First law of thermodynamics applied to Compression and Expansion

In summary, the first law of thermodynamics states that at constant temperature, the change in internal energy of a gas is equal to the heat added to the system plus the work done on the system. This applies to both compression and expansion of the gas. However, this only holds true for ideal or perfect gases, which have a constant internal energy dependent on temperature. The concept of a perfect gas is supported by the Joule experiment and the Joule-Thompson experiment.
  • #1
hms.tech
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How does the first law of thermodynamics apply to the Compression And expansion of a gas at constant temperature ?

If the gas is at a constant temperature and is being compressed, then
ΔQ = 0
ΔU = +
W(on the system) = +


Are those(^^) correct ?

The equation being :
ΔU = ΔQ + W(on the system)
 
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  • #2
Compression or expansion at constant temperature means heat is extracted or added.
 
  • #3
At constant temperature ΔQ is not zero. ΔU is zero, for ideal gas.
 
  • #4
nasu said:
At constant temperature ΔQ is not zero. ΔU is zero, for ideal gas.

would u explain "why" ?
 
  • #5
would u explain "why" ?

Originally this was an experimental fact about gasses. Nowadays there is maths to back it up, and we know that it only applies to ideal or perfect gasses.




http://demonstrations.wolfram.com/JouleExperimentOnFreeExpansion/

If you can cope with the maths google has lots on the Joule experiment and the Joule-Thompson experiment.

If not you can take it as a definition of a perfect gas.

The internal energy of a perfect gas depends only on its temperature.
 

1. What is the first law of thermodynamics?

The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or converted from one form to another. In other words, the total energy of a closed system remains constant.

2. How does the first law of thermodynamics apply to compression?

When a gas is compressed, work is done on the gas which increases its internal energy. This increase in internal energy is equal to the work done on the gas, as well as any heat transferred to the system during compression.

3. How does the first law of thermodynamics apply to expansion?

When a gas expands, work is done by the gas which decreases its internal energy. This decrease in internal energy is equal to the work done by the gas, as well as any heat transferred out of the system during expansion.

4. Can the first law of thermodynamics be violated?

No, the first law of thermodynamics is a fundamental law of nature and cannot be violated. It has been observed and tested in countless experiments and is a cornerstone of thermodynamics.

5. How is the first law of thermodynamics related to the conservation of energy?

The first law of thermodynamics is essentially a statement of the conservation of energy. It states that the total energy of a closed system remains constant, which is another way of saying that energy is conserved. Any changes in the energy of the system are accounted for by work done on or by the system, as well as any heat transferred into or out of the system.

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