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Homework Help: DC Circuits - Power?

  1. Mar 14, 2010 #1
    DC Circuits -- Power?

    I have a physics question I have been working on for quite some time, but have not yet been able to figure out. I am assuming it probably has more to do with unit conversions than with actual calculations, but I am rather stuck... Here is the question:
    An electric clothes dryer draws 15 Amps at 220 Volts. If the clothes put into the dryer have a mass of 7 kg when wet and 4 kg when dry, how long does it take to dry the clothes? (Assume all heat energy goes into vaporizing the water; latent heat of vaporization = 2.26 x 10^6 J/kg)

    I figured this was probably a question that had to do with power, since I was given current and voltage, so I used the fact that power=(current)(voltage). Since I know an amp is a Coulomb per second and a volt is a joule per Coulomb, I multiplied 15 Coulombs/second and 220 joules/Coulomb to get 3300 joules/second (or watts, but I had a feeling joules per second would be of more use to me).

    I am stuck there though, quite unsure of what to do concerning the weight of water and the latent heat of vaporization. The answer is supposed to be 34.2 minutes. Could someone please give me a hand?
  2. jcsd
  3. Mar 14, 2010 #2


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    Re: DC Circuits -- Power?

    The power used by the dryer is (as you said) = 220Vrms * 15 = 3300J/s

    Now how much energy do you need to dry the clothes?
    You need to evaporate 3kg (7-4kg) of water.
    You are told it takes 2.26MJ/kg

    How many Joules?, you have Joules/second, so how many seconds?
  4. Mar 16, 2010 #3
    Re: DC Circuits -- Power?

    Thanks! I really appreciate it :].
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