Second Law of Thermodynamics: Entropy and Systems

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  • 0:07 Second Law of Thermodynamics
  • 3:44 Application of the Second Law
  • 5:38 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

In this lesson, we'll learn about the second law of thermodynamics. By studying this law, you'll understand how energy works and why we're all moving towards eventual chaos and disorder.

Second Law of Thermodynamics: Law of Entropy

Do you ever wonder how the universe came into existence? The Big Bang theory tell us that the universe began as an infinite amount of energy exploded and began to spread into the universe as we know it today. Alright, that's great, but what's energy? Energy is simply the ability to do work, and work is the movement of something by some force.

At the beginning of time, all the energy in the universe was contained in a single relatively small location. This intense concentration of energy represented a massive amount of what we call potential energy, where potential energy is stored energy due to its location or position, and it is now equal to the total amount of energy in the universe today. As time goes on, the energy has spread over the vast expanse of our universe.

On a much smaller scale, a reservoir of water held back by a dam contains potential energy, as its location gives it the potential to flow over the dam. In each case, the stored energy, once released, spreads out and does so without any applied effort or force. In other words, the release of potential energy is a spontaneous process. A spontaneous process is simply a process that occurs without the need for additional energy. Another way of putting that is that it happens automatically once you give it a little bit of a kick. As the energy spreads out, some of it is converted into usable energy and gets the work done that we need. The rest of the energy is converted into unusable energy, simply referred to as heat.

The potential energy stored in the universe or in a dam spreads spontaneously once released.
Stored Energy

As our universe continues to spread out, it contains less and less useful energy. As less useful energy is available, less work can get done. As water flows over a dam, it contains less useful energy as well. This decrease in useful energy over time is referred to as entropy, where entropy is the amount of unusable energy in a system, and a system is simply a collection of objects that make up a whole.

Entropy can also be referred to as the amount of randomness or chaos in a system - less organization. As usable energy decreases over time, disorganization and chaos increase. Thus, as stored potential energy is released, not all of it is converted into usable energy. All systems experience this increase in entropy over time. This is very important to understand, and this phenomenon is referred to as the second law of thermodynamics.

As you may have guessed, the second law of thermodynamics follows the first law of thermodynamics, which is commonly referred to as the law of energy conservation, and it states that energy can't be created and it can't be destroyed. In other words, the amount of energy in the universe, or any system, is constant. The second law of thermodynamics is commonly referred to as the law of entropy, and it holds that energy becomes less usable over time. Therefore, while the quantity, based on the first law, of energy remains the same, the quality of energy decreases over time, based on the second law.

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