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: Charge flowing through a capacitor

  1. Nov 12, 2007 #1
    1. The problem statement, all variables and given/known data

    Initially, the switch in the figure is open and the capacitor is uncharged.


    How much charge flows through the switch after the switch is closed?

    2. Relevant equations

    Q = C*ΔVC

    3. The attempt at a solution

    Using the given values:

    Q = (10*10-6) * 1.5 = 1.5*10-5

    which is incorrect..

    in another calculation i got 1.5*10-4, also incorrect

    any help?

  2. jcsd
  3. Nov 12, 2007 #2


    User Avatar
    Homework Helper

    In what way is 1.5·10^-5 C incorrect? When the capacitor is fully charged, the voltage across it equals the battery voltage; the charge will be Q = CV_batt . Since the capacitor started out uncharged, that should be the amount of charge that flowed through the switch on its way to the positive plate.

    Is this a problem on a computer system like WebAssign? Is your statement of the problem exactly what was asked? The reason I ask this is that I've worked with students who had to use systems like this in the past; one of the reasons they often had answers rejected was because they weren't answering the question as stated. One of those reasons was that they weren't using the requested units. Do they want the answer in Coulombs, or perhaps in microcoulombs?
  4. Nov 12, 2007 #3
    yea its MasteringPhysics

    i forgot to mention that it asks for microcoulombs
  5. Nov 12, 2007 #4
    aaahhhhhhhhh you are right!!!! it was the UNITS!!!!! LOL i was putting it in as Coulombs instead of microcoulombs

  6. Nov 12, 2007 #5
    Answered before I got here...
  7. Nov 12, 2007 #6


    User Avatar
    Homework Helper

    I thought it might be something going on like that. Be very careful in working with inputs to the questions; read the problem statements closely. Every dang problem on those systems is like taking a multiple choice test -- you get no credit for doing the problem correctly if you don't answer it just right! (Computers are stupid like that...)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook