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Homework Help: Calculate RMS current?

  1. Apr 12, 2008 #1
    [SOLVED] calculate RMS current!?

    1. The problem statement, all variables and given/known data

    The maximum potential difference across certain heavy-duty appliances is 240 V. Assume that the total resistance of an appliance is 190 ohms.
    (a) Calculate the rms potential difference.
    169.68V <--CORRECT

    but for

    (b) calculate the RMS current,

    i have no idea what to do!?
  2. jcsd
  3. Apr 12, 2008 #2


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    How did you obtain your answer for the RMS voltage? Show some work, please.
  4. Apr 12, 2008 #3
    i did 240V * .707 (i got .707 from Vrms)
  5. Apr 12, 2008 #4


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    Ok. Here's a hint:

    HINT: RMS voltages and RMS currents across a resistive load follow rules analogous to their DC counterparts. So, do you remember how DC voltage and current across a resistor are related?
  6. Apr 12, 2008 #5
    i really don't,
    and honestly i guess for part A.

    i dont know why im taking physics, but im stuck with it.
    its not that i dont try...
    i just dont understand :\
  7. Apr 12, 2008 #6
    i know DC and AC have something to do with transformers
  8. Apr 12, 2008 #7


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    Then you should look up the answer if you do not remember. You must have done DC circuits if you are discussing AC concepts like these. I'm telling you that the RMS voltage and current values across a resistor follow the same laws that DC voltages and currents across a resistor follow.

    The fact that you don't remember that law means that you should probably review your notes from previous classes or look it up in your book. I do not see how me just giving you the law helps you in your physics education in the long run.

    I'll ask you again, just so you are clear on what your looking for:

    What law relates the DC voltage across a resistor to the Current through it? The RMS Values you are working with also follow the same law.
    Last edited: Apr 12, 2008
  9. Apr 12, 2008 #8
    i didnt say i dont remember, because i never knew in the first place....but is it Ohm's law? V=IR?
  10. Apr 12, 2008 #9


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    It is indeed Ohm's law. Apparently you did know. You just didn't know you knew.

    Now that you know that the RMS Values of voltage and resistance follow Ohm's law, can you find the RMS current from the information you have?
  11. Apr 12, 2008 #10
    again, a guess..

    so, if V=IR

    then 169.68=I*190? =.89

    i think?
  12. Apr 12, 2008 #11
    thats the answer. thanks!!!!!!
  13. Apr 12, 2008 #12


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    That is correct. Good Job.:smile:

    Just a word of advice for your future posts on PF. You may have a bad teacher, or you may just not be a physics person, but that does not mean that you deserve to be given solutions or methods to solve your problems more than anyone else here. Here, we try to help you figure out how to solve the problems. We do not solve them for you or just give you the method to get to the solution. You would learn nothing from that. Now, you solved this problem yourself with only a little guidance from me. You should feel good about that. Would you have felt as good, or learned anything if I just told you to use Ohm's Law?
  14. Apr 12, 2008 #13
    i understand thanks!
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