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Homework Help: Working Resistance, Vo or VRMS

  1. Apr 14, 2008 #1
    [SOLVED] Working Resistance, Vo or VRMS

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

    Hey everyone, i'm new here and just wondering if you can give me a helping hand. In my Physics homework, the following question is stated:

    "A heating element for an electric fire consists of a single strand if nichrome wire wound around an insulator. The heater is required to produce 1.2kW when connected to the 230V-RMS ac mains.

    (i) Calculate the working resistance of the nichrome wire.

    Okay, so it's not that i don't understand how to do the question, it's just that i don't know what it's asking for. Normally it would be okay, but because the 'RMS' was thrown in, it's put me off a bit.

    2. Relevant equations

    Equations used:

    V0 = V-RMS × √2 ------Part 1
    P=IV, therefore I=P/V ------Part 2
    V=IR, therefore R=V/I ------Part 3

    3. The attempt at a solution

    Part 1 ---- V0 = 230v × √2 = 325.269v...
    Part 2 ---- I = 1200w/325.269v... = 3.689A...
    Part 3 ----325.269v.../3.689A..., therefore R = 88.2 ohms

    Simply, what voltage do i use? Do i convert the V-RMS into V0, or leave it alone. I've tried both ways, the latter (not seen here), gives a resistance of 44.06 ohms.

    All help is greatly appreciated. Thanks.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Apr 14, 2008 #2

    Chi Meson

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    Science Advisor
    Homework Helper

    RMS means "root mean square." Or the square root of the average of the squares of the the values. It is another method of finding an "average." When doing ac circuits, you use the RMS "average" values for current and voltage, etc, when plugging into the standard Ohm's Law and Power equations.

    So, in short, use the RMS voltage.
  4. Apr 15, 2008 #3
    Thanks very much for your response, now you say it, it has become much more clear as to why i would use that value. Again, thank you Chi Meson.
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