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!

Homework Help: Pd across the capacitors

  1. Mar 15, 2010 #1
    1. The problem statement, all variables and given/known data

    A parallel plate capacitor is charged by a battery to 20 V and then disconnected . A simple pendulum consists of a metal bob which is suspended by a nylom thread is placed in the middle of the plates of the capacitor . Then the bob is oscillated towards the negative plate of the capacitor . Each time when the bob touches the negative plate , 10 % of the charge at the negative plate will be transferred to the bob and these charges are then neutralised when the bob touches the positive plate of the capacitor . Determine the potential difference between the plates of the capacitor after 5 complete oscillation of the bob .

    2. Relevant equations

    3. The attempt at a solution

    so i found that 0.1111 of the negative charges have been neutralised and 0.8889 of the -ve charges is left and i am not sure what to do next .
  2. jcsd
  3. Mar 15, 2010 #2
    The pd on the capacitor depends on the charge. If it has lost a certain fraction of its charge it should have "lost" the same fraction of its pd.
  4. Mar 16, 2010 #3
    thanks , am i correct in finding the percentage of -ve charge lost from the plate ? When it says the +ve charge from the other plate neutralise the -ve charge , is the +ve charges also decreasing at the same rate as the -ve charges ?
  5. Mar 16, 2010 #4
    Yes. If you imagine a charged capacitor that has +10 coulomb on one plate (A) and -10 on the other (B), we say that the capacitor has been charged to 10 coulomb. (Not 20)
    If you transfer 1 coulomb of negative charge from plate B to plate A, then plate B has lost 1 coulomb and now has -9, while plate A is now +9. (The one negative charge having neutralised one of the positives.)
    The result is that the capacitor now has a charge of 9 coulombs. +9 on the one plate and -9 on the other.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook