EK Chem internal energy

[browncomputer]

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On page 53, in EK gen chem, it says "if we have a closed system at rest with no electric or magnetic fields, the only energy change will be in internal energy and 1st law of thermodynamics can be rewritten as ∆U = q + w"

Won't there be potential energy in the closed system and isn't potential energy mechanical energy, so how can the energy change only be internal energy?
thanks!

 

[ae0124]

First we want to ask the question What is a System?
Anwser : interacting objects being observed
Then what is internal energy?
Anwser: Energy of the system due to its relative position
If the system is closed that means no energy can come in and no energy can come out
So total energy = total KE + total PE
So yes.... your right the PE can change based on its relative position but as the PE changes the KE changes accordingly so that total energy is conserved. Also I think your forgetting a system can include multiple objects the Earth is a system our body is a sytem. SO yes the potential energy can change but the KE or mechanical energy released would just do work on anther component in the system. IF the System is CLOSED no energy is coming IN our OUT.
Hope that helps a little bit.

 

[browncomputer]

If the system is closed that means no energy can come in and no energy can come out

actually, a closed system is a system in which mass does not change in the system but there can be energy transfer between system and surroundings. An isolated system is one in which no energy can come in or go out.

so here is what I'm thinking. the internal energy is the total potential energy in the system. the potential energy in the system has been converted into internal energy (motion of molecules) via heat and work processes, which is why we can rewrite the 1st law as such when there is no electric or magnetic fields. If there are electric or magnetic fields, the atoms could be repelled, which would lead to accounting for these energies.

let me know if this interpretation is correct, thanks!