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Calculate Current (RMS)

  1. Nov 13, 2015 #1
    1. The problem statement, all variables and given/known data
    Sorry for what will likely be considered a noob question but I've been stuck on it for a while and its only at the start of my assignment :(
    Simple circuit
    Vac = 50Sin200πt, R1 = 40Ω
    Calculate the value of the RMS current

    2. Relevant equations
    Vt = Vmax sin2πft = Vmax sinωt

    3. The attempt at a solution
    I can't figure out how to do this when we are given no value for time at all. I think i have it worked out at say 1ms
    Vt = 50sin((200π)(0.001))
    = 50sin(.628318)
    = 54.83V

    Vrms = 54.83 * .7071 = 38.77

    Irms = Vrms / R = 38.77 / 40 = 0.969 amperes

    We have been told that time is intended to be an unknown variable. It's been a long time since I've look at any of these types of problems, can someone help me out on what I'm missing here please

  2. jcsd
  3. Nov 13, 2015 #2
    The r.m.s. can be calculated just from the peak value if the current is sinusoidal, as is the case here. You can calculate the peak current from the peak voltage. Also remember that the average of the sin^2 function is half. Can you take it from here?
  4. Nov 13, 2015 #3
    Thanks for the reply PWiz...any formula I can find for these calculations seem to require a time.
    The only one I can find without t is Vpeak = Vrms√2...=> Vpeak/√2 = Vrms
    Vrms / R = Irms

    I was thinkin maybe I could half the Vac given in the question, but that still involves a t variable...so frustrating :(
  5. Nov 13, 2015 #4
    That's pretty much what I was talking about.
  6. Nov 13, 2015 #5
    Still a bit confused, so can I say
    Vpeak = 50Sin200πt / 2 = 25Sin200πt
    Vrms = 25Sin200πt / √2
    This still requires a value for t, no?
  7. Nov 13, 2015 #6
  8. Nov 16, 2015 #7

    Could someone confirm that my thinking on this question is correct for me please? I'm basically just disregarding the Sin200πt and calculating the Vrms and Irms derived from from the 50 value at the start of the expression, i.e 50V. This seems incorrect to me, could someone just check my logic here please
  9. Nov 16, 2015 #8


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    Staff: Mentor


    For a sinusoid (sine or cosine) the rms value is ##1 / \sqrt{2}## its peak value. This is entirely independent of the frequency of the sinusoid. In this problem you're given a sinusoidal voltage signal with a peak value of ##50 ~V##, so the rms voltage is ##50 / \sqrt{2}~~V##.

    It really is that simple for sinusoids. There are other relationships between peak and rms values for other signal shapes.
  10. Nov 16, 2015 #9
    Thanks for that gneill, much appreciated
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