Conversion help!

Discussion in 'The Projects Forum' started by Verónica, May 13, 2013.

  1. Verónica

    Thread Starter New Member

    May 13, 2013
    3
    0
    Hi guys, I'm a newbie here, so pls bear with me!
    What am I doing wrong in this exercise?

    >>Suppose the voltage is 250V and the current is 8.0 mA. The power dissipated by the potentiometer is
    >>(a) 31 mW
    >>(b) 31 W
    >>(c) 2.0 W
    >>(d) 2.0 mW


    I know the right answer is (c) (...right?), but I've run into some trouble converting units.
    Here is how:
    If I convert mA to A i seem to get the right result:
    P = EI
    P = 250V x 0.008A = 2.0W

    But this is what I don't get: If I convert V to mV I get something totally wrong... but shouldn't I be getting the same result? It's been such a long time since I left school, so maybe there is just some basic principle I'm missing...
    P = EI
    P = 250,000mV x 8mA = 2,000,000mW = 2,000W

    What am I doing wrong? I just don't get it... any help is much appreciated, and sorry for such a basic question!
     
  2. #12

    Expert

    Nov 30, 2010
    16,330
    6,818
    A milli times a milli is a micro. You forgot to multiply units.
    Another way to do it, which you did first, is get rid of the milli's by moving the decimal point and using whole amps and whole volts as the units.
     
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  3. Verónica

    Thread Starter New Member

    May 13, 2013
    3
    0
    Thank you for such a quick answer! I thought I was going crazy, but hopefully it is clear now :)
     
  4. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    Since milli is 10^-3, replace the 250mV with 250*10^-3V, does this give you 250V? No, because the prefix specifies where the decimal point is on the number, relative to the unit.

    So, try 250,000mV * 8mA = ((250*10^3)*10^-3)V * 8*10^-3A = 250V * 0.008A = 2W.
     
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  5. Verónica

    Thread Starter New Member

    May 13, 2013
    3
    0
    Hey! Thanks for the answer. I still need to polish up my math skills but thank you for the great input :)
     
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