
#1
Oct2313, 12:30 PM

P: 95

Hello,
Please i have a question about the internal energy: The Internal Kinetic energy is the motion of molecules and it's a random motion. But at macroscopic level the whole object formed by those molecules move in a same direction? how it can be? In a flowing fluid not static? thank you 



#2
Oct2313, 12:56 PM

Sci Advisor
Thanks
P: 2,951

Even in a gas or a liquid, it is possible that all the molecules have the same average velocity over time, even though at any given moment many of them will have small random deviations from that average. 



#3
Oct2313, 01:14 PM

P: 95

Thank you.
So when we say that the internal energy is related to the microscopic kinetic energy what is the velocity taken here? 



#4
Oct2313, 01:43 PM

Sci Advisor
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P: 2,951

Internal Kinetic Energy 



#5
Oct2313, 01:56 PM

Mentor
P: 11,230

Another way to put it is to imagine an observer from whose point of view the total momentum of the molecules is zero. That is, the molecules are flying around in random directions so that their momenta cancel out completely when you add them together.
According to that observer, the total KE is the "internal KE." 



#6
Oct2313, 03:07 PM

Sci Advisor
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P: 2,951





#7
Oct2413, 07:08 AM

P: 95

Just to see if i understand. If the fluid has a velocity of 5m/s for example then its molecules has an average of 5m/s. the internal kinetic energy is calculated by imagining an object that has the same velocity of the fluid and the molecules' velocity are related to 5m/s ?




#8
Oct2613, 02:05 AM

P: 95

Just a Last question please. In a flow of a fluid the temperature is related to the microscopic kinetic energy or to its macroscopic? And is there a dependency between the internal and macroscopic kinetic energy? or in another way:
if a fluid is static and has a temperature of 18^{o}C then its internal kinetic energy is equal to 3/2KT. But when this fluid flows lets say it has the same temperature (18^{o}C) so it has the same internal kinetic energy as the static one but how come the average velocity(macroscopic) is not 0? 



#9
Oct2613, 07:23 AM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 38,879

The temperature of a fluid depends on the motion of individual molecules relative to one another (or to their center of mass). I think that is what you mean by "microscopic". When you say "the average velocity (macroscopic)" that must be given relative to some external frame of reference. The temperature of a fluid is NOT relative to an external frame of reference and so cannot be dependent on the "macroscopic" motion or energy.



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