Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Field Intensity of Flat Capacitor

  1. Dec 18, 2011 #1
    1. The problem statement, all variables and given/known data

    Two identical flat capacitors with no dielectric material inside are connected in series to a voltage source. The electric field intensity inside the capacitors is E. If the space inside one of the capacitors is filled with a dielectric material of ε=9 how strongly did the field intensity change if:

    a) the circuit was connected to the voltage source when the capacitor was filled with a dielectric

    b) the circuit had been disconnected from the voltage source before the capacitor was filled with the dielectric

    2. Relevant equations

    Electric field intensity becomes reduced by:

    E = Eexternal / ε

    C = ε0εS/d

    3. The attempt at a solution

    The capacitance increases by a factor of 9

    Therefore the charge on the capacitor must increase by a factor of 9 therefore the charge on the other series capacitor increased by a factor of 9

    Therefore the electric field inside the filled capacitor must increase by a factor of 9

    Even if the above is correct what difference does having the source connected or disconnected do? I'm assuming it must be related to the fact that the two are in series therefore the charge on each must be the same.
  2. jcsd
  3. Dec 18, 2011 #2


    User Avatar

    Staff: Mentor

    If the voltage source remains connected then there is a complete circuit and current can flow when conditions change (like having a dielectric inserted in one of the capacitors). That means that the charges on the capacitors can change. The total potential across the capacitors must remain the same (equal to the voltage source).

    If the voltage source is disconnected before the dielectric is inserted, then there is no complete circuit and current will not flow. That means the individual charges must remain the same on the capacitors. On the other hand, there is no longer a restriction on the total potential difference across the series capacitors.

    You'll have to work within those constraints in order to determine how the charges and or voltages will change in each case.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook