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Topic: Temperature and pressure - Intensive?  (Read 16373 times)

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Offline 2810713

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Temperature and pressure - Intensive?
« on: February 21, 2005, 04:38:54 AM »
 
Hi,
[ chem. thermodynamics.]
Why are pressure and temperature intensive properties ?  
 - I  assume that  the pressure , temp. considered in the above statement is the same as in the equation PV= nRT  .
  So , P, V as well as T depend on quantity of substance , I mean -
                                              P=nRT/V
                                              T= PV/nR
                                              V= nRT/P

 The changes in pressure and volume may nutralize
each other to make T  an intensive  property. Is it correct?
But why is pressure intensive? It depends upon 'n'.
I think, V certainly depends upon 'n' , so for one of the remaining two variables to be intensive , the other one should nutralize the change in V , and for this , it should be extensive too. So, only one of the three variables can  be intensive. Isn't it?
Please help....

hrushikesh

 

Demotivator

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Re:Temperature and pressure - Intensive?
« Reply #1 on: February 21, 2005, 02:04:34 PM »
 It's  worthless arguing over this because they are really all interrelated.

Basically, I think, Volume is labeled as extensive because historically, the law of combining volumes establishes a relationship between volume and moles (assuming constant P and T). That leaves P and T as intensive, depending on each other.


dexangeles

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Re:Temperature and pressure - Intensive?
« Reply #2 on: February 21, 2005, 04:30:28 PM »
hint: intensive properties are those that do not depend on the amount of substance

eg. melting point -- ice will melt at 0 Celcius whether it's an ice cube or an ice berg (mass and volume are extensive while density is intensive)




savoy7

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Re:Temperature and pressure - Intensive?
« Reply #3 on: February 21, 2005, 07:14:32 PM »
I'm with the Demotivator - "It's  worthless arguing over this because they are really all interrelated."
Intensive properties are dependent on the amount of the substance

Temperature - depends on the molecular kinetic energy of a substance, not the amount of the substance

Pressure is dependent on many things - the force/area, so the amount of particles may affect the force, but that's not the only way to do it - so that's why some say that pressure is an intensive properties

savoy

dexangeles

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Re:Temperature and pressure - Intensive?
« Reply #4 on: February 21, 2005, 07:55:32 PM »
Intensive are independent you mean????

here's some text I'm quoting <--- had to mention that so i dont accussed of plagarizing  ;)

Quote
Thermodynamics deals with the relationship between the properties of a substance and their changes between two states. A thermodynamic property is any measurable quantity that is unique to a thermodynamic state. Some of the thermodynamic properties of interest to us are pressure, specific volume or density, temperature, internal energy, enthalpy, constant volume and constant pressure specific heats, and entropy. When a substance is in thermodynamic equilibrium there is no change in any of its properties. In thermodynamic equilibrium, an intensive property is uniform and does not depend on the mass of the substance but an extensive property varies with mass. Pressure and temperature are examples of intensive properties. Total volume depends on the mass of the substance and is an extensive property. Intensive properties can be derived from extensive properties by expressing such properties on a unit mass basis by dividing  he extensive property by the mass of the substance. Thus, specific volume (volume per unit mass) is an intensive property based on the total mass and volume of the substance.

that came from:
http://www.mfg.mtu.edu/~nvsuryan/ch4.pdf#search
« Last Edit: February 21, 2005, 07:56:33 PM by Ad Majorem Dei Gloriam »

Demotivator

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Re:Temperature and pressure - Intensive?
« Reply #5 on: February 21, 2005, 08:49:19 PM »
I can see that argument as it pertains to solids and liquids.
As gases, why can't pressure be viewed extensively and volume intensively?
So, it appears that  pressure is taken as intensive to maintain a  consistent viewpoint across all phases.

savoy7

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Re:Temperature and pressure - Intensive?
« Reply #6 on: February 21, 2005, 11:56:37 PM »
oops - typed too fast, I made a mistake - thanks for the catch

"Intensive are independent you mean????"

YES intensive properties are INDEPENDENT of the amount of the substance

I think that Demotivator has made some valid points.  I concure with his points.  I think that pressure can be affected by the number of particles, but that's because it affects the force on an area.  Let's look at this a different way:  If a person is producing a force of 500 N and is standing flat footed (lets say 325 cm2), they exert 1.5N/cm2.  Now that same person with the same amount of particles stands on one of their tip toes.  Now 500 N is pushed on a smaller area, lets say 6.5 cm2, the pressure has increased to ~77N/cm2.  The number of particles has not changed, but the pressure has.  Another example with gases.  I exhale hard and due to the size of my lungs I can only exhale a certain amount of air (volume relates to particles).  That exhale can exert a pressure.  Now take that same volume of exhale and place a pen with a spit ball on it.  By narrowing the mouth's opening, the area the exhaled particles are pushed through has decreased, causing more pressure which can propel the spit ball.  This is the same as a compressed air injector for giving shots.     Looking at it with these examples, pressure may change by adding more particles which affects the force like the Demotivator said, but can greatly change by changing the area. I think that most think it is intensive because it is based on force/area and can independently change without the # of particles changing.  

As for volume, in solids and liquids  - if one changes the amount of particles, one changes the volume.  We come to some issues when we deal with gases.  If we look at gases at the same temp and pressure, Avogadro's law comes into play (basically - equal #s of volume of gases have equal #s of particles).  Many times we see at STP that 22.4 L of a gas/1 mole of gas.  All of this data points to volume being extensive or dependent of the # of particles.  Issues present themselves when we deal with Ideal gas lawes and real gases.    

Offline 2810713

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Re:Temperature and pressure - Intensive?
« Reply #7 on: February 28, 2005, 12:28:42 AM »

 I think that most think it is intensive because it is based on force/area and can independently change without the # of particles changing.  -savoy
 
 Yes,
 The examples you give tell that P depends upon, not just ammount of gas , but also upon the area. But, the way we have defined  the intensive properties , we can not call P an intensive property, today!  Its not that important point ,but I wanted to see if I have any misconceptions as I am not getting the fact.
Thanks for the responses. :)

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