Which one of the following expression is true for Tds equal toa)dh - V...
Tds = du+ PdV— First Tds equation .
Tds - dh- VdP— Second Tds eauation
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Which one of the following expression is true for Tds equal toa)dh - V...
Understanding Tds and Thermodynamic Relationships
In thermodynamics, Tds (where T is temperature and ds is the change in entropy) is a fundamental expression that relates various state functions. To understand why option 'A' (Tds = dh - VdP) is correct, we need to explore the relationships between enthalpy (h), pressure (P), and volume (V).
Key Thermodynamic Definitions
- Enthalpy (h): Defined as h = u + Pv, where u is internal energy and Pv is the pressure-volume work.
- Entropy Change (ds): This represents the change in the disorder or randomness of a system.
Using the First Law of Thermodynamics
- The first law connects changes in internal energy (du), heat added (Q), and work done (W):
du = dQ - dW.
- For a reversible process, the heat transfer can be expressed as Tds.
Derivation of Tds = dh - VdP
1. Starting Point: Begin with the first law in differential form.
2. Enthalpy Differential: The differential of enthalpy can be expressed as:
dh = du + PdV + VdP.
3. Substituting for du: Utilizing the first law,
du = Tds + PdV, we can substitute this into the enthalpy equation.
4. Rearranging Terms: After substituting and simplifying, we arrive at:
Tds = dh - VdP.
Conclusion
This expression succinctly captures the relationship between changes in entropy, enthalpy, and pressure in a thermodynamic system. Thus, option 'A' is indeed the correct expression, demonstrating a fundamental concept in mechanical engineering thermodynamics.