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Work done by fan question, please help!

  1. Aug 23, 2007 #1
    1. The problem statement, all variables and given/known data
    Been at this question for some time an needing a double check to see if things are right.

    A fan that consumes 20W of eletric power when operating is claimed to discharge air from a ventilaed room at a rate of 0.25 kg/s at a discharge velocity of 8 m/s. Determine if this claim is reasonable?


    2. Relevant equations
    KE of wind = 0.5 mv^2
    20W = 20 j/s


    3. The attempt at a solution
    When i worked it out i calculated it would be 8J. This seems to be posible for a 20W fan to do this work. Is this correct??
     
  2. jcsd
  3. Aug 23, 2007 #2

    mgb_phys

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    You probably mean 8J/s - but yes, 40% efficency sounds reasonable.
     
  4. Aug 23, 2007 #3
    Sorry, I did mean 8J/s.

    Thankyou for the reply
     
  5. Jul 26, 2011 #4
    Another question, related to the written above -
    Is that correct to estimate the "usefull power" of a fan in the following way:

    0- Estimating Force applied on fan. we get F.
    1 - Estimating the velocity of the wind near the fan. we get Vo.
    2 - using this estimation alone (ignoring wind decay & turbulences & whatever happens far from fan) to look at the picture from the wind's frame of reference- where the fan is moving with speed of -Vo, thus calculating P=Vo*F.

    It's stage 2 I'm not completely sure of .. is that stage's logic OK?
     
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