Definition of first law of thermodynamic?

In summary, the first law of thermodynamics states that the change in internal energy of a closed system is equal to the heat that enters the system plus the work done on the system. However, it is convention to write the equation as Q = ΔU + W, with W representing work done by the system. This can lead to confusion when using different textbooks, but as long as all internal energy changes, heat flow, and work done are accounted for, the law applies to both closed and open systems. It is important to note that heat flow and work done can be negative, and this may have been the reason for the confusion and incorrect marking mentioned in the conversation.
  • #1
mugen715
9
0

Homework Statement



What is the definition for first law of thermodynamic? I was confused and which one is correct?

(a) The CHANGE in internal energy of a closed system is equal to the heat that enters a system and the workdone on the system

OR

(b) The INCREASE in internal energy of a closed system is equal to the heat that enters a system and the workdone on the system

Homework Equations



ΔU = ΔQ + ΔW

The Attempt at a Solution



In the exam, I use definition (a), but my teacher marked it wrong. But when i referred to all my textbooks, both definition can be used. So could anyone out there figure out which is correct?
 
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  • #2
mugen715 said:

Homework Statement



What is the definition for first law of thermodynamic? I was confused and which one is correct?

(a) The CHANGE in internal energy of a closed system is equal to the heat that enters a system and the workdone on the system

OR

(b) The INCREASE in internal energy of a closed system is equal to the heat that enters a system and the workdone on the system

Homework Equations



ΔU = ΔQ + ΔW

The Attempt at a Solution



In the exam, I use definition (a), but my teacher marked it wrong. But when i referred to all my textbooks, both definition can be used. So could anyone out there figure out which is correct?
The common convention is: Q = ΔU + W. This means the heat flow into a system is equal to the change in internal energy of the system + the Work done BY the system.

If one uses Q = ΔU - W, the W is the work done ON the system.

Since Q and W are not state variables, there can be no change in Q or W.

P.S. The reason for your mark may be that you did not make it clear that heat flow into the system and work done BY the system can be negative. If you made that clear (and if you were using b. and stated that the increase in U can be negative) it would be correct.

Re: closed system: so long as you account for all the internal energy changes, heat flow and work done, the system does not have to be closed. A steam engine is a closed system but an internal combustion engine is an open system. The first law applies to both.

AM
 
Last edited:

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. How does the first law of thermodynamics relate to heat and work?

The first law of thermodynamics is a general energy conservation law, meaning it applies to all forms of energy including heat and work. It states that the total energy of a closed system remains constant, which means any energy gained or lost by the system must be in the form of heat or work.

3. Can you give an example of the first law of thermodynamics in action?

One example of the first law of thermodynamics in action is a car engine. The chemical energy in the gasoline is converted into heat energy, which is then converted into mechanical work to move the car. Despite the various conversions, the total energy of the system remains constant.

4. What is the significance of the first law of thermodynamics?

The first law of thermodynamics is significant because it is a fundamental principle that governs the behavior of energy in all physical systems. It helps us understand how energy is transferred and transformed, and allows us to make predictions about the behavior of complex systems.

5. How does the first law of thermodynamics apply to living organisms?

The first law of thermodynamics applies to living organisms just like any other physical system. It explains how organisms obtain and use energy for their cellular processes, and how energy is conserved within the body. It also helps us understand the efficiency of biological processes and how they contribute to the overall energy balance of the planet.

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