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Electricity problem, is high potential at

  1. Apr 30, 2013 #1
    1. The problem statement, all variables and given/known data
    A proton of mass 1.67 * 10^-27 kg moves from a location where V1 = 75 V to a spot where Vf = -20V.

    a) Calculate the change in the protons kinetic energy
    b) Replace the proton with an electron, determine its change in kinetic energy


    2. Relevant equations



    3. The attempt at a solution

    The kinetic energy does not matter to me, but the sign does; so for a)

    The way I see it, the diagram looks like this:

    ++++++++++++++++++ (75 V)
    A

    B
    ----------------------- (- 20V)

    The proton moves from A to B, so it loses potential energy and it converts to kinetic energy. Kinetic energy is Positive, and this is correct!

    B is where the confusion begins :(

    b)

    This is how I see the diagram in my head
    ++++++++++++++++++ (-20 V)
    B

    A
    ----------------------- (75 V)

    The electron moves from A to B. It's potential energy decreases and converts to kinetic energy! What? I got the same answer as A, positive kinetic energy! But the answer is negative kinetic energy!

    The only way to explain this is that for b my diagram was indeed wrong, meaning it should be 75 V at the positive terminal and -20 V at the negative terminal! But this goes against what my teacher taught me.

    Hopefully someone can clear up my confusion. Thank you for your time!
     
  2. jcsd
  3. Apr 30, 2013 #2
    Are you sure you mean kinetic energy and not potential energy?
     
  4. Apr 30, 2013 #3
    Nope, I don't mean potential energy. I mean kinetic energy. Are the diagrams I drew also correct? Will the "high" and "low" voltages switch positions depending on if you have an electron or proton between them?
     
  5. Apr 30, 2013 #4

    TSny

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    Is it the potential energy that decreases or the potential that decreases? How do you relate change in potential to change in potential energy?

    Also, note that you want to determine the change in kinetic energy, not the kinetic energy itself.
     
  6. Apr 30, 2013 #5
    The question states the potential decreases, but so does the potential energy when I look at it position wise on my little drawing.

    So the potential energy should convert into kinetic.

    The change in kinetic would be ekfinal - 0 which would just be ekfinal

    Are my drawings accurate? Or will the top positive plate ++++++++++ always be at 75 V? My teacher says it changes depending on if theres an electron in it or not, but this contradicts with the books answers
     
  7. Apr 30, 2013 #6

    TSny

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    The electron starts at position A where the potential is +75 V. How would you find the potential energy of the electron at position A?
     
  8. Apr 30, 2013 #7
    Well, I could use E = kq1q2/r But since this information is missing I could subtract B from A to give me -20 V - 75 V = -95 J/C then multiply it by the electron constant change to get a positive number for potential energy, which would give me a negative number for the change in kinetic energy.

    However, this would contradict the diagram, because:

    ++++++++++++++ (-20V)
    B

    A
    ------------------- (75 V)

    The electron goes from A to B, it has more potential energy at point A (because the negative charge is pushing it up) and less at point B

    So it actually gains kinetic energy from going to point B from A

    The only conclusion that I can say is that the diagram has to be wrong

    For an electron, which is correct?

    a)
    ++++++++++++++ (-20V)
    electron here
    ------------------- (75 V)

    b)
    ++++++++++++++ (75V) <-- if this is correct, everything makes sense
    electron here
    ------------------- (-20 V)
     
    Last edited: Apr 30, 2013
  9. Apr 30, 2013 #8

    TSny

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    Why are you assuming that negative or positive charge is located at positions A or B? A and B are just points in space where you are given values for the potential. Take away all those -------- and +++++ in your diagram. You just have two points A and B and an electron. There is a simple formula for finding the potential energy of a charge when it is located at a point where the potential is known.
     
  10. Apr 30, 2013 #9
    Ah thank you, that clears it up.

    For some reason I was thinking they were between parallel plates

    It makes sense to me now.

    However, for another question; are my initial diagrams correct for low and high potential? (assuming it was between parallel plates)

    For an electron, would the high potential be at the negative plate and low potential be at the positive plate?

    And for a proton, it would be opposite, correct?
     
  11. Apr 30, 2013 #10

    TSny

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    The positive plate would be at the higher potential and the negative plate would be at the lower potential. That's true irrespective of whether you place a proton or electron at either plate.

    A proton will have higher potential energy at the postive plate compared to when it is at the negative plate.

    The electron will have higher potential energy at the negative plate compared to when it is at the positive plate.
     
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