1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Mass of object during heat exchange

  1. Aug 21, 2012 #1
    1. The problem statement, all variables and given/known data

    An aluminum container with a mass of 400 g contains 200 g of water at a temperature of 18°C. After a block of iron at a temperature of 90°C is placed in the water, the final temperature of the mixture changes to 28°C. Find the mass of the iron, assuming that there is no loss of heat to the surroundings.

    2. Relevant equations

    mass of aluminum = 0.40 g, heat capacity of aluminum = 910 J(kg.°C)
    mass of water = 0.20 g, heat capacity of water = 4200 J
    mass of iron = ? heat capacity of iron = 460 J

    (delta)t = change in temperature

    mAcA(delta)tA +mWcW(delta)tW + mIcI(delta)tI = 0

    3. The attempt at a solution

    This is the solution shown in the book:

    (0.40 kg) x (910 J/(kg.°C)) x (28°C - 18°C) + (0.20 kg) x (4200 J/(kg.°C)) x (28°C - 18°C) + (mI) x (460 J/(kg.°C)) x (28°C - 90°C) = 0

    3640 + 8400 - 28520 mI = 0
    12040 = 28250 mI
    mI = 0.422 kg

    What I want to know is where they got 3640 and 8400 from, when 0.40 x 910 = 364 and 0.20 x 4200 = 840. Why did they add a zero? Is this an error in the book? I also do not know where 28 520 is from. What am I missing...
  2. jcsd
  3. Aug 21, 2012 #2


    User Avatar
    Homework Helper

    (0.40 kg) x (910 J/(kg.°C)) x (28°C - 18°C) = 0.4 * 910*10 = 3640

    You are forgetting to multiply by the change in temperature i.e. 10.
  4. Sep 1, 2012 #3
    a block of aluminum 4 inch by 4 inch have and chamber of 1inch x 1 inch witch is heat faster the chamber or the whole block?
  5. Sep 1, 2012 #4


    User Avatar
    Homework Helper

    You should start a new thread with your question with all the relevant equations and terms in it. Otherwise, you are hijacking another person's thread which I think your question is not related to the OP's question.
  6. Sep 1, 2012 #5
    Hi Rock.freak667 what you mean I am hijacking onother person's thead,
    I just like to find out if a 1inch diameter long 4 inch heat faster then all aluminum block
    the block as reach the temperature of 100F can you help me ?
  7. Sep 1, 2012 #6


    User Avatar
    Homework Helper

    It means that you are posting a question in another person's thread which has nothing to do with your question.

    I advise you to start a new thread following the template so that you can input your question and what equations you think will be relevant.

    If you are at a loss with what equation would be relevant, look up heat transfer by conduction, as that is what you are essentially wanting to know.
  8. Sep 1, 2012 #7
    Thanks lot but do you know the answer? can you send to me? thanks
  9. Sep 1, 2012 #8


    User Avatar
    Homework Helper

    As per the forum rules, no on at PF can just give you the answer. You need to show some attempt at finding the answer. Once more, please start a new thread.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook