Saturday, December 16, 2017

Latent Heats Question & Yahoo Answers

Hi friends!

A while ago I thought to myself, hey, I should start a blog. Looking back, I would've liked it if I had continued to keep a blog going. Document my life, and whatnot. Life is short and I'd like to think that I got the most out of life that I can, and what's the best way to get your value's worth out of something but excessive documentation? Anyhow, for various reasons that didn't happen, mostly procrastination, but hey let's not talk about that now. 

Eh never mind, that's just me rambling. Anyhow, I figured, this might as well become a place for me to talk about things that I want to get into the world, specifically onto Google. Although I do have a domain where I host some projects (, it's nice to have a platform for putting stuff out there where I don't have to manually worry about styling or browser compatibility or SEO or stuff. Then I remembered - I have a blogspot!

To the point. Sorry if you're only here 'cuz you Google'd the question and read the above pointlessness. Physics 213 amirite. So (and yes I'm in college now, yay) I'm studying for my physics final in two days, and there's this problem in a past practice final:

A certain liquid is observed to boil at standard pressure (1.01 × 105 Pa) at a temperature of 42°C. When the pressure is lowered to 0.1 × 105 Pa, it boils at 38°C. What is its latent heat per molecule?

I couldn't for the life of me figure it out for a while. Yahoo Answers didn't exactly help, as even though it's on YA, - they used some formula that we didn't explain in class. Then, after a bit of browsing the lectures, I did figure it out, and it's actually pretty neat. I tried answering the Yahoo Answers thread that was originally posted 7 years ago, to help people in my shoes this year and the next hopefully, to find that it's closed. TIL that Yahoo Answers threads can be closed. I guess it makes sense, they have a literal ton (7+ years worth at least) of open threads, with probably not much moderation. Still though, I'd've liked to have answered it differently, and I even password-reset my old Yahoo account to try. Hence this new post, I guess. 

Anyhow, instead of using the Clausius Clapeyron equation, or whatever, you can use chemical potential as explained in class. The chemical potential of the liquid stays the same when the pressure changes since pressure only affects the gas (ideally), and in equilibrium the chemical potential of the gas should equal to the chemical potential of the liquid. So, the before-and-after chemical potentials of the gas should be the same. Using the formula on the formula sheet, (I'll figure out how to type equations on here later), chemical potential mu_i = kT*ln(ni/nTi) and ni/nTi = P/Pq where Pq is the quantum density pressure. We don't know that, but we can calculate that using the two pressures and two temperatures we have. And, after calculating it, just find mu_i = -delta and that's your answer. 

Hope this helps. If not, feel free to shoot me an email or something. Actually, don't, that would be weird. It's probably some sort of character flaw that I have that I really can't say no when people ask me for help on stuff that I can actually help on. Like, it's made me waste a lot of time. Well, for certain definitions of waste... I mean one could argue that if a lot of one's utility rests on helping others cuz it makes 'em feel good, then it's not at all wasting time. Then again... eh this paragraph is getting too long. Never mind. 

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