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Homework Help: Proton moving in a magnetic field

  1. Feb 29, 2012 #1
    1. The problem statement, all variables and given/known data

    A proton (mass= 1.67×10-27 kg, charge= 1.6×10-19 C) traveling with speed 1×106 m/s enters a region of space containing a uniform magnetic field of 1.2 T.

    The magnetic field is coming out of the screen.

    What is the time t required for the proton to re-emerge into the field-free region?

    2. Relevant equations

    qvB = mv2/R

    3. The attempt at a solution

    I begin by solving for R using the above equation:

    R= (mv)/(qB)

    Since the proton moves in and then back out it completes a semi-circle. The distance traveled is then πR as it is half of the circumference of a circle.

    The velocity does not change for the entire distance traveled so dividing distance by velocity should give me the time.

    This comes out as incorrect though. I am unsure as to what I am doing wrong.
  2. jcsd
  3. Feb 29, 2012 #2
    in what direction is the magnetic field pointed relative to the initial velocity of the proton?
  4. Feb 29, 2012 #3
    Oh sorry, I edited the post. It is coming out of the screen. So it is perpendicular to its motion which means it will create a net force on the proton and cause it to undergo the circular motion.
  5. Feb 29, 2012 #4
    ah, okay. Well, what did you get for your time?
  6. Feb 29, 2012 #5
    Well the question involved some steps and one of the steps was to find the radius. It continued to tell me I am wrong. I decided to stop using that and work on it my own using my method and I managed to come up with the correct answer, which just involved me to not round to 2 sig figs but add on more decimal digits. My answer was 2.73*10^(-8). Before I was just putting in 2.7*10^(-8).
  7. Feb 29, 2012 #6
    Thanks for trying to help.
  8. Feb 29, 2012 #7
    what did you get for your radius?

    huh, actually that's the same time I got.
  9. Feb 29, 2012 #8
    I got 8.698*10^(-3) for my radius. The computer kept telling me it was incorrect but I managed to get the right answer with this value so I do not know what is wrong...
  10. Feb 29, 2012 #9
    so the time you got was correct, but the radius is not? Maybe it wants radius in terms of some unit like cm?
  11. Feb 29, 2012 #10
    No it was asking for metres which is what my answer is in.
  12. Feb 29, 2012 #11
    interesting... I don't know what to tell you :(

    did you try the same thing with what you did with time? Try changing the number of decimal points, or how you round your numbers when you solve for it.

    Sometimes I have these same problems with online homework things. They usually accept answers within a certain range of the correct answer, but can be really stubborn sometimes :P
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