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Homework Help: Magnetic Field Help

  1. Mar 12, 2006 #1
    Magnetic Field Help


    Hi, when it comes to physics, I am the stupidiest person on Earth. Thus I have three questions, if someone could walk me through them I would be so thankful it would be ridiculous.

    Problem 1.
    An electron travels with speed 1.0 x 10^7 m/s between two parallel charged plates. The plates are separated by 1.0 cm and are charged by a 200 V battery. What magnetic field strength and direction will allow the electron to pass between the plates without being deflected?
    (for this one I thought that if electrons travel parallel or antiparallel to a magnetic field it will always allow the electron to move. Like I said before I'm a physics idiot)

    Problem 2.
    The uniform 30 mT fiel points in the positive z direction. An electron enter the region of magnetic field with a speed 5.0 x 10^6 m/s and at an angle of 30 degrees above the xy plane. Find the radius r and the pitch p of the electron's spiral trajectory.
    (I have no clue on this one)

    Problem 3.
    Two springs each have a spring constant of 10 N/m. They are stretched 1.0 cm when a current passes throught the wire. How big is the current?
    Note: Magnetic field is 0.5 T, and length of wire is 20cm
    (for this one, I found the magnitude of the spring force, multiplied that by 2 since there are two springs. I then set that equal to the F(magnetic field) which is defined by ILB. I then isolated to solve for I. I get an answer of 20A, but my textbook gives an answer of 2.0A. This would lead me to believe my error is small, or that the answer in the book is wrong)

    Whowever can help me with these I would be very appreciative, thanks for your time!
    Last edited: Mar 12, 2006
  2. jcsd
  3. Mar 12, 2006 #2


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    Homework Helper

    prob.0: you'll get better help from forum regulars with a "positive" attitude than with a "pity me" plea.

    prob.1: see in your diagram that there's an E-field in the gap, pointing
    from the +Q on one plate to the -Q on the other plate. Compute this E-field.
    Which way would this E-field deflect an electron?
    Which way should you arrange the B-field to point, in the gap, so that
    the magnetic Force cancels the Electric Force?

    prob.2: You DO know the right-hand rule, right? Circular motion (mv^2/r)?

    prob.3: how many turns of spring are there?
  4. Mar 12, 2006 #3
    p3. That information is not given, and I didn't think it was relevant because it is not a solenoid question?

    and as far as the right hand rule goes, I have heard it but I am unsure of how it truly works.
  5. Mar 12, 2006 #4


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    Homework Helper

    point your right hand (fingers together, straight) in the direction of qv .
    these are electrons, so qv is OPPOSITE their velocity
    (qv is in the direction of the Indicated current ...)

    Sweep your fingers only (keeping hand in place) to point in direction of local B.
    You usually have to rotate your wrist around the qv axis to do this.
    Fingers sweep through the angle "theta" in F = qv x B = qvB sin(theta).

    Thumb sticks out perpendicuklar to hand and fingers, points in direction of F.

    The springs themselves are carrying the current?
    adjacent coils ATTRACT one another, so it should SHORTEN.
    My guess was that there must be an external B-field, pushing on EACH coil.
  6. Mar 12, 2006 #5
    What I did for problem 1, was use E=V/d to calculate the electic field. I then set that equal to qvB. When I solve for B I get an answer that is way too large. I think my mistake is in using E as F. What other calculations need to be done here?
  7. Mar 23, 2006 #6
    For Problem 2, just use the equation:

    [tex] r= \frac{m*v}{q*B}[/tex]

    except that we just want the velocity perpendicular to the B-field, so we use the equation:

    [tex] r= \frac{m*v*cos(\pi/6)}{q*B}[/tex]

    Not sure how to find the pitch though. I'm thinking maybe something like:
    p = r*tan(pi/6)

    If you can figure out the second part, please post.

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