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Let’s say I have a positive electric charge in middle of two plates, one is positively charged, the other one negatively with total intensity of electric field of 5 N/C. If the charge has 2 Coulombs, and is 3 meters from positive plate and 3 meters from negative plate, it will have potential energy relative to negative terminal 30 Joules, right?

But what potential energy will that charge have relative to the positive plate? -30 Joules? And what does that even mean? Can we speak of potential energy when comparing the positive charge with positive plate?

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It represents the energy required to move the charge closer to the positive plate. It is analogous to measuring the potential energy of a boulder at the bottom of a hill relative to the top of that hill.

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The initial force between the two charges separated by a distance d is

  • F=2kQ²/d²

  • Q=CU and E=1/2CU² [J]

  • F(d)= E(d) * e for electric field e
  • The change in potential energy is related to the (constant) electric field by ΔU = −eE⋅Δd

Now go study harder.

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