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Homework Help: RC Time Constant Question (Easy)

  1. Oct 21, 2011 #1
    1. The problem statement, all variables and given/known data
    A 12.8 micro-F capacitor is connected through a 0.890 M-ohm resistor to a constant potential difference of 60.0 v.


    2. Relevant equations

    q=CE(1-e^(-t/(RC))
    i=(E/R)-(q/(RC))

    3. The attempt at a solution

    Compute the charge on the capacitor at the following times after the connections are made: 0 s, 5.0 s, 10.0 s, 20.0 s, and 100.0 s.

    Solved for q and according to masteringphysics (the homework program we use) I got all the q values correct. Here they are:
    0 , 2.7e−4 , 4.5e−4 , 6.4e−4 , 7.7e−4


    Second part, and this is the part that I need help on. I think I'm right but the program says I'm incorrect on the fourth term

    Compute the charging currents at the same instants. Calculated i:
    6.74e-5 , 4.37e-5 , 2.79e-5 , 1.12e-5 , -1.76e-7

    It says "Term 4: Very close. Check the rounding and number of significant figures in your final answer."

    What am I doing wrong?

    Thanks!
     
  2. jcsd
  3. Oct 21, 2011 #2

    SammyS

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    Probably too much too much round off.
     
  4. Oct 22, 2011 #3
    So am I right?
     
  5. Oct 22, 2011 #4

    SammyS

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    Close to right.

    A small round off in E/R makes a big difference and/or q/(RC) makes a big % difference in E/R - q/(RC)when E/R and q/(RC) are nearly the same size.

    The correct answer for current @ 20.0 s is ≈ 11.65 μA .

    Your answer for 100 seconds is nonsense, since it's negative.

    BTW:

    Another way to calculate the current at time, t, is to use [itex]I=(I_0)e^{-t/(RC)}[/itex], where I0 = E/R
     
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