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Homework Help: Air passes over an electrical heater at a steady rate of 2500cm^3 per second.

  1. Mar 22, 2012 #1
    1. The problem statement, all variables and given/known data

    Air passes over an electrical heater at a steady rate of 2500cm^3 per second. The inlet temperature of the air is 20^oC ( 20 degrees celcius ) and the steady outlet temperature is 40^0C ( 40 degrees celcius)
    a) What heat is absorbed by air passing over the heater in 2 hours
    b) Obtain an estimate for the power rating of the heater
    Density of air ( 1.2 kgcm-3)
    Specific heat capacity of air : 1000 J-1Kg-1K-1


    2. Relevant equations
    So I think that I should use the equation Q=mcΔθ


    3. The attempt at a solution
    Here's what I did :
    2500cm^3=2500g=2.5kg
    Since 2.5kg is heated every second then 2hrs of heating would heat 2.5x7200 = 18,000kg of air.
    ΔθOutlet-Inlet temperature = 40-20 = 20 degree celcius change in temperature,
    Specific Heat capacity of air : 1000 Jkg-1K-1
    Q=mxcxΔθ
    = 18000x1000x20
    =3.6x10^8
    Which is wrong because the answer at the back of the book had 4.32x10^5. I've tried, but I really need some help :) Thanks
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 22, 2012 #2

    tiny-tim

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    Science Advisor
    Homework Helper

    Welcome to PF!

    Hi Student-! Welcome to PF! :smile:

    (try using the X2 button just above the Reply box :wink:)

    Quickly dividing 4.32 by 3.6, my eye is drawn to …
    :wink:
     
  4. Mar 22, 2012 #3
    I think you have mis- calculated the mass of air.
    The density of air is 1.2kg/m^3 not 1.2kg/cm^3
     
  5. Apr 14, 2012 #4
    Thanks alot :D I got out the question and I saw my error- :)
     
  6. Apr 14, 2012 #5
    Re: Welcome to PF!

    Thanks I got it out eventually :D :)
     
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