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Solving for RMS current of a resistor, help?

  1. Mar 6, 2017 #1
    1. The problem statement, all variables and given/known data
    A resistor connected across an AC power supply has a current given by I=(1.35A)cos(300t) when connected to a power supply with emf 120 V rms.
    Find the RMS current.

    2. Relevant equations
    IRMS=Imax/√2

    3. The attempt at a solution
    IRMS=Imax/√2
    IRMS=0.477 A
    INCORRECT
    Not exactly sure what to do? I am not given any other data besides what i have written above. I do not have the Power, or the Resistance, and in fact will have to solve for those in part B. and C. of this problem. I know I am missing something here, and i have re-read the chapter in my textbook but it is not helping.
     
  2. jcsd
  3. Mar 6, 2017 #2

    gneill

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    Staff: Mentor

    How did you establish the value of ##I_{max}## that you used?
     
  4. Mar 6, 2017 #3
    I used the value of "I=(1.35A)cos(300t)" they gave as Imax.
     
  5. Mar 6, 2017 #4

    gneill

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    Staff: Mentor

    Okay, but that's not a value. What value (single number) did you use?
     
  6. Mar 6, 2017 #5
    I used (1.35A)cos(300t) = 0.675
    I think what I am missing is what the "t" component means?
     
  7. Mar 6, 2017 #6

    gneill

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    Staff: Mentor

    That is incorrect for a couple of reasons. First, the expression cos(300t) is a function of time t. Second, the "300" in the expression would have units of radians per second, not degrees per second.

    Read the maximum value from the coefficient: 1.35 A. The cosine function makes the current vary between the bounds -1.35 A and +1.35 A over time.
     
  8. Mar 6, 2017 #7
    Ahh Okay.
    So.
    Imax=1.35 A
    IRMS=1.35/√2=.955 A
    Excellent, thank you.
     
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