Thermodynamics - Help me understand phase diagrams

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Discussion Overview

The discussion revolves around understanding phase diagrams, specifically the T-V diagram for water, in the context of a homework problem related to thermodynamics. Participants explore how to interpret phase diagrams and locate specific states within them, addressing both theoretical and practical aspects of the topic.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • The initial poster expresses confusion about phase diagrams due to lack of labeled axes and seeks help in determining temperature from given pressure and specific volume.
  • Some participants suggest looking up T-V diagrams for water to find the corresponding point for the given state.
  • There is a concern about the accuracy of readings from logarithmically scaled graphs, with a participant questioning whether they should estimate the temperature between known values.
  • One participant advises that interpolation is necessary when using graphs and suggests familiarizing oneself with logarithmic scales for better accuracy.
  • Participants discuss the importance of identifying whether the state is in the superheated or saturated region based on the specific volume and pressure provided.

Areas of Agreement / Disagreement

Participants generally agree on the need to use T-V diagrams and interpolation techniques, but there remains uncertainty about the exact temperature estimation and the interpretation of logarithmic scales.

Contextual Notes

The discussion highlights limitations in the clarity of phase diagrams and the challenges of interpreting logarithmic scales, which may affect the accuracy of the answers derived from them.

Who May Find This Useful

Students studying thermodynamics, particularly those struggling with phase diagrams and graphical data interpretation.

Hercuflea
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Homework Statement



So my thermo professor has been putting a huge emphasis on phase diagrams, which I didn't really expect. He basically skips most of the analytical/calculus type problems and gives us the graphical phase diagram/ "real world" type of problems for homework. I don't understand phase diagrams because they never put any labels on the axes, and it is just like a little parabola and then he draws two horizontal lines through it, but nothing is labeled.

So here's the problem :

For H2O, determine the specific property at the indicated state. Locate the state on a sketch of the T-V diagram.

a) p = 300 kPa, v (specific volume) = .5 m^3/kg Find T, in Celsius.


Homework Equations



PV^n = k, k is a constant

The Attempt at a Solution



I have a table where I can look up these things but it says that water at 300 kPa (3 bar) is supposed to be .6058 m^3/kg not .5 m^3/kh. So I guess the water in this problem is not saturated and I can't use my table? How can I tell?

And once I find the temperature how do I draw the diagram?
 
Physics news on Phys.org
Google T-V diagram. One of the figures it will show you will be a t-v diagram for water. Click on it. Find your point on the diagram.
 
Ok I have done this. But the graph is scaled logarithmically or something and my answer would not be exact. Am I just supposed to just guess what the temperature is between 100C and 200C?
 
Hercuflea said:
Ok I have done this. But the graph is scaled logarithmically or something and my answer would not be exact. Am I just supposed to just guess what the temperature is between 100C and 200C?

Yes. That's how you would use any graph. You have to interpolate as best you can by eyeball. Are you uncomfortable with logarithmic coordinates? If so, get used to it. Find a piece of logarithmic graph paper with a more detailed grid on it to see how the numbers space out, or get yourself a slide rule (which has logarithmic scales on it). Try to find the location of your data point as closely as possible on the graph and see if you can at least see what region it is in qualitatively. For example, is it in the superheated region or in the saturated region?

chet
 

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