1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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


    User Avatar
    Science Advisor
    Homework Helper

    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?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook