First law of Thermodynamics and energy conservation

[mcastillo356]

Hi PF!
I don't understand the sentence: on one side says the energy is preserved, and, at the end, the total energy of the system will change if ##W## or ##Q## is added: ##\Delta{U}=Q+W##.
Greetings!

 

[etotheipi]

If a system is isolated, then ##W = Q = 0## and ##\Delta U = 0##, i.e. the energy of the system is constant. But if the system is not isolated, and ##W## and ##Q## are non-zero, then ##\Delta U## is not necessarily zero and the system energy not necessarily constant!

 

[KMCv]

There is difference between a closed and an open system when law of conservation of energy is considered. If we have an open system, there are some resultant external forces acting on the system. If system is closed, there are no resultant external forces acting on the system.

Lets say we have a cylinder that is containing some volume V of the gas. The gas will be the (open) system in this example. If we push a piston, the volume V of the system will decrease. The piston had done work W on the system. By that, the energy of system (the gas) increased by exactly the amount of work that piston had done on the system.
If our (closed) system includes the gas, the piston and us, the total energy of the system will be the same before pushing the piston and after pushing the piston.

But, the total energy is the same in both cases: for open system the total energy is the energy of system + work done on the system. The work done on the system was not created from thin air. In example above we pushed the piston and for that we used some our internal energy.

I hope this helps. Sorry for bad English, I'm not native speaker :)

 

[mcastillo356]

Thank you very much, etotheipi, KMCv! Well, let's see if I understood: example of open system: me; boiling water without a lid. Closed system: a pressure cooker, just before the safety valve opens. Isolated system: a thermoflask (this is not true; it's slower, but it exchanges heat with its surroundings). In terms of energy, there is a law, first proposed and tested by Émile du Châtelet, witch states that energy cannot be created nor destroyed; only transferred or transformed.
KMCv, your post is very revealing: the difference between open and closed system is the same in thermodynamics and Newton's dynamics; and in your example of the cylinder containing gas, we can apply ##\Delta{U}=Q+W## the same if the system is open or closed.
ethoteipi, thank you very much for explaining the definition of isolated system: ##\Delta{U}=0=Q+W##. No work nor heat (neither mass) can pass to the system.
Greetings

 

[KMCv]

I'm happy that helped! :)