Charge homework help problem

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1. A charge of 16.0 [C] moves from the top plate to the bottom plate in the structure shown
below. A 12 [V] car battery is connected between the two metal plates. Find the amount of
energy that the charge picks up in moving from the top plate to the bottom plate in each part
below. Indicate whether the charge will be speeding up or slowing down as it moves from the
top plate to the bottom plate.


This one is easy though. I just do 12 x 16 = 192J. But my question is how I know if it's speeding up or speeding down? a) has a battery connected to two plates. Negative to the bottom and positive to the top b) negative to the top and positive to the bottom plate.
b) is -192 J because it's "reversed" ?
 

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  • #2
berkeman
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1. A charge of 16.0 [C] moves from the top plate to the bottom plate in the structure shown
below. A 12 [V] car battery is connected between the two metal plates. Find the amount of
energy that the charge picks up in moving from the top plate to the bottom plate in each part
below. Indicate whether the charge will be speeding up or slowing down as it moves from the
top plate to the bottom plate.


This one is easy though. I just do 12 x 16 = 192J. But my question is how I know if it's speeding up or speeding down? a) has a battery connected to two plates. Negative to the bottom and positive to the top b) negative to the top and positive to the bottom plate.
b) is -192 J because it's "reversed" ?
Which way is the Electric field pointing between the plates? What force does this E field impart to the positive 16C charge? Force produces acceleration...
 
  • #3
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Which way is the Electric field pointing between the plates? What force does this E field impart to the positive 16C charge? Force produces acceleration...
I don't know. That is why I am asking. THe picture dosen't tell me.
 
  • #4
berkeman
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I don't know. That is why I am asking. THe picture dosen't tell me.
The picture does not show + and - symbols on the battery? Can you post a link?
 
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Yes, it does. That's what I said in the first posting though. Battery a has + connected to the top plate and -connected to the bottom plate. Battery b has - connected to the top and + to the bottom plate of battery b.

That's what I meant by "reversed". So Battery b is - because the flow is pointing to a different direction?
 
  • #6
berkeman
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Yes, it does. That's what I said in the first posting though. Battery a has + connected to the top plate and -connected to the bottom plate. Battery b has - connected to the top and + to the bottom plate of battery b.

That's what I meant by "reversed". So Battery b is - because the flow is pointing to a different direction?
Ah, I missed that before. So the force vectors point in opposite directions in the two parts. Since the + charge is moving in the direction of the E field in part a), and opposite to E in part b), then yes, in one case it is picking up energy (accelerating), and in the other is is losing energy (decelerating).
 

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