Aamc 9.. This answer is wrong!! Wtf

[bgoldst1]

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Please help.. I AM SO CONFUSED
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AAMC 9 Q6 PS

Which of the following best describes the motion of a negatively charged particle after it has been injected between the plates of a charged, parallel-plate capacitor? (Note: Assume that the area between the plates is a vacuum.)
A) It moves with constant speed toward the positive plate.
B) It moves with constant speed toward the negative plate.
C) It accelerates toward the positive plate.
Opposites attract, so the negatively charged particle will move toward the positive plate. Because there is a constant force qE on the particle, it will accelerate toward the positive plate. Thus, C is the best answer.
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MY JUSTIFICATION
The answer should be A.
It is true that as the negative charge approaches the positive, the attractive forces increase, thus providing a constant force on the charge. BUT is it not also true that as it moves farther from the negative plate, the repulsive forces are decreasing at exactly the same rate as the positive force is increasing to the positive plate. This should leave a net force of zero... which is why I am confused.

 

[Crash88]

think about it in terms of forces
A force results in acceleration


(negative plate)| -> P ----> | (positive plate)

Even if the repulsion force is decreasing (as indicated by the arrows), it is still working in the same direction as the attractive force. Meaning, no matter what, there is a net force in the direction of the positive plate. Net force = net acceleration. Why? F=ma.

 

[kobe200LATE]

Also, when the electron moves to the plate with a higher electric potential, its potential energy decreases (PE = e * V where e is negative). Because the change in kinetic energy = the opposite of the change in potential energy, the electron will gain in kinetic energy. Because delta KE is positive, KEfinal is greater than KEinitial, which means that it gains velocity and therefore accelerates.

 

[matth87]

If velocity is constant it means there is no acceleration. There is obviously an acceleration because it will move towards the positive plate.

velocity is not constant.

 

[bgoldst1]

oh okay i get it thx a lot guys... its accelerating, just at a constant rate, the acceleration is not increasing just the velocity is