Recent content by omberlo

  1. O

    Motion due to gravity without neglecting varying distance

    Hello everyone. In an attempt to brush up my Calculus skills and apply them to a physics problem, I made up an excercise which I tried to solve, but got stuck in the process. I'll write my attempt at a solution and I'd be grateful if someone could correct my approach and maybe even help me...
  2. O

    Thermodynamics - Irreversible Adiabatic expansion

    Homework Statement Closed system made of a cylinder containing air, compressed by a piston with weights on top of it. The piston is frictionless and its weight can be neglected. The system contains initially 10 kg of air at 27 °C and a pressure of 10 ata. The weights are suddenly halved...
  3. O

    Why is Flow Work defined as pV and not pdV?

    What do you mean that for a small amount, it simplifies? By following your math I get ##W=\int_0^L F ds##, introducing pressure ##W=\int_0^L \frac F A A \times ds## which in terms of pressure and volume is ##W=\int_0^V P dV## this is equal to ##P \Delta V## only if pressure is...
  4. O

    Why is Flow Work defined as pV and not pdV?

    Thanks everyone for the answers so far. From what I gather from all of you, flow work seems to be defined as pv because pressure is assumed to be constant. But isn't this assumption inaccurate for unsteady-flow processes?
  5. O

    Why is Flow Work defined as pV and not pdV?

    Hi! I understand what enthalpy is, but the book is trying to justify the use of enthalpy in the first law applied to flow processes by saying that in the energy balance internal energy of the fluid entering/exiting the control volume must be accounted for as well as the flow work, which is the...
  6. O

    Why is Flow Work defined as pV and not pdV?

    In the chapter relative to First law applied to flow processes, my book explains the "Flow Work" which appears in the energy balance, as work needed to push a volume of fluid into (and out of) the control volume. It also says that it's equal to pV, and then it is added to the internal energy U...
  7. O

    First Law of Thermodynamics for Irreversible Processes

    Yes, S is entropy. Yes, that equation does carry restrictions that apply to the first and second law, but (ir)reversibility should not be one of them, since the whole equation involves only state functions which behave equally well for both reversible and irreversible processes. So, can someone...
  8. O

    First Law of Thermodynamics for Irreversible Processes

    I'll answer my own question and raise more doubts: http://www.av8n.com/physics/thermo/boundary.html explains that the TdS term is really an interior property and not a boundary-flow term, thus dS must account for the total entropy change, i.e. even the entropy created from sctratch due to...
  9. O

    First Law of Thermodynamics for Irreversible Processes

    Hello everyone. I have found quite a lot of conflicting information about the first law applied to irreversible processes and in particular whether the dS term in the equation dU = TdS - pdV only accounts for the entropy transferred or for the total entropy, i.e. dS_transferred + dS_created...
  10. O

    Differential form of the first law of thermodynamics

    Let's see This part is clear. This statement is not so clear, why exactly do we want to reduce one of the terms to zero? What is the problem with having both independent variable changing? This part is clear except for the above doubts This part is not really intuitive to...
  11. O

    Differential form of the first law of thermodynamics

    I appreciate all your help Studiot :)
  12. O

    Differential form of the first law of thermodynamics

    Thank you Studiot, we are definitely making progress, now I understand where that equation comes from, but my doubt about HOW to use equations expressed in differential form still remains. Even if I look at the equation from the second law, which is a bit simpler than the one I asked about...
  13. O

    Differential form of the first law of thermodynamics

    Thank you for your answer and your time Studiot. Don't worry too much about my exams being imminent, I'll probably just wait until the next exams' session to prepare for it. I decided to stop studying for this course until I completely understand equations expressed in differential form...
  14. O

    Differential form of the first law of thermodynamics

    Hi everyone, I'm new here. I'm an italian student who has an exam in applied thermodynamics soon. Through the whole course as well as the Physics one I have faced a lot of equations expressed with differentials, basically all of them. I have never been taught how to use them though. For...
Top