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Electric current madness

  1. Jul 19, 2005 #1
    hey guys, here's my problem...

    coil connected to a 240 V (rms) ac line has a resistance of 34 ohms. what is the average power used

    (.5)V^2 /R so Pav = 847 W

    then it asks what are the min and max values of the instanteous power???

    this is where im stuck, i think i need to use the equation

    P(t)= IVsin^2 (omega(T)) but how do i know what t is, and the angular freq??? any help would be greatly appriciated,,,,also any knowledge on the topic would help, cause these ac circuits are kind of hard for me.... thanks
  2. jcsd
  3. Jul 19, 2005 #2
    V0 = root2 * Vrms

    got it
  4. Jul 19, 2005 #3
    me again.....im pretty sure i messed up every step on this problem...... still need help
  5. Jul 19, 2005 #4


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    The word "coil" usually implies some inductance, while "resistance" instead of "impedence" implies no inductance. If you have some inductance, then you need to rethink the problem. If the load is purely resistive, then the current and voltage are in phase and you can use your simple relationships between rms and peak voltage and current. You do not need the frequency or the specific times at which the instantaneous power is max and min; all you need is the maximum and minumum possible values of the squared sine function.
  6. Jul 19, 2005 #5
    no, no inductance.....you said use the square of the sine function, but doesnt that depend on angular frequency and time?,...... can i just use

    Vo = IoR ???
  7. Jul 19, 2005 #6


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    The sine function does depend on frequency and time, but you are not asked to find when the minimum and maximum occur, just the power values at the minimum and maximum. What is the maximum (minimum) possible value of sine squared?
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