4.1

## Hanyang University

Skip to 0 minutes and 16 secondsThis week in thermodynamics in energy engineering, we are still talking about the 2nd law thermodynamics, but we are going be looking at its application to real problems. At the very first part, we are going to be looking at the 3rd law of thermodynamics, which is essential to solve problems related to the entropy changes. The 1st law was the energy principle, and the 2nd law was the entropy principle. Here, the 3rd law is the temperature principle. The 3rd law is talking about the entropy of pure, perfectly crystalline materials at zero K. They have zero entropy by the 3rd law. Therefore, in classical thermodynamics, thermal motions entirely stop in particles at zero K.

Skip to 0 minutes and 59 secondsHowever, in quantum mechanics, every motion is governed by uncertainty according to the Heisenberg uncertainty principle. So we cannot ever measure the particles not having motion. Thus, absolute zero is never attainable Then based on the properties of entropy and the 3rd law, we are going to solve entropy changes in reversible and irreversible processes. For examples, we are going to be looking at entropy changes including phase transitions and freezing of water at zero degree and -10 degree Celsius. The freezing of water at -10 degree Celsius represent the irreversible process with positive entropy changes.

# The week ahead

Welcome to the 4th week of the course.During this week, we are still talking about the second law of thermodynamics, but we are going be looking at its application to real problems.

To calculate entropy, the third law of thermodynamics is essential. The first law is the energy principle, the second law is the entropy principle. Now, the third law is the temperature principle. The third law leads to a conclusion that absolute zero temperature is never attainable. Based on the properties of entropy and the third law, we are going to solve entropy changes in reversible and irreversible processes.