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Homework Help: Power supply voltage homework

  1. Jun 30, 2006 #1
    Need a little help here....getting started. I don't really understand what to use here.


    If two 1000W toasters were to be connected in series to a dc power supply, what power supply voltage would be neccessary for normal operation? Assume each toaster was designated to a 120V ac circuit.

    A. 110V
    B. 120V
    C. 170V
    D. 240V

    I'm guessing it is above 120V...so C or D. Just because I know that dc power takes more voltage to get 120V to ac.

    Thanks,
    Brad
     
  2. jcsd
  3. Jun 30, 2006 #2

    chroot

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    Find the resistance of the toasters, and solve for the DC voltage necessary to have each dissipate 1000W.

    - Warren
     
  4. Jun 30, 2006 #3
    Ok....so here is what I did.

    2000W/240V = 25/3.......using P=IV

    2000W/(25/3)^2 = 28.8Ohm......using P=I^2R

    Now, using R = V/I........28.8Ohm * (25/3) = 240 V

    Is that right?? If so.....then my answer would be D.
     
  5. Jun 30, 2006 #4

    chroot

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    I agree with your answer. I would have calculated the resistance for each toaster independently, though.

    Also, I strongly suggest using units more often. Intermediate answers like "25/3," without units, make your work hard for anyone to follow, including yourself.

    - Warren
     
  6. Jun 30, 2006 #5
    Warren.....I do use units a lot, but am unfamilar with these electric units.....I didn't know what my units were on the 25/3....was it still ohms?
     
  7. Jun 30, 2006 #6

    chroot

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    Power / voltage = current (in amperes). Ohm's law, which you quoted as "P = IV," has power in watts, potential difference in volts, and current in amperes.

    - Warren
     
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