Thermodynamics concept question

In summary: Therefore, both properties can be used to calculate the heat transfer, but in this case, using the change in specific internal energy is more convenient.
  • #1
Rumpelstiltzkin
17
0
Here's the question:
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A 0.5m3 rigid tank contains refrigerant-134a initially at 200kPa and 40% quality. Heat is now transferred to the refrigerant until the pressure reaches 800kPa. Determine (a) the mass of the refrigerant in the tank and (b) the amount of heat transferred. Also, show the process on a P-υ diagram with respect to saturation lines.
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The solution for this question was already released, and I know the working to do it, but I can't understand the concept for part (b). In the solution, the amount of heat transfer was given as Q = (mass) * (change in specific internal energy, u).

I'm confused: Why can't we use change in enthalpy, h? Doesn't the change in enthalpy also mean heat transfer? I know it has something to do with the volume being constant ("rigid tank"), but I don't see how exactly.

Would someone care to explain what's the difference between these two properties?

Thanks.
 
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  • #2
In this case, the volume is constant, so the change in enthalpy (h) is equal to the change in specific internal energy (u). The heat transfer can be calculated by multiplying the mass of the refrigerant with the change in internal energy. This is because the heat transfer is equal to the amount of energy that is being transferred between the system and its surroundings. Since the volume is constant, the change in enthalpy is equal to the change in internal energy.
 
  • #3


The change in enthalpy, h, does indeed represent heat transfer in a constant pressure process. However, in this problem, the tank is rigid, meaning the volume remains constant. Therefore, the pressure and temperature will increase as heat is added, but the volume will not change. This means that the specific internal energy, u, is a more appropriate property to use in this case, as it takes into account the constant volume of the system. The change in specific internal energy, u, is equal to the change in enthalpy, h, minus the change in the product of pressure and specific volume, PΔv. This is known as the work done on the system, and in this case, it is equal to zero since the volume is constant. Therefore, the amount of heat transferred can be calculated using the change in specific internal energy, u, as shown in the solution. I hope this helps clarify the difference between these two properties in this specific scenario. Additionally, on a P-υ diagram, the process would be shown as a vertical line since the volume is constant, and it would intersect the saturation line at the final pressure and quality state.
 

Related to Thermodynamics concept question

1. What is thermodynamics and why is it important?

Thermodynamics is a branch of physics that studies the relationship between heat, temperature, energy, and work. It is important because it helps us understand and predict the behavior of physical systems, such as chemical reactions and engines.

2. What are the laws of thermodynamics?

The laws of thermodynamics are fundamental principles that govern the behavior of energy in a thermodynamic system. The first law states that energy cannot be created or destroyed, only transferred or converted. The second law states that the total entropy of a closed system will always increase over time. The third law states that the entropy of a perfect crystal at absolute zero temperature is zero.

3. What is the difference between heat and temperature?

Heat is a form of energy that is transferred between objects due to a temperature difference. Temperature, on the other hand, is a measure of the average kinetic energy of the particles in a substance. Heat and temperature are related, but they are not the same thing.

4. How does thermodynamics impact everyday life?

Thermodynamics has a significant impact on many aspects of our daily lives. For example, it helps us understand how our bodies regulate temperature, how refrigerators and air conditioners work, and how energy is produced and used in various industries. It also plays a crucial role in fields such as chemistry, engineering, and environmental science.

5. Can the laws of thermodynamics be violated?

The laws of thermodynamics are considered to be universal and have not been found to be violated under normal circumstances. However, there are some instances where they may appear to be violated, such as in systems with extremely high or low temperatures. These cases require a deeper understanding of the laws and their limitations.

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