Posted on Categories:Thermodynamics, 热力学, 物理代写

# 物理代写|热力学代写Thermodynamics代考|Finding the coefficient of performance

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## 物理代写|热力学代写Thermodynamics代考|Finding the coefficient of performance

In a cycle, the working fluid returns to its starting condition after the heat rejection process is complete. The heat transfer and work processes in a refrigeration cycle are repeated continuously to move heat from the lowtemperature reservoir to the high-temperature one.
The performance of a refrigerator isn’t measured by efficiency like a heat engine. Instead, it’s determined by the coefficient of performance (COP). The coefficient of performance is a measure of how much heat is transferred by the amount of work put into the refrigerator. In general, any measure of performance is (what you want $) /($ what you provide). For a refrigerator, what you want is to transfer heat. What you provide is work. The coefficient of performance is calculated much like the efficiency. The coefficient of performance is expressed as follows:
$$C O P=\frac{\text { Desired output }}{\text { Required input }}$$
For a refrigerator, the desired output is the amount of heat absorbed from the low-temperature reservoir $\left(Q_1\right)$. The heat removed from the reservoir equals the heat input to the refrigerator $\left(Q_i\right)$. The work into a refrigerator $\left(W_{\text {in }}\right)$ is equal to the difference between the heat output $\left(Q_{\infty}\right)$ and the heat absorbed $\left(Q_{\mathrm{n}}\right)$ by the refrigerator. That is, $W_{\mathrm{in}}=Q_{\text {out }}-Q_{\mathrm{in} \text {. }}$. The coefficient of performance for a refrigerator is calculated using this equation:
$$C O P_{\mathrm{R}}=\frac{Q_{\mathrm{in}}}{W_{\text {net.in }}}=\frac{Q_L}{W_{\text {netin }}}=\frac{Q_L}{Q_H-Q_L}$$
For a heat pump, the desired output is the amount of heat rejected to the warm energy reservoir $\left(Q_\mu\right)$, the interior of a house. The heat added to the reservoir equals the heat output of the refrigerator $\left(Q_{0 u}\right)$. You calculate the coefficient of performance for a heat pump by using this equation:
$$C O P_{\mathrm{HP}}=\frac{Q_{\text {att }}}{W_{\text {ret, in }}}=\frac{Q_H}{W_{\text {ret, in }}}=\frac{Q_H}{Q_H-Q_L}$$
You can find the coefficient of performance for a refrigerator or a heat pump by working out the following example. Suppose you have a refrigerator that absorbs 1 kilowatt of heat from the cold reservoir and rejects 1.3 kilowatts of heat to the warm reservoir. You can find the actual coefficient of performance for the refrigerator with the following equation:
$$C O P_{\mathrm{R}}=\frac{1 \mathrm{~kW}}{(1.3-1) \mathrm{kW}}=3.3$$

## 物理代写|热力学代写Thermodynamics代考|What Is Entropy?

Remember when you first got a new desk? You arranged all your papers and knick-knacks on it. It looked nice and neat. But if you’re like most people, things started to pile up on your desk and before you knew it, books, candy wrappers, sticky notes, and empty coffee cups took over. Yes, one aspect of entropy is at work here. Things that start out neat and tidy naturally become disordered. You can picture the universe this way. In the beginning, it was much smaller than it is today; its energy was concentrated into a very small space. But as the universe ages and expands, it becomes more disordered. Making something ordered again takes effort; you have to do some work.
Entropy has many different interpretations. Its definition depends on who you’re talking to. In principle, entropy is used by physicists, theologians, engineers, philosophers, information specialists, and economists, among other professionals. Entropy is often thought of as a measure of disorder of a system. But how can you quantify order or disorder? Entropy is a thermodynamic property of a substance that needs to be quantified in order to be useful.
In thermodynamics, you find microscopic and macroscopic perspectives on entropy.
Taking a microscopic view of entropy
On a microscopic level, entropy starts with the third law of thermodynamics, which I discuss in Chapter 2. At absolute zero temperature, the molecules in a substance have no energy to move, vibrate, or rotate. The entropy of the material is zero. As energy is added to a material, the entropy of the molecules increases because they become more energetic and more disorganized – the way your desk gets more cluttered the more you use it.

The entropy of a material increases as its temperature increases. Solid materials have less entropy than liquids, and liquids have less entropy than gases. As molecules in a material increase in temperature, they like to spread out and take up more room; that is, they become more disordered.

Pressure has the opposite effect on entropy of a material. As the pressure of a gas, liquid, or solid increases, the entropy decreases. However, liquids and solids are considered nearly incompressible, so the entropy decrease is minimal. Pressure forces molecules closer together; they become more ordered, so entropy decreases.

Scan through the thermodynamic property tables in the appendix to see how entropy increases with temperature and decreases with pressure.

## 物理代写|热力学代写Thermodynamics代考|Finding the coefficient of performance

$$C O P=\frac{\text { Desired output }}{\text { Required input }}$$

$$C O P_{\mathrm{R}}=\frac{Q_{\mathrm{in}}}{W_{\text {net.in }}}=\frac{Q_L}{W_{\text {netin }}}=\frac{Q_L}{Q_H-Q_L}$$

$$C O P_{\mathrm{HP}}=\frac{Q_{\text {att }}}{W_{\text {ret, in }}}=\frac{Q_H}{W_{\text {ret, in }}}=\frac{Q_H}{Q_H-Q_L}$$

$$C O P_{\mathrm{R}}=\frac{1 \mathrm{~kW}}{(1.3-1) \mathrm{kW}}=3.3$$

## MATLAB代写

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