# Energy interactions of a stationary closed system

#### mech-eng

Summary
A stationary closed system will exchange energy with its surroundings.
A stationary closed system such as an air in a room or a water in tank can exchange energy with its surroundings, such as receiving heat, fan work, electrical work, shaft work. These energy interactions cause a change in the total energy E of a system. This total energy can be comprised of different sorts of energies such as kinetic (maybe if the air or water mass experiences acceleration or deceleration) KE, since the system is assumed at constant elevation there are no potential energy PE interactions, internal energy (probably only based on the temperature).

What kind of the energy changes does the energy exchange of a constant volume, fixed-elevation closed system experience?

That is it will energy exchange with the surroundings cause a change in kinetic, internal or other kinds of energy of the system

Thank you.

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#### anorlunda

Mentor
Gold Member
Your system is not moving meaning no net kinetic energy, and fixed-elevation which means no change in gravitational potential energy. So how does energy change the total internal energy? The Wikipedia article on that question is not bad.

https://en.wikipedia.org/wiki/Internal_energy#Internal_energy_changes said:
Thermodynamics is chiefly concerned only with the changes, ΔU, in internal energy.

For a closed system, with matter transfer excluded, the changes in internal energy are due to heat transfer Q and due to work. The latter can be split into two kinds, pressure-volume work Wpressure-volume, and frictional and other kinds, such as electrical polarization, which do not alter the volume of the system, and are called isochoric, Wisochoric. Accordingly, the internal energy change ΔU for a process may be written[3]

When a closed system receives energy as heat, this energy increases the internal energy. It is distributed between microscopic kinetic and microscopic potential energies. In general, thermodynamics does not trace this distribution. In an ideal gas all of the extra energy results in a temperature increase, as it is stored solely as microscopic kinetic energy; such heating is said to be sensible.

A second mechanism of change of internal energy of a closed system is the doing of work on the system, either in mechanical form by changing pressure or volume, or by other perturbations, such as directing an electric current through the system.

If the system is not closed, the third mechanism that can increase the internal energy is transfer of matter into the system. This increase, ΔUmatter cannot be split into heat and work components. If the system is so set up physically that heat and work can be done on it by pathways separate from and independent of matter transfer, then the transfers of energy add to change the internal energy:

If a system undergoes certain phase transformations while being heated, such as melting and vaporization, it may be observed that the temperature of the system does not change until the entire sample has completed the transformation. The energy introduced into the system while the temperature did not change is called a latent energy, or latent heat, in contrast to sensible heat, which is associated with temperature change.

#### mech-eng

So how does energy change the total internal energy?

Aren't we talking about the following possibilities?

Summary: A stationary closed system will exchange energy with its surroundings.

A stationary closed system such as an air in a room or a water in tank can exchange energy with its surroundings, such as receiving heat, fan work, electrical work, shaft work.
I don't think so because the individial gas molecules might accelerate because of energy interactions. This might cause them have kinetic energy. Over time, the individual gas molecules would collide each other. This might cause them warm up which might mean the kinetic energy is converted into internal energy.

#### Chestermiller

Mentor
I don't think so because the individial gas molecules might accelerate because of energy interactions. This might cause them have kinetic energy. Over time, the individual gas molecules would collide each other. This might cause them warm up which might mean the kinetic energy is converted into internal energy.
The internal energy U of a gas is equal to the sum of the random kinetic energies of the molecules (relative to their mean velocity) plus the potential energy associated with their mutual interactions.

#### mech-eng

plus the potential energy associated with their mutual interactions
Would you please expand on this? What are these "mutual interactions"?

#### Chestermiller

Mentor
Would you please expand on this? What are these "mutual interactions"?
Mutual interactions are conservative forces between molecules as they get closer together (e.g., van der Waals forces).

#### anorlunda

Mentor
Gold Member
I don't think so because the individial gas molecules might accelerate because of energy interactions. This might cause them have kinetic energy.
I think you are getting confused between kinetic energy and net kinetic energy.

The molecules in a gas move. They have kinetic energy. We call that temperature. At the temperature absolute zero, motion stops.

But kinetic energy is a vector quantity with both magnitude and direction. If we add up the vector kinetic energies of all the molecules in a static container of gas, the sum is zero. Zero is the net kinetic energy of the system.

If the container was moving with all the gas inside, then the sum would be nonzero.

I hope you see the difference. The kinetic energy of the system and the kinetic energies of individual particles are not the same thing.

Edit: On second thought, the conclusion is right, by my explanation was wrong. K.E. has no sign but momentum does. If the sum of the momenta of the particles is zero, then the motion of the center of mass is zero, and the total system has no K.E.

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"Energy interactions of a stationary closed system"

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