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Latent Heat equation

  1. Dec 7, 2011 #1
    A 3 kW kettle contains 2.0 kg of water at a temperature close to 100oC.


    Latent heat of vaporisation for water: Lv=2256 (kJ kg^-1)

    Q= Lv x mass



    Ok I understand this problem because I now the answer but I don't understand the process.

    Like my teacher wrote 2256x10^3 why he wrote 10^3 ?? I don't understand !!!!! .. so he plugged into the equation and he get 2256x10^3 x 1.5kg = the first answer is 3.38x10^6 J But again I understand why he wrote 10^6??? whats the point ... because If put in my calculator 2256 x 1.5 = 3384 .... , and the final answer is 3.38 MJ what happen to the 10^6 ?? why he convert to MJ I really confused help me please!
    Thank you..
     
  2. jcsd
  3. Dec 7, 2011 #2

    gneill

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    Staff: Mentor

    Unit prefixes and powers of ten: read here
     
  4. Dec 7, 2011 #3
    Cheers but I still struggling how can i get 3.38x10^6
     
  5. Dec 7, 2011 #4

    gneill

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    Staff: Mentor

    MJ = megaJoules = 106 Joules
     
  6. Dec 7, 2011 #5

    Ok but that doesn't make sense because I have another problem with the same type of answer
    E =1.5kg * 4190 J kg-1 K-1 x 85o = 5.34*10^5 J = 534 kJ. and there is no 10^5 in the decimal prefixes ...
     
  7. Dec 7, 2011 #6

    gneill

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    Staff: Mentor

    534 kJ = 534 x 103 J = 5.34 x 10^5 J

    The reason it was specified as 534 kJ is because there's no 105 prefix.

    Standard prefixes are for powers that are multiples of three (kilo and above).
     
  8. Dec 7, 2011 #7
    So the answer is in Kj because is 10^5 and is between 10^5 and 10^3 ??
     
  9. Dec 7, 2011 #8

    gneill

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    Staff: Mentor

    Essentially, yes.

    Sometimes you'll see values like 0.341 kJ or 1500 MW where the convention is "bent" a bit. Usually this is done to make the particular number have common units with other like-values for comparison (like entries in a table of values), or where one particular prefix would not handle all of the values to be shown.
     
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