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How come potential energy is negative when charges attract

  1. Apr 26, 2013 #1

    x86

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    Insights Author

    and positive when they repel?

    It doesn't make any sense to me at all. To what are they referring to?

    The way I see it, if you're got....

    +.......-

    The closer they are, the less electrical potential energy they have. So it should still be positive.

    And if you've got two like charges

    C.......C

    The closer they are, they more electric potential energy they have. So it should also be positive.

    How come for my first example its negative? Mathematically its negative, ofc, since (-1) * (1) is always negative.. But I can't understand the concept of attraction being negative potential energy. It just doesn't make sense.

    I understand potential energy is all based on reference, but I just can't understand it. The book doesn't say why, it just basically wants you to memorize the facts. However, I know that you don't learn physics by memorizing, but by understanding. Hopefully someone can shed some light on this.

    Thanks
     
  2. jcsd
  3. Apr 26, 2013 #2

    Doc Al

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    Staff: Mentor

    Potential energy depends on your reference point. It is customary, and mathematically advantageous, to take "infinity" as your reference where PE = 0. Then, using your own reasoning, you should be able to see why two similar charges must have a positive PE for any finite distance, while two opposite charges must have negative PE.

    (The same is done for gravitational PE between two masses.)
     
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