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


Conservation of Energy

Many of the most important and powerful scientific laws are conservation laws - the amount of some physical quantity or variable is found to remain a constant; it cannot change. Energy is one of the physical quantities that is conserved in our universe. This can be stated as follows: Energy cannot be created nor destroyed. The one exception is in nuclear reactions where mass is turned into energy, and as we have seen, this occurs only in the center of stars and in nuclear reactors.


Let's look at an example of an electric light bulb. A light bulb changes electrical energy into light energy. If we were to measure these energies for one second, we might get energy numbers that look something like this:
 
Electrical
Energy In
100 J (joules)
right arrow graphic
Light
Energy Out
70 J (joules)

This would suggest that energy is not conserved? Where did the other 30 J of energy go?
electric light bulb graphic

Remember that energy can change into more than one form simultaneously. And if you feel a light bulb it is very hot. The "missing" energy must have gone into heat energy. So, the actual energies were more like this:
Electrical
Energy In
100 J (joules)
right arrow graphic
Light
Energy Out
70 J (joules)
+
Heat Energy Out
30 J (joules)

This propensity for energy to change into more than one type of energy is extremely common. And the most common energy for this "missing" energy to go to is heat energy. Since our goal for the light bulb is for all of the electrical energy to go into light energy, the heat energy is really "lost" energy. We can rate our light bulb by measuring its efficiency, or percentage of energy that goes where we want it to go. The equation for efficiency is:

eff=energy out / energy in x 100
So for our light bulb above, the efficiency would be 70% as shown below:
eff=70J/100J x 100=70%
Even though we may "lose" energy in the form of heat, the total energy is still the same. Energy is conserved. So, if all energy is conserved, we can never run out of energy! Right? As we will see, our lives and the lives of energy are not that simple.


The animation below requires the Macromedia flash player. If you don't see the animation, you can download and install the flash player from the following site: Download Flash Player

In the animation below of a spring bouncing up and down, energy is constantly changing types. The three energies involved are: spring, or elastic potential energy (EPE); gravitational potential energy (or GPE), and kinetic energy (KE). The energies are all changing in differnet ways depending on how much the spring is stretched and how high the object attached to the spring is. Note, however, that energy is conserved, the total energy stays the same (the bar on the right).


Created by Tom Chandler, OME educator, 2001-02


For more information on the law of conservation of energy and other conservation laws, check out the websites below.

Basics of Life Law of Conservation of Energy.
Conservation Laws Some diagrams of conservation laws and info on isolated systems.