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Homework Help: Work Done ON or BY the System [Conceptual Question]

  1. Oct 6, 2012 #1
    1. The problem statement, all variables and given/known data
    A student performs a combustion experiment by burning a mixture of fuel and oxygen in a constant-volume metal can surrounded by a water bath. During the experiment, the temperature of the water is observed to rise. Regard the mixture of fuel and oxygen as the system.
    a) Has heat been transferred to/from the system? How can you tell?
    b) Has work been done by/on the system? How can you tell?
    c) What is the sign of ΔU of the system? How can you tell?
    d) 1g of fuel is placed in the can for combustion. During combustion, the 200mL water bath is observed to increase in temperature by 35.5°C. What is the heat of combustion (energy released per gram) of the fuel?

    2. Relevant equations
    ΔU = Q - W
    where ΔU = change of internal energy in system, Q = heat gained by system, W = work done on the surrounding BY the system.

    3. The attempt at a solution
    Now I know that for part a), heat has been transferred from the system, as the temperature of the water bath rises (meaning, heat has been transferred from the system to the surroundings). However, I am a little confused about part b). I am not sure if the work is done by the system or not. I am guessing, since the metal can is at constant-volume, no work has been done, but I may be wrong. Please correct me if I am wrong.
    I am stuck at this point. I can't attempt the other parts of the problem unless I understand part b).

    Thank you.
  2. jcsd
  3. Oct 6, 2012 #2
    Your understanding of b) is correct.
  4. Oct 7, 2012 #3


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    Staff: Mentor

    The unit of work is the Joule. Work is done whenever energy is exchanged. Heat is measured in Joules. The chemical energy of the contents of the can is reduced. The heat energy of the water jacket increases. Work has been performed by the system on the water.
  5. Oct 7, 2012 #4
    Imagine pushing a box.
    Work is done ON the box, but BY who? Ya, BY you.
    You are exerting kinetic energy towards the box to make it move :)
    Imagine putting food into oven.
    Work is going to be done ON the food (To cook it) BY the oven.
    Note: There is only heat energy, no "cold" energy.
    So when it comes to a fridge, heat energy is being transferred from the food to the fridge. Work done by the food :D
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