ENTHALPY and INTERNAL ENERGY in 12 Minutes!

Summary

This video explores thermodynamic processes involving constant volume and constant pressure heating, using examples like pressure cookers and rigid tanks. It explains how these processes relate to changes in internal energy and enthalpy, emphasizing the first law of thermodynamics. The video also introduces key thermodynamic properties such as specific internal energy (u) and enthalpy (h), and demonstrates their use in practical scenarios. A step-by-step example of a heat transfer process in a closed tank is presented, showing how to calculate the heat transferred and the effects of heating on a saturated liquid-vapor mixture.

Takeaways

• 😀 A constant volume process, such as heating in a pressure cooker, occurs in a rigid tank where the volume does not change, and heat causes the pressure and temperature to increase.
• 😀 In the case of heating at constant volume, there is no work done since the volume remains the same, and the heat added results in a change in internal energy.
• 😀 The relationship between heat transfer and internal energy change is key. The first law of thermodynamics in constant volume processes shows that the heat added is equal to the change in internal energy.
• 😀 Specific internal energy (u) is an intensive property, while total internal energy (U) is an extensive property, meaning it depends on the mass of the substance.
• 😀 In a constant pressure process (such as heating in a piston-cylinder system), the work done is not zero because the volume changes, and this requires the inclusion of both internal energy and work in the energy equation.
• 😀 Enthalpy (H) is introduced as the sum of internal energy (U) and pressure-volume (PV) work, and it reflects the capacity of a system to gain or release heat during non-mechanical work processes.
• 😀 The conservation of energy equation is applied to both constant volume and constant pressure processes to determine heat transfer, with the enthalpy formula used in constant pressure processes.
• 😀 The use of property tables, like specific internal energy and enthalpy values, is vital for solving thermodynamic problems involving phase changes and heating at constant volume or pressure.
• 😀 The key distinction between heating at constant volume and constant pressure lies in the role of enthalpy: at constant volume, energy is only in the form of internal energy, while at constant pressure, both internal energy and pressure-volume work contribute.
• 😀 In an example problem, a rigid tank containing a water-vapor mixture is heated, and the quality of the mixture changes, requiring the use of property tables to calculate temperature and heat transfer based on pressure and specific volume.
• 😀 When solving thermodynamic problems, it's important to use known values like quality and specific volume, as well as the saturation tables, to determine the changes in internal energy, temperature, and heat transfer.

Q & A

What is the focus of the video script?

The video focuses on heating at constant volume and compares it with heating at constant pressure. It also introduces the concepts of enthalpy and internal energy.

How does heating at constant volume differ from heating at constant pressure?

In heating at constant volume, the volume of the system remains fixed, and only the temperature and pressure change. In contrast, heating at constant pressure allows both temperature and volume to change while maintaining constant pressure.

What is an example of a constant volume process?

An example of a constant volume process is a pressure cooker, such as an Instant Pot, where the volume of the tank is rigid and does not change, but heat is added to change the temperature and pressure inside.

What is the relationship between heat transfer and internal energy in a constant volume process?

In a constant volume process, the heat transfer is equal to the change in internal energy since there is no work done (because the volume is constant).

What is the significance of the equation Q = mcΔT?

The equation Q = mcΔT is used to calculate the heat transferred when the temperature of a substance changes. It is derived from the principle that the change in internal energy is proportional to the heat added in the case of constant volume.

What is the definition of enthalpy?

Enthalpy is a thermodynamic property that combines internal energy and the product of pressure and volume. It is particularly useful in constant pressure processes and is defined as H = U + PV.

How is enthalpy related to heating at constant pressure?

Enthalpy is used to describe heating at constant pressure. The heat transfer in this process is equal to the change in enthalpy, which is the sum of the change in internal energy and the work done by the system (PV work).

What is the purpose of property tables in thermodynamics?

Property tables provide values for intensive properties such as specific internal energy (u), specific enthalpy (h), and specific entropy (s) at different temperatures and pressures. These tables are essential for solving thermodynamic problems involving fluids in various phases.

What is the role of quality in a saturated liquid-vapor mixture?

Quality (x) represents the proportion of the mass of the mixture that is in the vapor phase. It is used to calculate the specific internal energy and specific volume of a two-phase mixture, where the quality value helps determine the state of the mixture between saturated liquid and saturated vapor.

How is the heat transferred in the example with the rigid tank and two-phase mixture of water?

In the example with a rigid tank, heat is added to a two-phase mixture of water (liquid and vapor) at constant volume. As heat is added, the pressure and temperature increase, but the volume remains constant. The heat transfer is calculated using the change in specific internal energy (u2 - u1), based on the initial and final states of the mixture.