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Homework Help: Electron in equilibrium between two plates

  1. Feb 4, 2014 #1
    1. An electron is placed between two charged parallel plates. The electron is in equilibrium between the electric force and the gravitational force.
    a) Make a drawing of the experimental setup. Label the direction of the gravitational field, the placement of the plates, and indicate which plate is positively or negatively charged.
    b) What is the value of the electric field? (Necessary constants can be found in the book)

    How do I know where the electron is in equilibrium between the forces??:eek:
  2. jcsd
  3. Feb 4, 2014 #2
    Does it mean that there is electrostatic equilibrium?
  4. Feb 4, 2014 #3
    Where the electron is in equilibrium is irrelevant to the question, you can figure it out, but you don't need to. First off draw a free body diagram, and equilibrium in force question almost always means the same thing.
  5. Feb 4, 2014 #4
    - - - - - - -


    + + + + + + +
  6. Feb 4, 2014 #5
    That didn't show up quite right i had the electron in the dead center
  7. Feb 4, 2014 #6
    In the set up the plates are like = i.e one on top of the other ,not || i.e placed side by side .

    And,no the electron is not in electrostatic equilibrium .It cannot be .Positive plate attracts it,whereas the negative plate repels it .There is a net electrostatic force on the electron .But this force is balanced by the gravitational pull ,hence causing an overall equilibrium .
  8. Feb 4, 2014 #7
    i had them set up horizontal. Is the electron just in the middle of the plates?
  9. Feb 4, 2014 #8
    It can be anywhere .It doesn't matter .You can place it where ever you like .What matters is that the two forces are equal and opposite .
  10. Feb 4, 2014 #9
    The gravitational field would be going down correct?
  11. Feb 4, 2014 #10
    or does that just cancel out?
  12. Feb 4, 2014 #11
    Yes..The electrostatic force will be upwards whereas gravitational pull downwards.
  13. Feb 4, 2014 #12
    Is the electric field value just 0 then? or do i add the electrons charge to 0
  14. Feb 4, 2014 #13
    No...The electric field between the plates of a parallel plate capacitor is σ/ε0 where σ is the charge density on the plates .
  15. Feb 4, 2014 #14
    E= σ/2ε0
  16. Feb 4, 2014 #15

    E= σ/2ε0 is due to only one plate .The net electric field is σ/ε0 .
  17. Feb 4, 2014 #16


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    Check the sketch in post #4. If the negative plate is on top and the positive plate below the electron, where does the electric field on an electron point to? Is that right?
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