Magnetic Field Problem! HELP NEEDED

  1. [SOLVED] Magnetic Field Problem! HELP NEEDED!!!!!!!

    1. The problem statement, all variables and given/known data

    A proton of mass mp and charge e is in a box that contains an electric field E, and the box is located in Earth's magnetic field B. The proton moves with an initial velocity vertically upward from the surface of Earth. Assume gravity is negligible.

    (a) On the diagram above, indicate the direction of the electric field inside the box so that there is no change in the trajectory of the proton while it moves upward in the box. Explain your reasoning.

    (b) Determine the speed of the proton while in the box if it continues to move vertically upward. Express your answer in terms of the fields and the given quantities.
    The proton now exits the box through the opening at the top.

    (c) On the figure above, sketch the path of the proton after it leaves the box.

    (d) Determine the magnitude of the acceleration a of the proton just after it leaves the box, in terms of the given quantities and fundamental constants.

    2. Relevant equations

    F=IL*B = qv*B

    B= uI / 2piR u=4pi*10-7

    I don't have the diagram but i will try to find a picture of it on the internet.
    Last edited: Apr 16, 2008
  2. jcsd
  3. Kurdt

    Kurdt 4,851
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  4. well that is becasue i am very confused.
    i don't even know where to start, that is why i haven't posted anything, usually i do, but i just am lost!

    help? :(
  5. Kurdt

    Kurdt 4,851
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    Well what have you been doing? I'm sure you have some sort of notes or a book that can give you a slight clue as to what the answer to part a) should be.
  6. i should use the curved hand rule right?
    Last edited: Apr 19, 2008
  7. If the magnetic field is also in the box, you need to balance the force by the magnetic field with the force of the electric field, so the trajectory within the box is straight.To do this, you need to think about which way the force from the magnetic points initially, and how to place an electric field so the force on the charge is in the opposite direction. Once you have figured this out, you can start working with some formulas. Also, In such a problem its reasonable to neglect gravity i would think.
  8. thank you engageengage!

    okay so for part a) the electircal field is in the postive z direction (out of the page) right?

    and for part b) do i use the formula Fm=qv*B ? Where v is the velocity or in other words, the speed? (by the way, Fm=Force magnetic)

    PLEASE help! thanks to all who have helped me till now. buti still need to understand how to do the rest of it.
  9. Doc Al

    Staff: Mentor

    First figure out the direction of the magnetic force on the proton. (You'll need to use the right hand rule.)
  10. well if i use the right hand rule, the magnetic force is in the -x direction (to the left).

    is that correct?
  11. Doc Al

    Staff: Mentor

    Good! So which way must the electric force act in order to cancel out the magnetic force? So which way must the electric field point?
  12. in order to cancel the magnetic force, the electric field must act in the positive x direction.

    so the elctric field should point in the positive z direction?
  13. Hootenanny

    Hootenanny 9,676
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    Now why would you say that? How does the electric field relate to the force experienced by a charged particle?
  14. do they attract each other?
  15. Doc Al

    Staff: Mentor


    You need to understand how to find the force on a charge due to both electric and magnetic fields. Look it up in your textbook.
  16. we don't have a textbook.
  17. can someone explain to me "how to find the force on a charge due to both electric and magnetic fields. " ????? Please?
  18. Doc Al

    Staff: Mentor

    Say what? What do you have?
  19. Hootenanny

    Hootenanny 9,676
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    If you haven't got a textbook, read your class notes.

    Edit: Sorry Doc, didn't see you there.
  20. Please don't be rude to me.

    Our teacher said that physics is better learned w/out a textbook. he justs gives us problems to work.

    Does this have anything to do with induction?

    Currently, i am studying my class notes. i have been, i just cna't seem to find anything about electic fields.
    in my class notes it says taht E= Fe/q
    Last edited: Apr 20, 2008
  21. Doc Al

    Staff: Mentor

    This is what textbooks are for. If you don't have one, get one!

    A good resource is hyperphysics. For example: Lorentz Force Law
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