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# Which of the following statements about internal energy is/are true watch

1. I) The internal energy is the sum of the Kinetic and Potential Energies of the molecules of the system
II) The internal energy of the system is proportional to temperature of the system
III) A change of phase does NOT affect the internal energy of the system because it happens at a constant temperature

I am thinking I and II are true
because
I is basically the definition of internal energy
II - internal energy=kinetic energy + potential energy. Since kinetic energy is proportional to kinetic energy, then internal energy will also be proportional to temperature

I am unsure about these though. Can someone let me know if I am correct or provide me with a correct solution
Thanks
2. (Original post by ksrw395)
I) The internal energy is the sum of the Kinetic and Potential Energies of the molecules of the system
II) The internal energy of the system is proportional to temperature of the system
III) A change of phase does NOT affect the internal energy of the system because it happens at a constant temperature

I am thinking I and II are true
because
I is basically the definition of internal energy
II - internal energy=kinetic energy + potential energy. Since kinetic energy is proportional to kinetic energy, then internal energy will also be proportional to temperature

I am unsure about these though. Can someone let me know if I am correct or provide me with a correct solution
Thanks
I is true.
II is false.
III is false

I is true by definition.

II is false since internal energy is not proportional to T, in general (it is only the KE part that is proportional to T)

III is false since, at a phase change (e.g. water to steam), we add energy to the system. By conservation of energy, this must manifest itself as an increase in the internal energy. Since we see no rise in T, we can further assert that, at a phase change, there is no increase in the KE of the system, so all of the energy must go to increasing the PE.
3. (Original post by atsruser)
I is true.
II is false.
III is false

I is true by definition.

II is false since internal energy is not proportional to T, in general (it is only the KE part that is proportional to T)

III is false since, at a phase change (e.g. water to steam), we add energy to the system. By conservation of energy, this must manifest itself as an increase in the internal energy. Since we see no rise in T, we can further assert that, at a phase change, there is no increase in the KE of the system, so all of the energy must go to increasing the PE.
Thanks alot!

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