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Homework Help: Quick question 1st law thermodynamics

  1. Oct 11, 2005 #1
    What is the equation to find the change in heat of a system that is isothermal?

    And also, for a PV graph, if the state of the system loops back onto itself, as in it starts at a and ends at a, is the total change in internal energy always 0?
    Last edited: Oct 11, 2005
  2. jcsd
  3. Oct 11, 2005 #2

    Andrew Mason

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    Since PV=nRT, if T does not change, PV is constant. So the internal energy of the system does not change. As a result, there has to be heat flow to or from the system when work is done by or to the system (if it is compressed, heat is lost to the system; if it expands isothermally, it absorbs heat). The heat flow is: [itex]\Delta Q = \Delta U + P\Delta V = P\Delta V[/itex].

    Yes. If PV is the same, T is the same (PV=nRT). Since [itex]U = nC_vT[/itex], if the temperature is the same and n does not change, internal energy is the same.

    Last edited: Oct 11, 2005
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