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

Discussion in 'MCAT: Medical College Admissions Test' started by Lacipart, May 7, 2007.

1. ### Lacipart M1 at UW-Madison 5+ Year Member

Joined:
May 2, 2007
Messages:
291
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 Kaplan MCAT Instructor 5+ Year Member

Joined:
Feb 1, 2007
Messages:
70
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 FacultyModerator Emeritus 10+ Year Member

Joined:
Aug 5, 2006
Messages:
1,254
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 M1 at UW-Madison 5+ Year Member

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

6. ### 12thandSouth Junior Member 5+ Year Member

Joined:
Sep 26, 2005
Messages:
29
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 Kaplan MCAT Instructor 5+ Year Member

Joined:
Feb 1, 2007
Messages:
70
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.