# Why Is Entropy Considered a State Variable?

In summary, Brad, a graduate electrical engineering student, went back to his textbook Physics for Scientists and Engineers with Modern Physics to review fundamentals of physics. He specifically wanted to understand more about entropy and its relation to thermodynamics. He found that entropy is a state variable and can be calculated using a reversible path from one state to another. However, he is still unsure about why it is a state variable and how it applies to irreversible processes. He asks for clarification on this topic. Another user responds by explaining that while the entropy of the total universe may not be reversible, the entropy of a particular system can be reversed in a controlled manner, resulting in an increase of entropy in the universe.
Hi,

My name is Brad and I am a graduate electrical engineering student. Recently I wanted to go back and review and patch some holes in my head about fundamentals of physics. Therefore, I went back to my textbook Physics for Scientists and Engineers with Modern Physics by Serwey and Jewett, 7th ed. I wanted to understand a few questions I had about thermodynamics from years ago, and more specifically, about entropy.

In the text, it states that entropy is a state variable; therefore, we can find some path of reversible process that leads from one state to the next to calculate it. It never stated why it is a state variable or why the reversible path will work for irreversible or real processes. I can certainly calculate it. However, my understanding about this particular section is frustratingly vague. Please enlighten me Q_Q... Thank you.

i guess entropy of the total universe if not reversible, but entropy of a particular system is reversible.

so if you made your desk messy, you can make it tidy again in a completely reversible fashion, but in the process introducing more entropy into the universe.

## 1. What is entropy and why is it important in science?

Entropy is a measure of the disorder or randomness in a system. It is important in science because it helps us understand the direction and efficiency of physical and chemical processes, and is a fundamental concept in thermodynamics.

## 2. How does entropy relate to the second law of thermodynamics?

The second law of thermodynamics states that the total entropy of a closed system will always increase over time. This means that systems tend to move towards a more disordered state, and that it takes energy to maintain order and organization. Entropy is a way to quantify this tendency towards disorder.

## 3. Can entropy be reversed or decreased?

In a closed system, the total entropy will always increase. However, it is possible for localized decreases in entropy to occur. For example, when a living organism creates order and structure within itself, it is decreasing its own entropy. However, this decrease in entropy is always accompanied by an overall increase in the entropy of the surrounding environment.

## 4. How does entropy relate to information theory?

In information theory, entropy is a measure of the uncertainty or randomness in a system. It is used to quantify the amount of information contained in a message or signal. The concept of entropy in information theory is closely related to the concept of entropy in thermodynamics.

## 5. How can we apply our understanding of entropy in everyday life?

Our understanding of entropy can help us make informed decisions about energy usage and resource management. For example, we can minimize waste and increase efficiency by designing systems that reduce the amount of energy lost to entropy. Additionally, understanding entropy can help us understand the natural processes and cycles that shape our world.

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