Dahriel Aron 3A
Posts: 60
Joined: Fri Sep 25, 2015 3:00 am

Regarding question 8.15 which asks, in an adiabatic process, indicate whether which statements are true, false, or uncertain in a closed system
a) internal energy equals 0
b) heat equals 0
c) heat is less than 0
d) internal energy equals heat
e) internal energy equals work

I was wondering if someone could explain the logic through answering this question and how to determine when the energy is transferred as heat

Chem_Mod
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Joined: Thu Aug 04, 2011 1:53 pm
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It's important to note that in the question, it asks for changes in internal energy and heat which is by definition energy in motion due to a temperature difference.

That being said, we define an adiabatic process as one in which energy in the form of heat is not transferred. Since the change of internal energy of a system is the sum of heat and work, and we know that heat (q) = 0 by definition of an adiabatic process, we must conclude that the change in internal energy is equal to work that is performed by the system or onto the system. Now lets go through each scenario.

a. change in internal energy is 0: Since internal energy is determined solely by work in an adiabatic process, this is true, unless no work is being performed.
b. heat equals 0: True, by definition of the adiabatic process, q=0
c. heat is less than 0: False, by definition of the adiabatic process, q=0
d. change in internal energy equals heat: Since q=0 is a given, for this scenario to take place, the change in internal energy must equal 0. Since the only other determinant of changes in internal energy is work, this can be true when also equals 0 since change in internal energy is the sum of heat and work.
e. change in internal energy equals work: q=0 is given, so by definition of an adiabatic process, change in internal energy must equal work. The previous answer is a special scenario of this one.