AAMC 9 Physical Sciences, Q 6

[Natasha1989]

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6. Which of the following best describes the motion of a negatively chargeed particle after it has been injected between the plates of a charged, parallel-plate capacitor? (Note: area between is a vacuum)

a) it moves with constant speed towards the positive plate
b) it moves with constant speed towards the negative plate
c) it accelerates toward the positive plate
d) it accelerates toward negative plate

I put a), but the answer is c). I thought the speed is constant in vacuum. Can someone please explain?

 

[kentavr]

The electron moves because electric force act on it all the way.
Any body under force will accelerate. (a=F/m).

The interesting part of it is that between two plates this force is constant. More formal, Force affecting electron is
F=e*E,
where e-electron charge and E is a vector of electric field.

For parallel (infinite) plates this vector E is constant.
E is easy to calculate, but I am not sure that it is in MCAT, so I skip it. The important fact is that it is constant.

Therefore the force is constant.
Dynamically this model is not very different from free fall of the body to the ground. HTH

 

[ownage]

Speed is constant for object in space if there is no force acting on it.

If there is force acting on something, then their acceleration because F = ma
In parallel plate capacitor, there is electric field, which exerts force on electron.

Thus, electron accelerates. Acceleration is constant, however.

 

[MCAT4life]

I think I missed it too, isn't it fun for AAMC to write up questions like that.. Such an easy concept, with integration of Newton's 2nd law, yet if you are not careful it will get you lol..

 

[luctoretemergo]

The electron moves because electric force act on it all the way.
Any body under force will accelerate. (a=F/m).

The interesting part of it is that between two plates this force is constant. More formal, Force affecting electron is
F=e*E,
where e-electron charge and E is a vector of electric field.

For parallel (infinite) plates this vector E is constant.
E is easy to calculate, but I am not sure that it is in MCAT, so I skip it. The important fact is that it is constant.

Therefore the force is constant.
Dynamically this model is not very different from free fall of the body to the ground. HTH

does this hold true for charged capacitors too, i'd assume?
also, how would one decrease charge in a charged capacitor (vs inc distance or decreasing area in parallel plate cap)