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Homework Help: Electron enters uniform magnetic field and takes circular path

  1. Apr 24, 2014 #1
    1. The problem statement, all variables and given/known data
    A time t = 0 an electron enters a region of uniform magnetic field B = 0.010 T and has kinetic energy of 6.40E-16 J. It goes through a half-circle, exits the field and then accelerates across a gap with a potential difference of 2000 V, increasing in speed. It then hits a collecting cup. At what time t does it hit the cup?

    2. Relevant equations

    3. The attempt at a solution
    I can find the time to go through the half circle using the first equation above. It is 1.788E-9s. I need to find the time to get to the cup so I can add the two times up to get the total time.
  2. jcsd
  3. Apr 24, 2014 #2
    I have attached a diagram of the problem.

    Attached Files:

  4. Apr 24, 2014 #3


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    I think you need to find the radius of the circle using the force of the field on the electron. That will let you get the arc length.

    Remember, B fields do no work.

    Just out of curiosity, what is T in you're equation ?

    One more thing, I don't believe you can solve this without knowing the size of the gap, or at least that it's negligbly small.
  5. Apr 24, 2014 #4


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    DOH! Should've read the diagram.

    A) What is a Volt?
    B) How does a Volt relate to distance and time? (Hint* express a Volt in more fundamental units)

    answer these 2 questions and I think you can handle this question.
  6. Apr 24, 2014 #5
    The gap from when the electron leaves the B field to the cup is 25 cm, its in the diagram. T is the period. I don't think the radius and arc length are needed.
  7. Apr 24, 2014 #6


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    Yea I think your equation should work, but I'm sticking to what I said about expressing Volts in fundamental units.
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