Physics Help Forum Find the potential energy lost by an electron as it moves from B to A (with solution)

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Jan 27th 2011, 06:07 PM   #1
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Find the potential energy lost by an electron as it moves from B to A (with solution)

My question (along with its solution) is under #25.15 of the attached document.

I know this may sound very dumb to those who aren't knew to this stuff but technically I haven't even covered this in class yet .

My problem is:
What does it mean when the book says "Notice that V is a potential rise from B to A." and how does one determine when it is a potential rise or drop?

Any input would be GREATLY appreciated!
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 tempQ.pdf (71.2 KB, 5 views)

Jan 27th 2011, 08:22 PM   #2
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Join Date: Dec 2009
Posts: 209
Hi there

 Originally Posted by s3a My question (along with its solution) is under #25.15 of the attached document. I know this may sound very dumb to those who aren't knew to this stuff but technically I haven't even covered this in class yet . My problem is: What does it mean when the book says "Notice that V is a potential rise from B to A." and how does one determine when it is a potential rise or drop? Any input would be GREATLY appreciated! Thanks in advance!
Actually what it should say is that the electron loses potential energy going from plate B to A, no rise, a drop. What they mean is going from plate B to plate A there is an increase in the potential to do work on objects. In other words the voltage increases as you move from plate B to plate A. V = Ed. So as you move from plate B covering a distance d, getting to plate A with some constant E field, you get an increase in voltage. Voltgae is an electrical potential to do work. So it is not wrong what they are saying, it is actually correct, but an electron will lose potential energy as it moves from plate B to plate A. When at plate B it is being pulled on by plate A, or looked differently is being pushed on by like charges on plate B to move to where there is less like charges at plate A. So it has the potential to do something while at plate B. It can run into another electron and knock it out of the way. So at plate B, while being held there it has a potential to do work. So it has a potential energy. When it is released that potential energy is being converted into kinetic energy as it gains speed and by the time it reaches plate A it no longer has any potential energy but all that it did have, assuming it lost none getting there, was converted into kinetic energy.

It is much like holding a stone above your head. Gravity wishes to pull that stone onto the ground. So the stone, with respect to the ground, has a potential to do work on something. When it is dropped that potential energy is converted into kinetic energy and by the time it strikes the ground it has no more potential energy as all that it had is at that point kinetic energy.

So if you are wondering when or how to decide if something has a potential energy. Look at it like, does it have the potential to do work on something if it is say released and moves towards something else. If it does, then in getting to that some place else it has a potential energy. If you want to give something a potential energy, place it some where that if released it can freely move and do work on something in its way when released.

I hope that helps,
Craig

Last edited by clombard1973; Jan 27th 2011 at 08:29 PM.

 Tags electron, energy, find, lost, moves, potential, solution

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