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Qualitative meaning of electric potential

by ehabmozart
Tags: electric, meaning, potential, qualitative
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ehabmozart
#1
Mar29-13, 10:43 AM
P: 194
Allright, when I say what is electric potential energy, someone will say it is kq1q2/r ... This answer doesn't satisfy me because what I am searching for is this.. Electric potential energy denoted by U is the work done by an external force to bring a charge from infinity to zero" ...That is why keeping two positive charges, the force done by hand would be positive, therefore U possessed by the test charge realtive to infinity is positive. Likewise, I am asking my new question now... WHAT IS ELECTIC POTENTIAL... I don't need formulas, I need the qualitative meaning! ... Thanks for anyone woshares his ideas.. I owe a lot to those people!
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msbell1
#2
Mar29-13, 12:22 PM
P: 22
I think it's kind of just an abstraction to make it easier to think about and solve problems. Really, the basic thing here is the force between two charges, given by Coulomb's Law. From that, we can begin to talk about an electric field (force per unit charge)---something that can exist with only one charge (whereas Coulomb's Law requires at least two charges to find a force). That electric field tells us something about how other charges will move when placed in the electric field (similar to the concept of a gravitational field, which tells us how matter will move). But, electric fields can be messy, as they are vectors. As you learned in your mechanics class, sometimes it is much easier to solve a problem if you forget about forces, and just make use of conservation laws, one of the most commonly used being the conservation of energy. The concept of energy allows us to solve some problems using just scalar quantities. We can consult Coulomb's Law again to calculate an electric potential energy for two charges, using the concept of work (integral of force dotted with displacement). Looking at changes in energy between multiple charge configurations can be helpful for problem solving, but what if we would like a more general description of what would happen if we add a charge to the system? Then it would be nice if we didn't have to always do our calculations with at least two charges. One would suffice. Or we could have some kind of a charge distribution. Both of these, of course, would create an electric field. But if we didn't care about vectors to solve our problem, we could compute an electric potential energy per unit charge. Then we can predict the behavior of any charge that we add to the system. (The charges we add will try to move from areas of high potential energy to low potential energy, or, equivalently, from areas of high electric potential to low electric potential (for positive "test" charges) and from areas of low electric potential to high electric potential (for negative "test" charges).)

I don't know that an electric potential is a "real" thing. It is a useful, abstract quantity derived from the real thing, which is Coulomb's Law. Perhaps the best way to think about an electric potential is as a sort of elevation. Moving to lower potential is like moving down a hill (for positive charges). Moving to higher potential is like climbing a hill---positive charges will require either some initial kinetic energy or some work done on them to move up these "hills".

Well, hopefully that is a bit helpful.
sophiecentaur
#3
Mar29-13, 07:25 PM
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Potential is as "real" as force. It just tells you the energy needed to bring a unit charge from infinity to the point in question. That's no less real than expressing the effect in terms of acceleration on a notional unit mass with that unit charge. The fact that one is a vector and the other is a scalar is not particularly significant. We may find 'force' a bit more easy to deal with because we can feel forces a bit more directly but that's only a matter of relative familiarity.
To ask what something "is" is more than one can really expect. Do not confuse familiarity with understanding. All through history, people have argued against good Science on the grounds that it didn't make sense when tested against familiar ideas of the time. A few decades later, that bit of Science was accepted as making excellent sense because people got used to it. Potential is just a way of looking at a situation and assigning a 'value' in order to calculate and predict things.

ehabmozart
#4
Mar30-13, 11:02 AM
P: 194
Qualitative meaning of electric potential

IN our definition of electric potential energy, we said it is the work done by hand to move a unit charge from infinity to r (point of interest). How then would potential have almost the same difference. Moreover, I don't get the issue of how positive and negative charges get affected by potential. Can I have an elaborate explanation. Thanks!
sophiecentaur
#5
Mar30-13, 12:27 PM
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Quote Quote by ehabmozart View Post
IN our definition of electric potential energy, we said it is the work done by hand to move a unit charge from infinity to r (point of interest). How then would potential have almost the same difference. Moreover, I don't get the issue of how positive and negative charges get affected by potential. Can I have an elaborate explanation. Thanks!
Stop right there, go back to the definitions and just follow it through, ignoring any existing subjective feelings you may have. Electric Potential is defined in terms of the behaviour of a unit of positive test charge. So the potential of that charge with respect to another positive charge is positive (repulsion, needing work to be put in to get closer) and with respect to a negative charge, it's negative (needing negative work to be done in order for it to get closer).
ehabmozart
#6
Mar30-13, 12:48 PM
P: 194
Quote Quote by sophiecentaur View Post
Stop right there, go back to the definitions and just follow it through, ignoring any existing subjective feelings you may have. Electric Potential is defined in terms of the behaviour of a unit of positive test charge. So the potential of that charge with respect to another positive charge is positive (repulsion, needing work to be put in to get closer) and with respect to a negative charge, it's negative (needing negative work to be done in order for it to get closer).
I have a question here. It is said that negative charges gain kinetic energy by moving to lower potential. How can you explain this?
sophiecentaur
#7
Mar30-13, 02:24 PM
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Quote Quote by ehabmozart View Post
I have a question here. It is said that negative charges gain kinetic energy by moving to lower potential. How can you explain this?
What is the DEFINITION of potential? Work it out for yourself and don't insist that what you have been told is wrong. The answers have all been given to you in various forms. It's up to you now.
Just think how you can give a negative charge more KE by moving towards a lower electrical potential. Does that make sense? You are confusing KE with PE, for a start.


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