# What is internal energy? Am I right?

• Engineering
lee123456789
Homework Statement:
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Relevant Equations:
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Am i right
Is internal energy for engineering just the sum of energy stored with a system of a substance. which is only affect by heat and work coming out of the system

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What do you think and why? On this forum, you don't get help until you show some effort.

Mentor
It is a physical property of the material comprising the system, and is equal to the sum of the random kinetic energies of the molecules and their potential energies of interaction.

Gold Member
2022 Award
What do you think and why? On this forum, you don't get help until you show some effort.
It is a physical property of the material comprising the system, and is equal to the sum of the random kinetic energies of the molecules and their potential energies of interaction.
OK, I was wrong. You DON'T have to show any work yourself

lee123456789
OK, I was wrong. You DON'T have to show any work yourself
well
i thought it was, the sum of energy stored with a thermodynatic system of a substance. which is only affect by heat input and work coming out of the system.

Mentor
well
i thought it was, the sum of energy stored with a thermodynatic system of a substance. which is only affect by heat input and work coming out of the system.
That is correct, but it is the short version. I though you wanted more detail.

Tom.G
lee123456789
That is correct, but it is the short version. I though you wanted more detail.
Sorry i was getting confused. Was reading in some engine÷ng websites that it excluded potention and kinetic energy and on overs that it included.

lee123456789
That is correct, but it is the short version. I though you wanted more detail.
Is that the engineering defination. Some websites say excluding sum potenital and kinetic energy if motion for the system.

Mentor
Yes. That is how it is done in engineering. The more complete version of the 1st law is $$\Delta E=\Delta U+\Delta (PE)+ \Delta (KE)=Q-W$$ so that U is strictly the random kinetic energy and potential energy of interactions of the molecules (and is a fundamental physical property of the gas).

lee123456789
Yes. That is how it is done in engineering. The more complete version of the 1st law is $$\Delta E=\Delta U+\Delta (PE)+ \Delta (KE)=Q-W$$ so that U is strictly the random kinetic energy and potential energy of interactions of the molecules (and is a fundamental physical property of the gas).
Thank u
Is it true that it can only be affect by heat or heat and work

Mentor
Thank u
Is it true that it can only be affect by heat or heat and work
Yes, as far as I know.

lee123456789
Yes, as far as I know.
this is the statement i found. which confused me.
The internal energy of a thermodynamic system is the energy contained within it. It is the energy necessary to create or prepare the system in any given internal state. It does not include the kinetic energy of motion of the system as a whole, nor the potential energy of the system as a whole due to external force fields, including the energy of displacement of the surroundings of the system.

also wouldn't matter as well help dtermine the internal energy. eg in a open flow system

Mentor
this is the statement i found. which confused me.
The internal energy of a thermodynamic system is the energy contained within it. It is the energy necessary to create or prepare the system in any given internal state. It does not include the kinetic energy of motion of the system as a whole, nor the potential energy of the system as a whole due to external force fields, including the energy of displacement of the surroundings of the system.

also wouldn't matter as well help dtermine the internal energy. eg in a open flow system
Well, the 1st sentence is confusing. But the 2nd sentence makes some sense, at least up to the comma.

Your statement about matter is a little confusing. The first law, as you have written it applies exclusively to a closed system.

lee123456789
Well, the 1st sentence is confusing. But the 2nd sentence makes some sense, at least up to the comma.

Your statement about matter is a little confusing. The first law, as you have written it applies exclusively to a closed system.
ow ok.
So the internal energy is the sum of energy the resides with the thermodynatic system. It is affected by work and heat. the potential and kinetic energy is minimial in the system so it is excluded. the internal energy keeps account of the energy input and output so the overall internal energy change is balanced so equals zero.

Mentor
ow ok.
So the internal energy is the sum of energy the resides with the thermodynatic system. It is affected by work and heat. the potential and kinetic energy is minimial in the system so it is excluded. the internal energy keeps account of the energy input and output so the overall internal energy change is balanced so equals zero.
If you mean that the total amount of energy is conserved, then, yes.