When do you use Cv vs Cp if the question does not explicitly say which conditions it is under?

[Mizuno_Mikaela_1D]

When do you use Cv vs Cp if the question does not explicitly say which conditions it is under?

For example in midterm 2013 Q3A, A blacksmith putting a 1.45kg carbon steel katana at 314.0 C into water at 25C. The steam does not heat past 100C. The q=mCdeltaT equation uses Cp for both the water and katana. How do you know to use Cp?

[Mizuno_Mikaela_1D]

I just read the part that says it was a poorly worded question in case students assumed constant volume..

But then in the Harry Potter question, when both volume and temperature were changed, I understand you can used Cv assuming the volume and temperature changed happened separately to calculate change of entropy, but why wouldn't you use Cp?

[Hannah_El-Sabrout_2K]

I could be wrong, but I'm pretty sure you can use either as long as you're consistent in all of the parts.

[stephanieyang_3F]

Mizuno, you wouldn't use Cp because you have to keep volume constant. When you're changing two variables at a time like volume and temperature, you have to keep two of the three variables constant in order to calculate for entropy or else it'd be too complicated. In the Harry Potter question, step 1 keeps the temperature constant to calculate the change in entropy for a volume change, while for step 2 you'd need to keep the volume constant to calculate the change in entropy for a temperature change. I hope that made sense. We ended up adding the steps together and we can divide the calculate of entropy change into two steps because entropy is a state function.