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 something
like this:
Electrical
Energy In
100 J (joules)
Light
Energy Out
70 J (joules)
This
would suggest that energy is not conserved? Where did
the other 30 J of energy go?
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)
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:
So for
our light bulb above, the efficiency would be 70% as shown below:
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
For more information
on the law of conservation of energy and other conservation laws,
check out the web sites below.
