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Homework Help: Transfer of Heat: mass of liquid helium that boils?

  1. Nov 23, 2005 #1

    MKM

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    Liquid helium is stored at its boiling-point temperature of 4.2 K in a spherical container (r = 0.251 m). The container is a perfect blackbody radiator. The container is surrounded by a spherical shield whose temperature is 72.8 K. A vacuum exists in the space between the container and shield. The latent heat of vaporization for helium is 2.1 x 104 J/kg. What mass of liquid helium boils away through the venting valve in 19.2 hours?


    I have tried calculating the net Powere first withthe equation Pnet =e sigma (5.67*E -8) A(.251*.251*pi) ( T^4-Tenvirnment^4) this gives me
    .315210793711 as my Pnet. then I plug it into the formula Q=Pt Q=P*69120 secs) = 21787.3700613. My last step is to plug it into Q=mLv which is my new specific heat divided by my liquid heat of vaoprization of helium which is 2.1*E4 I get 1.03749, which is incorrect. I am not sure where I am going wrong . should I be calculating seperate Q values from the beginning with different e values?
     
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  3. Nov 23, 2005 #2

    Astronuc

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    For one thing the area of a sphere is 4[itex]\pi[/itex]r2.

    So one calculates the P (W) from the Stefan-Boltzmann Law - using the correct area.

    Then with power * time = Q, and one can then use the latent heat to find the mass.
     
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