1. Hey Guest! Check out the 3 MCAT Study Plan Options listed in the 'stickies' area at the top of the forums (BoomBoom, SN2ed, and MCATJelly). Let us know which you like best.

    Also, we now offer a MCAT Test-Prep Exhibitions Forum where you can ask questions directly from the test-prep services.
    Dismiss Notice
  2. Download free Tapatalk for iPhone or Tapatalk for Android for your phone and follow the SDN forums with push notifications.
    Dismiss Notice

Quick physics question. Electric field's effects on KE?

Discussion in 'MCAT Discussions' started by Lacipart, May 7, 2007.

  1. Lacipart

    Lacipart M1 at UW-Madison
    5+ Year Member

    Joined:
    May 2, 2007
    Messages:
    291
    Likes Received:
    0
    Status:
    Medical Student
    *EDIT* Questioned was answered below, thanks!

    A magnet cannot do work. But I found a question online asking something along the lines of "a magnetic field can't affect which of the following?".

    I narrowed it down to momentum or K.E.

    Both have velocity and mass in their equation. Only algebraic difference is KE has a V^2. Hmm, any idea?
     
  2. Note: SDN Members do not see this ad.

  3. Rofeh20

    Rofeh20 Kaplan MCAT Instructor
    5+ Year Member

    Joined:
    Feb 1, 2007
    Messages:
    70
    Likes Received:
    0
    Status:
    Other Health Professions Student
    Momentum is a vector, and since magnetic fields can change the velocity vector of moving charges momentum is definitely not your correct answer.

    On the other hand, kinetic energy is a scalar term.
     
  4. RPedigo

    Physician Faculty Moderator Emeritus 10+ Year Member

    Joined:
    Aug 5, 2006
    Messages:
    1,248
    Likes Received:
    45
    MDApps:
    Status:
    Attending Physician
    In a magnetic field, you can move stuff around in a circular path. You can't do work, so you can't accelerate in one direction, but if you go in a circle, the average acceleration becomes zero via vector addition.

    So since kinetic energy is a scalar, and doesn't care where it's oriented, you can't change that-- it has no directional property, so any change in kinetic energy would have to be due to a change in its velocity.

    Edit: On one hand, my post contradicts the person above. On the other hand, p223 in the Princeton Review Physical Sciences Review says:

    "Since the magnetic force a charge feels is always perpendicular to the velocity of the charge, magnetic forces do no work. Recall that is a force F is perpendicular to the displacement d of an object, then this force F does zero work, because W = Fdcos(90) = 0. Since magnetic forces never do work, they can never change the kinetic energy of a charged particle. This follows from the work-energy theorem, W = d(KE); if W = 0, then d(KE) = 0 also, so KE is constant."
     
  5. Lacipart

    Lacipart M1 at UW-Madison
    5+ Year Member

    Joined:
    May 2, 2007
    Messages:
    291
    Likes Received:
    0
    Status:
    Medical Student
    Ooo, that's kinda tricky. Totally forgot that one isn't scalar. Thanks a bunch guys! :)
     
  6. 12thandSouth

    12thandSouth Junior Member
    5+ Year Member

    Joined:
    Sep 26, 2005
    Messages:
    29
    Likes Received:
    0
    This question was on the April 12th MCAT. They gave you a bunch of superfluous information, including a diagram, and finished with something like "The KE of the object is x when it enters the field -- what is its KE upon exiting the field?"

    Those tricky MCAT writers!
     
  7. Rofeh20

    Rofeh20 Kaplan MCAT Instructor
    5+ Year Member

    Joined:
    Feb 1, 2007
    Messages:
    70
    Likes Received:
    0
    Status:
    Other Health Professions Student
    We don't contradict. We agree. You explained it differently than I did. I just saw right away that momentum could not possibly be the answer b/c it is a vector quantity and since the velocity vector will constantly change that cannot be the answer. But I didn't explain why the right answer was right, and you nailed that. Nice job.
     

Share This Page