Confusion regarding the First Law of Thermodynamics

In summary: I was thinking of the gas expanding freely, doing no work. Thanks for clearing that up.In summary, the gas does not do any work on the surroundings as it expands freely.
  • #1
Smith
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Is forcing a closed system to expand (e.g. by pulling out a piston), causing it to cool, work done to the system or work done by the system? I assume it was work done to the system, but that means the first law of thermodynamics formula no longer balances if you assume an adiabatic change. With adaibatic, Q = 0, so:
-w = + delta u.
-w is equal to work done to the system, but delta u should be negative, not positive as the formula would make it.

Any ideas what I'm doing wrong.
 
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  • #2
Smith said:
Is forcing a closed system to expand (e.g. by pulling out a piston), causing it to cool, work done to the system or work done by the system? I assume it was work done to the system
No, it corresponds to work done by the system. It is equivalent to reducing the external pressure and letting the system do the work.
 
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  • #3
Thanks very much, that makes sense.
 
  • #4
Physicists and physical chemists use opposite signs for the work done by the system. But that has to be accounted for when writing the first law as ##\Delta E = q + w## or ##\Delta E = q - w##.
 
  • #5
Smith said:
Is forcing a closed system to expand (e.g. by pulling out a piston), causing it to cool, work done to the system or work done by the system? I assume it was work done to the system, but that means the first law of thermodynamics formula no longer balances if you assume an adiabatic change. With adaibatic, Q = 0, so:
-w = + delta u.
-w is equal to work done to the system, but delta u should be negative, not positive as the formula would make it.

Any ideas what I'm doing wrong.
It is not clear from your scenario whether the expanding gas does any work. If it is the external force rather than the gas that does all the work on the surroundings in expanding the volume for the gas, the gas will not cool. The gas freely expands doing no work. By the first law if Q = 0 and W = 0, then ΔU = 0 which means ΔT = 0. In that case, the work is not done to the system by the external force. Rather the work is done on the surroundings.

AM
 

Related to Confusion regarding the First Law of Thermodynamics

1. What is the First Law of Thermodynamics?

The First Law of Thermodynamics, also known as the Law of Conservation of Energy, states that energy cannot be created or destroyed, only transferred or converted from one form to another.

2. What are the implications of the First Law of Thermodynamics?

The First Law of Thermodynamics has important implications for various fields of science, including physics, chemistry, and engineering. It allows us to understand and predict the behavior of energy in various systems and processes.

3. How does the First Law of Thermodynamics relate to heat and work?

The First Law of Thermodynamics states that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system. This means that energy can be transferred as heat or work, but the total amount of energy in the system remains constant.

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 tested and proven to hold true in countless experiments and observations.

5. How does the First Law of Thermodynamics differ from the Second Law?

The First Law of Thermodynamics deals with the conservation of energy, while the Second Law of Thermodynamics deals with the direction of energy flow and the concept of entropy. The First Law tells us that energy is conserved, while the Second Law tells us that energy tends to disperse and become less organized over time.

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