The Third Law of Thermodynamics & Its Application to Absolute Entropy

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  • 0:00 What is the Third Law?
  • 1:45 Definition of Absolute Entropy
  • 2:24 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to explain what the third law of thermodynamics is, and how that relates to the concept of absolute entropy. A short quiz will follow.

What is the Third Law?

Thermodynamics is the study of the movement of heat. It covers everything from how heat transfers during melting and boiling, to what temperature means, to whether and how heat flows between cold and hot places. Thermodynamics has various laws, and today we're going to talk specifically about the third law of thermodynamics.

The third law of thermodynamics says that the entropy of a perfect crystal at absolute zero is exactly equal to zero.

But hold on a minute. What exactly is entropy? Well, entropy is a measure of disorder in the universe. An object or substance with high entropy is highly disordered. When you put things in order, such as, putting your child's toys away in a box, you're decreasing entropy. Or when you look at the result of a farmer's market at the end of the day, that's a lot of entropy. Unfortunately, you're also producing entropy through the heat in your muscles. In fact, entropy in the universe can only increase. It can never decrease. A solid is more orderly than a liquid, because a solid contains molecules in nice, neat rows.

Going back to the third law: it says that entropy at absolute zero is zero. Or in other words, things are most orderly when they're really cold. Absolute zero is the temperature at which molecules stop moving or vibrating at all. Sounds pretty orderly to me! So the third law of thermodynamics makes a lot of sense. This complete stop in molecular motion happens at -273 Celsius, which is defined as 0 kelvin, or absolute zero.

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