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Homework Help: Magnetic FOrce

  1. Apr 1, 2008 #1
    1. The problem statement, all variables and given/known data

    A charged particle is moving in a uniform magnetic field. Which of the following statements concerning the magnetic force exerted on the particle is false?

    A) It increases the speed of the particle.
    B) It does no work on the particle.
    C) It changes the velocity of the particle.
    D) It does not change the kinetic energy of the particle.
    E) It can act only on a particle in motion.

    2. Relevant equations

    3. The attempt at a solution

    C isnt it? because when a charged particle is in a magnetic field the magnetic force forms two right angles which will cancel each other and velocity will stay constantM
  2. jcsd
  3. Apr 1, 2008 #2


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    Homework Helper

    The magnetic force causes the charfed particle to move in a circular path. If it moves in a circular path what can you say about its speed and velocity? (Remember that velocity is a vector quantity!) ---> Your answer about speed will tell you about its K.E.
    Is the magnetic force exerted in the same direction as the diplacement for work to be done on it?
  4. Apr 1, 2008 #3
    umm there wont be work because the force and motion are in the same direction?
  5. Apr 1, 2008 #4
    You are right that no work is done, but it's because the force and motion are always perpendicular. Centripetal force is always directed radially inward, whereas the direction of motion in a circle is always tangent. In fact, all of the statements are true except for one of them. rockfreak gave you a BIG clue about which one.
  6. Apr 1, 2008 #5
    Ok so the velocity wil change therefore changing the kinetic energy...making the answer D
    KE = 1/2mv^2 so if velocity changes so will KE
  7. Apr 1, 2008 #6
    Kinetic energy does not depend on velocity, it depends on speed.
  8. Apr 1, 2008 #7
    Ok i got it wrong cant get marks for it can someone explain this to me please
  9. Apr 1, 2008 #8
    It sounds to me like you aren't quite solid on the difference between speed and velocity. Figure that out and it should all make sense.
  10. Apr 1, 2008 #9

    Shooting Star

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    Absolutely nothing. That is not the key point here.

    (Dear n77ler, please don't double post like this, within a short time.)
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