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Homework Help: Heat question

  1. Dec 10, 2009 #1

    itr

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    1. The problem statement, all variables and given/known data

    A girl wishes to make a lead casting of an object which will require 2kg of melted lead. if the lead was stored in her basement at approximately 55 degrees F, and it is melted to a 1/4" thick steel pot of approximate cross sectional area of 12in^2, how long will it take to melt the lead if its placed in a kiln at 617.5 degrees F, the melting point of lead?

    2. Relevant equations

    Q=mc delta t
    Q=mc

    Constants for lead: Lf=.25E5 J/kg Lv=8.7E5 J/kg c=130J/kgCelsius
    Constants for steel: k=40 J/s times m times Celsius

    3. The attempt at a solution

    Q=2(130)(562.5)
    Q=1.46E5
    Qv=2(8.7E5)
    Qv=1.74E6
    Qf=2(.25E5
    Qf=50000

    Not sure where else to go from here...
     
  2. jcsd
  3. Dec 10, 2009 #2
    A long one to go on this I am afraid. First off your data is in farenheit and your constants in celcius. What course is this--i'm thinking you have overall energy requirements posted above, but nothing about how fast the process will occur.
     
  4. Dec 10, 2009 #3

    itr

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    oh man...the temp units are off. a complicated problem you loose track of the basics...

    I gave all of the info that i have...and its for my calc based physics class
     
  5. Dec 10, 2009 #4
    Thats reassuring. To solve this will require some math. There should be some coefficient of heat transfer, etc.Not sure what Lf and all the other constants represent, the only one that I see which has a time dimension is the last.
     
  6. Dec 10, 2009 #5

    itr

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    lf=latent heat of fusion
    lv=latent heat of vaportation?
     
  7. Dec 10, 2009 #6
    Sorry, I'm not sure I can help. Someone else may be able to.

    You're given a mass of lead and the shape it will become. You also know the lead is just melted and not vaporized, so know need to worry about lv. I believe the temp of the steel can be assumed to be constant, so you know exactly the number of calories it will take to make 2 Kg of solid lead to molten lead. You're given a bunch more info, but nothing else? Take an example with water--you take a liter of very cold ice and dump it into a steel pot at 0 degrees--don't you need to know something about the shape of ice block and NOT the pot, as well as rate of heat transfer. Wish I could help more, problem just doesn't have enough info imo.
     
  8. Dec 10, 2009 #7

    itr

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    it is alright, thank you for attempting though..
     
  9. Dec 11, 2009 #8

    itr

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    Any other thoughts?
     
  10. Dec 11, 2009 #9

    itr

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    =[.....
     
  11. Dec 11, 2009 #10

    Wish I had.
     
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