1. Limited time only! Sign up for a free 30min personal 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!

What happens if plates of charged capacitor gradually separated?

  1. Jun 13, 2013 #1
    Say you have a charged, parallel plate capacitor, isolated from any external circuit. If you were then to gradually seperate the two plates (or thicken the dielectric) what would happen to the charge on the two plates ?
     
  2. jcsd
  3. Jun 13, 2013 #2

    phyzguy

    User Avatar
    Science Advisor

    What do you think would happen? Where could the charge go?
     
  4. Jun 13, 2013 #3
    My guess is that excess charge must radiate. perhaps arcing or coronal discharge
     
  5. Jun 13, 2013 #4

    phyzguy

    User Avatar
    Science Advisor

    What "excess" charge? Why should it go anywhere?
     
  6. Jun 13, 2013 #5
    If you think of a capacitor like that in a perfect vacuum (theoretical conditions) there is no where for the charge to go and just by separating the plates doesn't make the charge to vanish.
    Indeed what you would get is a increase in charge strength because now the energy that was stored in the electric field goes back to the plates.A capacitor works by that as it stores energy in electric field , hence separating the plates further and further would diminish the field between the plates.
    + you are doing work on the plates as you are separating them.
     
  7. Jun 13, 2013 #6
    Thanks crazymechanic. Yes, an increase in charge strength makes sense. Is there a quantum physics description of what would be going on here ?
     
  8. Jun 13, 2013 #7
    The correct answer is that the charge remains constant. The capacitance decreases as the plates are separated, and the voltage increases in inverse proportion. I.e., if you halve the capacitance, the voltage doubles.
    Since the energy stored is proportional to capacitance and to voltage squared, the energy is increased! Where does the extra energy come from? It comes from the work you do in separating the plates because you are working against the attractive force. This is the principle of the Electrophorus and of the Wimshurst Machine (q.v.).
     
  9. Jun 13, 2013 #8
    Thanks. I never thought it'd add energy. I have many questions about capactance.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: What happens if plates of charged capacitor gradually separated?
Loading...