giving you the answer... I've broken down the problem here:

    1) Find the magnetic field due to the inner solenoid
    2) Calculate the emf on the outer coil due to this magnetic field.

    you should first read about solenoids... then try to do part 1... tell me where you get stuck, and we'll go from there...
  8. Oct 8, 2007 #7
    ok i understand it a little bit now.

    1) when i plugged everything in i got

    6.9E5(B)= (6.9E5)^2
    3.5E-26 ________

    For q would that just be +1 since the charge is +1?

    After i work it all out i get 3.54E-20 T, and the computer says that is wrong.

    3) so to find magnetic field i use F=qvB
    and to calculate the emf i would use emf= -N delta (magnetic Flux)/delta t right?

    Thanks again for helping i know i'm an idiot.
  9. Oct 8, 2007 #8


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    Did you forget to plug in q? q = 1.6*10^-19

    That formula is not the one you need to get B... I highly recommend looking up solenoids... there's a formula for solenoids and B...

    The second part is right... but they just want the magnitude so you don't need the '-'... for delta magnetic flux... remember that flux is magnetic field*A... magnetic field starts at 0... then it becomes some value call it B... so magnitude of emf = N (BA - 0(A))/delta t so emf = N*B*A/delta t... where B is the field when the current is fully running... remember that N is the number of turns in the outer coil... A is the area of the inner long thin solenoid...

    You know the A, N and delta t... you just need B...

    No, don't say that. This stuff takes a while to get used to... Magnetic field problems can be tricky. Keep doing the problems and your understanding will improve...
  10. Oct 8, 2007 #9
    Ok i got the first one..Deadline just passed so i couldnt work on #3, Thanks for all of your help it was greatly appreciated.
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