3 replies to this topic

[lucy skies]

Can someone please explain to me how to solve this problem?

A 10m high open cylinder with a cross-sectional area of .1m^2 contains 20C water above and 2 kg of 20C water below a 198.5 kg insulated floating piston. Atmos. Pressure is 101 kPa. Heat is added to the water below so that it expands pushing the piston up, causing the water on top to spill over the edge. The process continues until the piston reaches the top of the cylinder. Find the final state of the water below the piston (P, v, T) and the heat added in the process. (Q=22340 kJ is the answer.)

[Art Montemayor]

Lucy:

The first and best thing you can do to help us help you is to completely fill out your Member's "about me" information form. The more we know about you and your background - especially your university training and the courses you have taken - the better and faster we can help. We don't want to talk down to you; we also don't want to start talking over your head. That would all be a waste of time and would only insult or confuse you. All we know about you now is that you are a woman and not a man. And that doesn't explain why you should be having a problem understanding this problem. The problem may be too sophisticated for your level of learning; or, you may be just too lazy to apply what you have already been taught and only want someone else to do your work for you. This happens all the time on this Forum.

Your problem is definitely not a Thermodynamics problem, but a relatively simple one that involves heat transfer and fluid statics -- with a little physics thrown in.

Show us that you have put in an effort to solve your problem - for example, draw us a good, detailed and understandable sketch of the problem in an Excel spreadsheet, complete with all the basic data given. I've already done this and have the answer.

Await your reply.

[lucy skies]

Your problem is definitely not a Thermodynamics problem, but a relatively simple one that involves heat transfer and fluid statics -- with a little physics thrown in.

Um, that problem was out of my Thermodynamics textbook.

[Doggert]

Can someone please explain to me how to solve this problem?

A 10m high open cylinder with a cross-sectional area of .1m^2 contains 20C water above and 2 kg of 20C water below a 198.5 kg insulated floating piston. Atmos. Pressure is 101 kPa. Heat is added to the water below so that it expands pushing the piston up, causing the water on top to spill over the edge. The process continues until the piston reaches the top of the cylinder. Find the final state of the water below the piston (P, v, T) and the heat added in the process. (Q=22340 kJ is the answer.)

It looks like your system is doing work on the surroundings via expansion of the water underneath the piston - thus, you're going to have to find out what that work is and relate it to the energy put into the water.

Since you have no entering or leaving streams (taking the water under the piston as being the system), you are looking at this application of the 1st law:

dU = Q + W

Since you can calculate the volume of the water under the piston at the end state, this should give you your temperature and pressure readily. All of these together will help you get your W value, which will help out with the Q value on this equation. Is there a drawing along with the problem to help visualize this?

That'll get you started. Then you'll be on a hunt for the toughest part of thermo - the little relations/equations that help you resolve all this. I would dive into that to help, but I'm in a bit of a time crunch today.

Good luck.