Introduction

The Energy Story

  Energy Is Born
  Energy Types
  Energy Changes
  Energy Generation

The Energy Problem

  Conservation of Energy
  Aging of Energy
  Finite Resources
  The Oil "Crisis"
  Energy Pollution
  Discussion Topics

The Energy Solution

  Conserving Electricity
  Appliance Efficiency
  Heating Conservation
  Renewable Energy

Web Links

Teacher Guide

About the Author


Secret Lives Title - The Energy Problem


Aging of Energy

So, energy is conserved, and is neither created nor destroyed. The study of thermal or heat energy is called thermodynamics, and the law of conservation of energy is known as the first law of thermodynamics. The first law indicates that the amount of energy in the universe is the same as it was on e day ago, one week ago, or millions of years ago. Unfortunately, there is also what is known as the second law of thermodynamics, which deals with a physical quantity known as entropy. The second law indicates that while the amount of energy stays the same, the usefulness of that energy is not the same. Energy is essentially aging as it changes from one type to another.



Entropy
The second law of thermodynamics states: Natural processes go in a direction that increases the total entropy of the universe. Entropy is a measure of randomness. The more ordered a system is, the less the entropy. The more random, scattered, and disorganized a system is, the greater the entropy. Let's look at a simple example.

hot and cold boxes graphic


Above we see two boxes. The box on the left contains molecules at a high temperature. These molecules have a high velocity, a lot of KE, and therefore a lot of heat energy. The box on the right contains molecules at a low temperature. These molecules have a low velocity, low KE, and therefore little heat energy.

These two boxes illustrate a condition of low entropy because they are very ordered. Most of the "hot" molecules are on the left and most of the "cold" molecules are on the right. These are not random. Remember, the more randomness, the higher the entropy.

Because the entropy is low, this energy is in a very useful state. For example, we could use the "hot" molecules to heat a room, since the temperature of 120 degrees F is well above room temperature of around 70 degrees F. One of the results of the second law of thermodynamics is that heat energy always moves from the hot temperature to the cold temperature. Likewise, we could use the "cold" molecules to cool a room, since the temperature of 20 degrees F is well below 70 degrees F.

What happens after we use our energy to heat or cool our room? It will be much the same as mixing the two boxes above. When we are done, all of the molecules will have an average temperature of around 70 degrees F as shown below.

mixed hot and cold graphic

Now the randomness has increased, since the molecules are no longer separated into "hot" and "cold." The entropy has therefore also increased. Not only that, but the usefulness of the heat energy has decreased as well. The molecules at 70 degrees F would not heat a room as well as the molecules at 120 degrees F.

So, the second law of thermodynamics basically says that the universe MUST run downhill. The net result of any energy changes must be an increase of entropy and the resulting energy will be less useful and more difficult to use than before. As energy changes form, as it does almost constantly, it ages. This means that any energy resource must be finite, and must run out if used often enough over a long enough period of time.



For more information on the concept of entropy, check out the websites below.

Entropy Text based info on the laws of thermodynamics and entropy.

The Page of Entropy

For some wacky examples of the results of entropy try The answer is entropy on this page.

Thermodynamic Equilibrium A simulation showing how the temperatures change when a door is opened between two chambers.
Qualitative Statements Statements that come from the 2nd law of thermodynamics.
Entropy and the Universe A number of pages on entropy.
Entropy A simulation illustrating how entropy changes with particles in a box.