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Homework Help: Magnetic fields and Circular Orbits Question

  1. Jul 27, 2004 #1
    Hi, does anybody know what approach to take with the following problem.

    An electron is accelerated from rest through a potential difference of 500V, then injected into a uniform magnetic field. Once in the magnetic field, it completes half a revolution in 2 ns. What is the radius of the orbit? And what is the magnetic field?

  2. jcsd
  3. Jul 28, 2004 #2

    Doc Al

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    Staff: Mentor

    You'll need to know several things. First, how fast is that electron going when it enters the magnetic field? (Think potential energy changing to kinetic: [itex]q\Delta V = 1/2 mv^2[/itex].) Then you'll need to combine your knowledge of the magnetic force on a moving charge (F = qvB, assuming the field is perpendicular to the velocity) and centripetal force ([itex]F = mv^2/r = m\omega^2 r[/itex]). Good luck!
  4. Jul 28, 2004 #3
    Thanks you, I think I got the right answer, my magnetic field is just a little huge, like 16.4 T which just doesn't seem right, but I took the approach you put and also since T = 2pir/v and we know how long it took to complete half a revolution, I solved it. Thanks again Doc.
  5. Jul 28, 2004 #4
    The question may have been designed to test your ability to integrate circular motion, work and energy and electrodynamics. So the large answer may not be a major factor...

  6. Jul 28, 2004 #5
    Thanks for the assurance vivek. Did I need to integrate? I found the radius from the amount of time it took to complete half a circle. I found the velocity from change in voltage(charge) = 1/2 mv^2. and then I found the mag field from r =vm/qB. That's the correct approach to take right?

  7. Jul 28, 2004 #6


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    Integration wasn't involved in the problem. I think he meant it in a literal rather than mathematical sense.
  8. Jul 29, 2004 #7

    I meant integration of various topics/ideas of physics...as Gza understood correctly :smile:

  9. Jul 29, 2004 #8


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    16T magnets are quite common...why we have a couple in my lab.
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