1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A positron moving from a Charged Plate to Another

  1. Dec 20, 2008 #1
    A positron moving from a Charged Plate to Another [SOLVED]

    Can you solve the question a and b and explain it to me ?

    What i know ? ( I'm not sure though );
    Kinetic Work = Kinetic Energy = 1/2 m . v^2
    Electrical Work = Electrical Energy = q.V

    So at the begin, the positron isn't moving,
    so it has Potantial energy maybe...

    At the point K its Energy;
    3V.q.(1/3d).d = V.q (?)

    At the point L its Energy;
    Vq - 4V.q.(1/2d).d = -Vq (?) ( How can energy be negative ? Oops ? )
    a)EnergyK/EnergyL = ?
    b) speedK / speedL = ?
     

    Attached Files:

    Last edited: Dec 20, 2008
  2. jcsd
  3. Dec 20, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What's the potential difference (volts) between M and K? When the positron goes from M to K, how much energy does it gain?

    Use similar thinking between M and L.
     
  4. Dec 20, 2008 #3
    The Potential difference between the start and K isn't given in the question.
    I dont know too...
    But i have found the energy gain between the start and K ;
    q.3V.(1/3d).d =qV


    The energy gain between start and L is;
    3qV gain (between start and Y Plate) + 2 qV Loss ( between Y plate and K) = 1 qV Net Gain
    Start --> K = qV gain
    Start --> L = qV gain

    The electrical gain means, the kinetical gain,
    a)
    Total_Kinetic_At_K / Total_Kinetic_At_L = qV / qV = 1...
    b)
    Speed is related with kinetical Energy; The positron has same kinetical energy at both points, so has Same Speed at both points... Speed_K/ Speed_L = 1

    Can you please check ? All right ? Any mistake ?

    if all above in this message are right :) Then i have a question to you;
    How you calculate the Voltage Difference between the Start and K ? ( between X plate and K point )
     
  5. Dec 20, 2008 #4

    Doc Al

    User Avatar

    Staff: Mentor

    Sure it is. You should be able to deduce it from the diagram. Hint: Between X and Y is 3 volts. (The field between the plates is uniform.)
     
  6. Dec 20, 2008 #5
    So between X and K is 1 Volt ?
    Or between X and K is still 3 Volts ?

    I think;
    Anywhere between X and Y plates are under 3 Volt potential difference...
    But the Potential gain is depended with how much you have moved between X and Y...
    If you move from X to K then you gain 1 Volt... Or if you move from Y to K then you lose 2 Volts.So; we can't say that the potential difference between X to K is 1 volts right ?
    Or can we say ? Exactly im not sure about the difference between;
    Potential Difference ( Voltage Difference ) and Potential Gain ( Electrical Gain - Voltage Gain)

    1) Please, clean my mind about the above statment.
    2) Did i solve the problem right ?
     
  7. Dec 20, 2008 #6

    Doc Al

    User Avatar

    Staff: Mentor

    Between X and Y is 3 volts. Since K is 1/3 the distance between the plates, the voltage between X and K is 1 Volt. (Note that X is 3 volts above Y.)

    How about this. Let's call the voltage of plate Y to be zero (the reference point is arbitrary). So, the potential of X is +3, the potential of K is +2, the potential of Y is 0, the potential of L is +2, and the potential of Z is +4. When the positron moves from X to K, it loses PE and gains KE in the amount of 1 eV. (What's the drop in PE from X to L?)
     
  8. Dec 20, 2008 #7
    Sorry, English isn't my native language.
    What means eV. ?
    I think its shortening of something but i dont know...

    From X to L,
    PE Drops 3, gains 2 --> PE Drops 1 Volt

    From X to K,
    PE dropps 1Volt and KE gains it somehow. ( Gains it as speed ( V^2 ) )

    And yes i have exactly understood what you said except the last sentence i quoted.
    Thank you so much :)
     
  9. Dec 20, 2008 #8

    Doc Al

    User Avatar

    Staff: Mentor

    eV stands for electron-Volt, a unit of energy. See: electron Volt


    Yes.

    Yes.

    (Energy is conserved. When the particle loses electric PE, it gains KE.)
     
  10. Dec 20, 2008 #9
    Understood everything...
    Thank you very much...
    I have no more questions related to this...

    If you have any question for me, you are welcome.

    //Problem Solved.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: A positron moving from a Charged Plate to Another
  1. Moving a charge from inf (Replies: 10)

Loading...