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Homework Help: Velocity of an electron

  1. Oct 23, 2004 #1
    i am just wondering, is it possible to find the velocity of a single electron that is accelerated by a certain potential difference?

    Also im having trouble with this homework problem:
    A coil with a magnetic moment of 1.40 A*m^2 is oriented initially with its magnetic moment antiparallel to a uniform magnetic field of magnitude 0.845T.

    What is the change in potential energy of the coil when it is rotated 180 degrees so that its magnetic moment is parallel to the field?

    I remember that antiparallel means when two vectors just have the same direction cept that they are opposite signs so wouldnt the change in PE just be 0?

    I'm not too sure about that one.
    Last edited: Oct 23, 2004
  2. jcsd
  3. Oct 23, 2004 #2
    anti parallel means perpendicular.
  4. Oct 23, 2004 #3
    Yes. You can find the velocity of an electron accelerated through a particular potential difference.

    Remember that:

    potential differerence (aka voltage) * charge = Change in potential energy (U)....

    Since you know the charge of an electron....you can simply multiply this by the potential difference to figure out the energy change.

    KE = U
    Now, since KE = 1/2 mv^2 ......you can plug in and solve for velocity.
  5. Oct 23, 2004 #4
    the problem states that what happens when it is rotated 180 degrees and it is parallel. if anti parallel meant perpendicular, then it wouldn't be parallel.
  6. Oct 23, 2004 #5
    I was also interested in the speed of an electron due to it's voltage so I plugged your formula into mathcad and when I use a voltage of 1 million volts I get a velocity of 5.931X108m/s. This is faster than the speed of light? What am I doing wrong?
  7. Oct 23, 2004 #6
    Well...the method I just gave you is based on classical mechanics, which basically works ok for anything << speed of light. However, anything remotely close to the speed of light (such as something with a speed of 0.1c)...would require the relativistic definitions....

    In classical mechanics:

    KE = 1/2 mv^2

    In relativistic mechanics:

    M = rest mass

    KE = (MC^2 * y) - (Mc^2)...

    with y = 1 / sqrt. (1 - v^2/c^2)
    Last edited: Oct 23, 2004
  8. Oct 23, 2004 #7
    Thanks, I'll give that a try.
  9. Oct 23, 2004 #8
    can anyone still help me with my second problem?
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