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Ohm's law power formula question

Discussion in 'Electronic Basics' started by ITSME.ULTIMATE, Feb 19, 2006.

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  1. Ok, using ohm's law, in a DC circuit, we can say that:

    power=
    (V^2)/R
    I^2 * R
    V * I

    But what if you have a one ohm resistor?

    Example, one ohm resistor with 0.25A going through.
    V=I*R, so V= 0.25 x 1 = 0.25A

    using the formula, power = V*I, you get 0.25W
    but using the other formula (V^2)/R, you get 0.0625W
    0.0625 != 0.25W

    Let's try 4 ohm resistor with 0.5A flowing through:
    V=I*R, V= 0.5 x 4 = 2V
    P= V*I = 2 x 0.5 = 1W
    P=(V^2)/R = (2^2)/4 =1W
    P= (I^2) * R = (0.5 x 0.5) * 4 =1W

    Why isn't it working and what did I not do right?
     
  2. Dan H

    Dan H Guest

    First example
    V = .25, I = .25 so V x I = .25 x.25 = .0625
     
  3. Ross Mac

    Ross Mac Guest

    Thats .25V
    V does not equal 1 !
    P=IE
    P=.25^2=.0625W

    You just messed up your variables......I assume you are a student and all I
    can say is you REALLY need to know these two formula's inside and out. They
    will serve you well into the future....good luck....Ross
     
  4. John Fields

    John Fields Guest

    ---

    E = IR = 0.25A * 1R = 0.25V

    _not_ 0.25A
    ---
    ---
    No. In the firtst case:

    P = EI = 0.25V * 0.25A = 0.0625 watt

    and in the second,

    E² 0.25A * 0.25A
    P = --- = --------------- = 0.0625 watt
    R 1R
    ---

    ---
    Let's see:


    E = IR = 0.5A * 4R = 2V

    P = EI = 2V * 0.5A = 1 watt

    and also:

    E² 2V * 2V
    P = --- = --------- = 1 watt
    R 4R



    and also:

    P = I²R = 0.5A * 0.5A * 4R = 1 watt.

    Those all come out right, so you made some transcription errors on
    your first try.
     
  5. Pooh Bear

    Pooh Bear Guest

    0.25 Volts actually !
    No P=V * I = 0.25 * 0.25 = 0.0625W
    Yes - correct. Your first instance was wrong.
    No P=V * I = 2V * 0.5A = 1W
    You've gone mad.

    Graham
     
  6. John Fields

    John Fields Guest

    ---
    No, he hasn't. He just made a simple error in the first part of his
    post which he couldn't reconcile.

    This is sci.electronics.basics, where anyone should be able to ask
    any questions they might want to, no matter how stupid those
    question might seem to be and, with the answers to those questions
    in hand, walk away a little more informed than they were when they
    first got here without feeling any guilt for their earlier
    ignorance.

    Your pejorative comments seem to be designed to denigrate the
    querant(s) into believing that your judgement of their submissions
    matters because they're stupider than you are.

    They may or not be true, but if they're asking for help and you
    smugly deny it, then what's to be said in your favor?
     
  7. Pooh Bear

    Pooh Bear Guest

    Well, actually he got 'confused' about the value of I.

    Ergo my response ! ;-)

    Graham
     
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