Can internal energy be calculated from equation of state?

In summary, the conversation discusses the calculation of ##\int PdV## using the equation of state and the expression for entropy as a function of temperature, volume, and pressure. It is mentioned that Maxwell's relations can be used to simplify the calculation, but the heat capacity is also necessary for a complete calculation.
  • #1
arpon
235
16
We know,
$$dU=TdS-PdV$$
##\int PdV## can be calculated if the equation of state is given.
I tried to express ##S## as a function of ##P ,V## or ##T## (any two of those).
$$dS=\left(\frac{\partial S}{\partial V}\right)_T dV+\left(\frac{\partial S}{\partial T}\right)_V dT$$
$$=\left(\frac{\partial P}{\partial T}\right)_V dV+\left(\frac{\partial S}{\partial P}\right)_V \left(\frac{\partial P}{\partial T}\right)_V dT~~~ [Using ~~Maxwell's~~ relation]$$
$$=\left(\frac{\partial P}{\partial T}\right)_V dV-\left(\frac{\partial V}{\partial T}\right)_S \left(\frac{\partial P}{\partial T}\right)_V dT~~~[Using ~~Maxwell's~ ~relation]$$
All the terms except ##\left(\frac{\partial V}{\partial T}\right)_S## can be calculated using the equation of state.
Any suggestion will be appreciated.
 
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  • #2
You can't do it solely in terms of the equation of state. You need to use the heat capacity as well.
 
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  • #3
Chestermiller said:
You can't do it solely in terms of the equation of state. You need to use the heat capacity as well.
Thanks. That's exactly what I wanted to know.
 

1. What is the equation of state?

The equation of state is a mathematical equation that describes the relationship between the physical properties of a substance, such as pressure, volume, and temperature. It is commonly used in thermodynamics to calculate the behavior of gases, liquids, and solids.

2. How is internal energy related to the equation of state?

Internal energy is one of the properties that is described by the equation of state. It is the sum of all the microscopic energy of a system, including the kinetic and potential energy of its particles. The equation of state can be used to calculate the internal energy of a substance at a given state.

3. Can internal energy be directly measured?

No, internal energy cannot be directly measured. It is a microscopic property and cannot be observed or measured directly. However, it can be calculated using the equation of state and other thermodynamic equations.

4. What are the limitations of using the equation of state to calculate internal energy?

The equation of state assumes that the substance being studied is in a state of thermodynamic equilibrium. It also assumes that the substance is homogeneous and has a constant composition. These assumptions may not be applicable in some real-world situations, leading to limitations in the accuracy of the calculated internal energy.

5. Can the equation of state be used for all substances?

No, the equation of state is specific to each substance and cannot be used for all substances. Different substances have different equations of state that are based on their individual physical properties. For example, the ideal gas law is an equation of state that is specific to ideal gases, while the van der Waals equation of state is used for real gases.

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