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Homework Help: Struggling with the rules behind electricity

  1. May 14, 2016 #1
    1. The problem statement, all variables and given/known data
    A moving particle encounters an external electric field that decreases its kinetic energy from 9480 eV to 8260 eV as the particle moves from position A to position B. The electric potential atA is -43.0 V, and that at B is +25.0 V. Determine the charge of the particle. Include the algebraic sign (+ or -) with your answer

    2. Relevant equations
    deltaE = q*deltaV

    3. The attempt at a solution
    I got the solution correct using that formula, I was just wondering why the solution is positive? I thought NEGATIVE charge should flow from low (-43.0 V) to high (+25.0 V) voltage.

    Also, does anyone know any good resources to learn the rules of electricity e.g. electrons flow from negative to positive
  2. jcsd
  3. May 14, 2016 #2
    Yes, you are right. In normal conditions, negative charge should flow from low to high voltage. In that case the charge would gain kinetic energy at the expense of potential energy.
    But if we force a positive charge to go from negative voltage to positive voltage, it will go as desired by losing its kinetic energy. Look at the problem statement, that is exactly the case. Kinetic energy of the charge particle has decreased. So, it should be positive charge which has moved.
    So it's not the normal case as you think. I hope it helped.
  4. May 14, 2016 #3

    Oh wow okay thanks man!
  5. May 14, 2016 #4


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    The key is that the field determines the direction of the force on the charge rather than the direction of motion itself. The force will cause an acceleration and that may eventually change the direction of motion but it depends on the initial conditions. It's a bit like throwing a ball upwards while gravity acts downwards.
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