This section is created as a project which covers the topic 'The Second Law of Thermodynamics'
within the scope of PHYS 201: Waves, Fluids and Thermodynamics course.

Introduction



In nature, many of the events take place in only one direction but not the other. For example, a hot cup of coffee that you leave in a room cools instead of increasing its temperature. Or, the air inside a flat tire goes out rather than stay in it. Furthermore, it is possible to convert mechanical energy totally into heat, but not the reverse. Have you ever wondered why these phenomena, which appear naturally to our eyes based on our daily experience, cannot happen in the opposite direction even though they do not violate the law of conservation of energy?

In order to answer these and such questions, we need to know The Second Law of Thermodynamics and entropy, which examines the directions in which thermodynamic events take place. The second law and entropy are very good tools to explain many natural phenomena, such as why it is easy to mix milk and coffee, but it is difficult to separate mixed milk coffee, or why we can only make predictions about the future while we can remember the past and build memories we can trust.

Now, let's look at the mixing milk and coffee example below. In both cases the time arrow from left to right.

Seems normal.

In the first case everything seems normal. Milk is slowly mixing into the coffee.

Does not seem normal at all.

However, in the second case something seems strange. The coffee and the milk turn to their unmixed state themselves. We all know that the second case is impossible and the impossibility can be explained by using the concept entropy.


Understanding the second law of thermodynamics and entropy, both theoretically and mathematically, and combining this new knowledge with the old ones will give you a deep perspective. Thus, I tried to make my site as simple and understandable as possible. I hope it helps you.


Contents


Directions of Thermodynamic Processes

Heat Engines

The Second Law of Thermodynamics

Entropy

Videos and Readings

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