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Homework Help: How much gas is required to heat a house

  1. Jul 17, 2009 #1
    1. Methane has a heat of combustion of 5.5 x 10^7 J/kJ.
    a)How much gas is required to heat a house that requires 2.0 x10^9 kJ for the whoel winter? Assume that the furnace is 80% efficient

    3. So i tried dividing using Q=mL
    but i don't seem to be getting the right answer (4.5 x 10^5)

    I also tried using the efficiency formula but that doesnt make sense =\

    There is also part b and c but those are reliant on this answer so i'm trying to focus on it but i simply dont understand what i should be doing
  2. jcsd
  3. Jul 17, 2009 #2

    Andrew Mason

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    Re: Efficiency

    (NOTE: Your units are wonky. The heat of combustion is: 5.5 x 10^7 J/kilogram)

    How many joules of combustion heat do you need (this is where the efficiency comes in).

    If you get that, it is a simple matter to figure out how many Kg of methane you will need.

  4. Jul 17, 2009 #3
    Re: Efficiency

    So..the house needs 2.0 x 10^9 kJ....
    so thats...( 2 x 10^12 x .8) = e/ m

    what woudl E be here - 5.5 x 10 ^7 ?
  5. Jul 17, 2009 #4


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    Re: Efficiency

    I am not sure what unit that is supposed to be. But I'd start by finding the energy input using the formula for efficiency.

    EDIT: well I was beaten to it, but Andrew Mason has the idea.

    Right, so that is how much energy the methane outputs
  6. Jul 17, 2009 #5
    Re: Efficiency

    Efficiency = useful output / input x 100%
    .8 = 2.0 x 10 ^12 J / input x 100%

    no i have no idea what im doing XD help?
  7. Jul 17, 2009 #6


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    Re: Efficiency

    Find the input energy perhaps.
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