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# 物理代写|热力学代写Thermodynamics代考|Finding the state at each end of a path: The state postulate

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## 物理代写|热力学代写Thermodynamics代考|Finding the state at each end of a path: The state postulate

Robert Frost is famous for his poetic statement:
Two roads diverged in a wood, and $I-$
I took the one less traveled by,
And that has made all the difference.
This poem conjures up an image of starting down a path and arriving at a destination. You may be in one state of mind at the beginning of the road and arrive with a completely different state of mind at the end. This idea shows up in every process in thermodynamics.
A state defines the properties (such as temperature and pressure) of a system or substance at each endpoint of a thermodynamic process. It describes what condition the system is in before the process starts and the condition of the system when the process is finished. Every process has an initial state and a final state. Without a state, you don’t know where you’re starting from or where you’re ending up with a process. The number of properties you need to describe the state of a system is spelled out in the state postulate. The state postulate says: Two independent intensive properties are necessary to completely define the state of a simple compressible system.
A simple compressible system doesn’t involve kinetic or potential energy or energy from magnetic or electric fields. If these energy forms are involved in a process, then you need to specify properties related to those forms of energy in addition to the two properties required by the state postulate.
Properties are independent if one property can change while the other one is held constant. You see this in Figure 2-2, where each path allows one property to vary while the other one remains constant. Among the many properties in thermodynamics, temperature and specific volume are always independent of each other. Temperature and pressure are independent in most cases, the exception being when a phase change occurs in a constantpressure process. If the pressure is constant during a phase-change process, then the temperature is constant. Specific volume changes during a constantpressure phase change, so it can be the other independent property. For a phase change in a rigid container, the specific volume is constant and both the pressure and temperature vary, so either temperature or pressure can be used as an independent property.

## 物理代写|热力学代写Thermodynamics代考|Connecting processes to make a cycle

In any thermodynamic cycle, the fluid goes through several different processes and returns to its initial state at the end of a cycle. Thermodynamic cycles use processes that either involve work or move heat, and Figure 2-3 shows a cycle with four typical processes:
Work input
Heat input
Work output
$\sim$ Heat output

The processes in a thermodynamic cycle are connected to each other at the initial and final states of each process; that is, the final state of one process is the initial state of the following process. In a typical thermodynamic analysis of a cycle, you need to find the state of the system at the end of each process in the cycle.

In the modern steam engine, for example, water goes through these thermodynamic processes to complete an ideal cycle:

1. A compression process using a work input increases the water pressure.
2. A heat input process generates steam from liquid water.
3. An expansion process in a piston or turbine gets work out of the engine.
4. A condensation process gets heat out of the system and turns the steam to its liquid state.
After Step 4, the water is ready to go around the cycle again.
In Chapters 10-13, I cover the thermodynamic analysis of many different cycles for engines and refrigeration.

## 物理代写|热力学代写Thermodynamics代考|Connecting processes to make a cycle

$\sim$热量输出

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