Force and Charge........

[DocDrakeRamoray]

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This question should be easy for most of you.

The force created by a magnetic field acts on an electron by moving it in a circular path. If the path traveled by the electron remains the same, and the electron's speed is doubled, the magnitude of the force must:

A. be halved

B. remain the same

C. be doubled

D. be quadrupled

 

[LeftyUM]

D. mv^2/r = F?

 

[DocDrakeRamoray]

D. mv^2/r = F?

Yes, so the force would be quadrupled. but why if I use F=qvB, I get doubled force?

why F=qvB should be applied here?

 

[ksmi117]

Yes, so the force would be quadrupled. but why if I use F=qvB, I get doubled force?

why F=qvB should be applied here?

I'm assuming you're asking why it shouldn't be applied. It's because it doesn't take into account the radius. Also, for the radius to stay the same and the velocity to double, the field strength must also change. The question never says that B doesn't change (since it does... it also doubles).

 

[amikhchi]

Since the qvB = mv^2/r (the magnetic force is the force creating the centripetal accel)

setting those equal you get qB = mv/r, solving for B and you get B = mv/(rq). So in using F = qvB if v doubles, then you get a 2 from the v in the eqn and a 2 from the v (within the B) giving you 4

I know this was confusing and pointless, but it goes to show you CAN use F = qvB if you really wanted to but as long as you know how to use it in the context of the question. Always think centripetal force when you're told Uniform circular motion

 

[rocketbooster]

Since the qvB = mv^2/r (the magnetic force is the force creating the centripetal accel)

setting those equal you get qB = mv/r, solving for B and you get B = mv/(rq). So in using F = qvB if v doubles, then you get a 2 from the v in the eqn and a 2 from the v (within the B) giving you 4

I know this was confusing and pointless, but it goes to show you CAN use F = qvB if you really wanted to but as long as you know how to use it in the context of the question. Always think centripetal force when you're told Uniform circular motion

ohhh nice. and this is exactly what you need to be aware of. the mcat splices multiple topics together.

 

[DocDrakeRamoray]

Thanks guys, I see it now.