Quick Question

Discussion in 'Homework Help' started by fishguts, Sep 19, 2007.

  1. fishguts

    Thread Starter Member

    Sep 3, 2007
    22
    0
    I am trying to find the time constant for a circuit.I want to make sure I am doing this right. My resistor is 10K while my capacitor is 5pf. this would give me:

    T=RC
    T=10X10^3X5X10^-12
    T=10,000X.000000000005
    T=.00000005s

    then .00000005s would convert to .05ns right?
     
  2. niftydog

    Active Member

    Jun 13, 2007
    95
    0
    You were right until the last step.
    0.00000005s = 50ns

    I prefer to stay away from all those zeros and use the sum of the exponents. In this case you just do this;

    10 x 5 = 50

    Now add the exponents;
    3 + -12 = -9

    ^-9 = nano so it's 50ns
     
  3. fishguts

    Thread Starter Member

    Sep 3, 2007
    22
    0
    alright thanks alot. I am not that great with exponents yet, but I am practicing.
     
  4. fishguts

    Thread Starter Member

    Sep 3, 2007
    22
    0
    I have oe more question. I am trying to figure out the capacitor coltage after a certain time but I am stuck on using the formula I cant understand it at all. Does anyone know anylinks that explain how to use the formula better? I tried reading the article on this website but it was not any help.
     
  5. niftydog

    Active Member

    Jun 13, 2007
    95
    0
  6. fishguts

    Thread Starter Member

    Sep 3, 2007
    22
    0
    The part that is confusing me is the exponent part where it is t/T. I have no idea how the examples are getting the numbers they are getting in their solution at that part
     
  7. niftydog

    Active Member

    Jun 13, 2007
    95
    0
    Small t is the time in seconds at which you want to know the voltage.

    Big T (actually the greek letter tau) is the RC time constant (T = R x C).
     
  8. fishguts

    Thread Starter Member

    Sep 3, 2007
    22
    0
    ok I got the formula down. I have one more question

    I have a problem that says to plot Vc for 0<t<4t using at least for points. what does it mean by this notation of 0<t<4t? I have never seen a problem like this before
     
  9. niftydog

    Active Member

    Jun 13, 2007
    95
    0
    t has to be defined before it can be used in this context, so unless they say "t=0.5s" or something then t is meaningless. You sure it's not "T" rather than "t"?

    "0<t<4t" just means to plot the "t" axis from 0 up to "4 x t".
     
  10. fishguts

    Thread Starter Member

    Sep 3, 2007
    22
    0
    here is an attachment if it helps
    it is number 3b.
     
    • hw4.pdf
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  11. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,164
    In your problem, you are going from time zero ( 0 < t ) to four time constants ( 4 tau ) .

    You were given that tau is 2 mS ... so they wanted the plot to 8 mS.
     
  12. fishguts

    Thread Starter Member

    Sep 3, 2007
    22
    0
    alrigt thats what I was kinda thinking. thanks for the help!
     
  13. niftydog

    Active Member

    Jun 13, 2007
    95
    0
    Yes, look closely, the t after the 4 is actually "T" or tau, not "t".
     
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