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Homework Help: Vrms and DC power dissipation

  1. Sep 30, 2013 #1
    1. The problem statement, all variables and given/known data
    So today in lecture, my professor talked about VRMS. I understand VRMS and its relationship to DC power dissipation, however, in lab today I became very confused.

    In lecture, we were told to use the VRMS value when applying Ohms laws/power equations (P=VI). Then, later, in lab we were observing an AC voltage (1Vpp) across to resistors (wired on a breadboard).

    The oscilloscope read 1Vpp drop across both resistors - which I was a little confused with... more importantly, I was confused because when I was using OHM's law (V=IR) to calculate the "calculated values" to compare them to the measured values, I used VRMS... since I thought that was what we could ONLY use... the Oscilloscope kept displaying Vpp... so I became very lose because even the text said that you had to use VRMS when using Ohm's laws - which would mean the oscilloscope was displaying the wrong values? or incorrectly using Vpp instead of VRMS?

    Can someone help me to understand when we HAVE to use VRMS and when VP or VPP is even ok in a calculation?

    Other lab information: We output the 1Vpp Voltage from a signal generated (sine wave) with a Frequency of 10kHz

    Thank you so much

    2. Relevant equations
    P = VRMS I

    3. The attempt at a solution
    I got the solution... I just do not understand, from my text and lecture, why Vpp is allowed in Ohm's law. It made sense to me that we had to use RMS values
    Last edited by a moderator: Sep 30, 2013
  2. jcsd
  3. Sep 30, 2013 #2


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    Hi Physixs! Welcome to PF! :smile:
    For P = VI we can use Vrms and Irms, or we can use Vp and Ip with an extra factor of 1/2.

    (for the detailed equation, see eg the pf library on impedance)

    Obviously, rms is more convenient! :wink:

    For V = IR, I don't see that it makes any difference, so long as you use the same type (rms or p or pp) for both V and I. :wink:
  4. Oct 1, 2013 #3

    That's not what my text said (it said I could ONLY use Vrms when using OHM's laws since the laws were based in DC...) so I was very confused. What you are saying makes a lot more sense!
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