Identify each energy exchange as primarily heat or work and
deter...

Question

Identify each energy exchange as primarily heat or work and
determine whether the sign of ΔE is positive or negative for the
system.
a. Sweat evaporates from skin, cooling the skin. (The evaporating
sweat is the system.)
b. A balloon expands against an external pressure. (The contents
of the balloon is the system.)
c. An aqueous chemical reaction mixture is warmed with an
external flame. (The reaction mixture is the system.)
Identify energy exchanges as primarily heat or work.

Accepted Answer

hey everyone in this example, we need to identify the sign of delta U. Or our change in internal energy for the indicated system. So in part A. We are told that our system is this gas sample that is being compressed A diabetic lee. So we should recall that because we're talking about our system being compressed, that means that our internal energy change is going to be equal to work. And we should recall that a diabetic lee means that there is no heat transfer. Recall that heat is represented by Q. So we would say therefore due to these two facts, we just wrote down our compression is being done on our system which is the gas and so therefore work is done on the system. And so our change in internal energy is going to be greater than zero. So this would be our answer for part A. So moving on to part B. We have our system being hot coffee. We're we're told that it's being cooled in a refrigerator. So because our coffee is being cooled, we would say that our change in internal energy is going to be equal to the heat that is being exchanged between our coffee and our refrigerator. And so because our coffee is the system in which our coffee is losing heat. So we lose Q. Which again represents heat. We would say therefore our change in internal energy of our system is going to be less than zero because we lose heat. And so this would be our answer to complete example B. We lose heat from our coffee. So our change in internal energy is going to be less than zero, it's going to be negative. So moving on to Part C. It says that a worker pushes a trolley along a ramp and our system is going to be the worker. So the key word here is pushes and that means that our internal energy is going to be equal to work. Our change in internal energy is going to be equal to the work being done by our system. And because this question is stating that the system is our worker work is done by the system because our worker is pushing the trolley and so therefore our change in internal energy is going to be negative or less than zero. And so this would be our final answer for part C. Of this question. So everything boxed and blue represents our three final answers for parts A through C. I hope that everything we reviewed was clear. But if you have any questions, please leave them down below and I will see everyone in the next practice video.

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