1. The SDN iPhone App is back and free through November! Get it today and please post a review on the App Store!
    Dismiss Notice
  2. 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
  3. Dismiss Notice
Dismiss Notice

Interview Feedback: Visit Interview Feedback to view and submit interview information.

Interviewing Masterclass: Free masterclass on interviewing from SDN and Medical College of Georgia

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

Discussion in 'MCAT: Medical College Admissions Test' 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:
    1,254
    Likes Received:
    50
    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