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    Basic theory/formula's

    Okay, I remember the E, I, and R formula according to Ohm's law, but can some one remind me of the Watt formula? I know E x I = W, but how do I figure resistance from 100 watts and 120 volts?

    #2
    Re: Basic theory/formula's

    PIE

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      #3
      Re: Basic theory/formula's

      Don't forget that you can substitute from E=IR to P=IE and vice-versa. An example would be to substitute for E, P=I(IR) becomes P=I^2R.
      Charlie Eldridge, Indianapolis, Utility Power Guy[COLOR=red][/COLOR]

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        #4
        Re: Basic theory/formula's

        The logical approach by only using one formula is...120 times 120 = 14400 divided by 100 = 144 ohms.

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          #5
          Re: Basic theory/formula's

          I find that an Ohm's law equation can always be solved by remembering one rule:

          I x R = E can be changed to

          E / R = I or

          E / I = R

          It might be clearer to use numbers:

          2 X 3 = 6 can be changed to

          6 / 3 = 2 or

          6 / 2 = 3

          Given any two variables the third can always be deduced. I also think of this as being able to "unmultiply" something by dividing it in the opposite direction. :cool:

          100w / 120v = .833333A and you need resistance
          that's E / I = R or 120v / .833333A = 144ohms

          [ May 09, 2003, 03:54 PM: Message edited by: physis ]
          Sam, San Francisco Bay Area

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            #6
            Re: Basic theory/formula's

            Watts = volts x amperes, P = E x I. however, this is only where you have a purely resistive load or unity power factor, no inductive loads. Where inductive loads are supplied, watts = volts x amperes x power factor, P = E x I x PF.

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              #7
              Re: Basic theory/formula's

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                #8
                Re: Basic theory/formula's

                Get yourself an Ugly's book, for under $20, it is a handy reference book full of electrical formulas, and other useful info.

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                  #9
                  Re: Basic theory/formula's

                  DOCTOR WATTS pocket guide is pretty good too by Mark Shapiro.
                  Bryan P. Holland, MCP
                  NEMA - Codes & Standards

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