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Question about electric potential energy?

  1. Sep 24, 2011 #1
    basically i don't understand the basic main idea of electric potential energy.
    since by definition, energy is ability to do work( to move an object across a distance).
    Negative source charge creates a uniform field around itself, and we place a positive test charge very near the source charge. why the electric potential energy so high? this confuses me because the test charge would just stick to source charge and do very little work.
    Now for instance image same test charge is place at 10 times farther than its previous location. electric potential energy is very small as r is increase but the test charge did more work because it has moved a greater distance.
    also please provide the main idea about electric potential energy
    thanks
     
    Last edited: Sep 24, 2011
  2. jcsd
  3. Sep 24, 2011 #2

    rcgldr

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    Homework Helper

    The confusing part is electric potential energy is a signed value (positive or negative) and it's not absolute but relative to some point of reference.

    For a negative source, the field is considerd negative. For a positve charge in a negative field, the potential energy is a negative value. That would mean that potential energy at a point near the negative source charge would be more negative than a point farther away. If the positive charge was accelerating towards the negative plate (and you ignore issue related to the moving positive charge and the field it generates, and this was in a vacuum), then it's kinetic energy increases, and it's potential energy decreases by becoming more negative.

    Note that gravitational potential energy is handled in the same way. For large distances, a common convention is to define a reference point at infinite distance from the gravitational source as zero potential energy, in which case gravitational potential energy becomes increasingly negative as you approach the center of the gravitational source.
     
  4. Sep 24, 2011 #3
    this means that the conservatin of energy holds here. But what doesn't more negative electric potential signifies.
    thanks for your concise response
     
  5. Sep 25, 2011 #4

    Drakkith

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    Staff: Mentor

    Hold on, did you know there is a difference between Electric Potential and Electric Potential Energy?
     
  6. Sep 25, 2011 #5
    Good catch, Drakkith.
     
  7. Sep 25, 2011 #6
    Let's start with the definition first:

    The reference zero for potential energy is usually at infinite separation and particles are at rest. The electric potential energy of a system is relative to this zero and positive work must be done to bring like (opposing) charges together. [You could define this other ways but this is the agreed upon convention.]

    As Post#2 notes, gravitational potential energy is handled the same way.

    Placing a positive charge near a negative charge, where the attraction becomes stronger, is of opposite sign to placing two charges of the same sign together, right?

    In your example, placing a positive charge near a negative charge, means potential energy is negative....the forces are attractive....you have to do work to keep the charges separated.
     
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