# Barrons - Model Exam C - Question 22

Discussion in 'MCAT Study Question Q&A' started by eedoctor, May 27, 2008.

1. ### eedoctor 5+ Year Member

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electric field points horizontally north.

horizontal magnetic field pointing west

electron traveling south, proton traveling north.

How does the magnetic field affect the speed of the electron and proton?

A) The speeds of both are not affected.
B) The proton speeds up and the electron slows down.
C) Both speed up due to the magnetic forces
D) The electron speeds up and the proton slows down.

Barron's claims the answer is A.

However, shouldn't the answer be C? The magnetic field will cause both the electron and proton to have a force pointing upward. Now the proton and electron will have two velocity vectors (one due to Electric field, and another due to Magnetic field)

speed = ( velocity_electric_field^2 + velocity_magnetic_field^2 ) ^ (1/2)

shouldn't the speed increase???

2. ### fileserver 10+ Year Member

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Magnetic field force only changes direction, not magnitude. If you only look at the magnetic field force acting on a proton, you know that the direction changes making the proton to travel in a circle. So the magnitude of velocity due to magnetic field in your equation is the same all the time. I believe the question asks, as time goes on how the magnetic field would affect the speed.

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