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Change in internal energy as a state function

Posted: Thu Jan 25, 2018 12:40 pm
by Christina Cen 2J
Can someone please explain to me how delta U is a state function and = w + q, but w and q are not state functions?

Re: Change in internal energy as a state function

Posted: Thu Jan 25, 2018 1:09 pm
by Vincent Tse 1K
As the delta in front of U suggests, U is a measure of the change of energy within the system; it's simply a measure of what it started as vs. what it ended as.

q and w, tho, depends on the path taken within the process itself. They are NOT measures of initial and final states; that's why there's no "delta" in front of them.
Even though q and w are not state functions independently, they are still related to each. Their combined "effects" on the system is hence defined as delta U.

Re: Change in internal energy as a state function

Posted: Thu Jan 25, 2018 4:59 pm
by Kayla Danesh 1F
So depending on the process by which a system gets from its initial to its final state, the amount of energy transferred by heat/work methods can vary. The distribution of energy transfer between heat and work depends on the process. This is why we say heat and work are "path-dependent."

On the other hand, the total amount of energy transferred through all methods is always the same--it always has to be ΔU (energy can neither be created nor destroyed). This is why ΔU is considered a state function.