In any discussion of energy we will encounter scientific terms which should be understood by the student.
The scientific definition of
energy
is “the ability to do work” or “to transfer heat”.
Work
, in the scientific sense, is only accomplished when an object is moved over a distance. The amount of energy required depends upon the mass of the object and the distance it is moved. For example, it would require twice as much energy to move 10 kilograms for a distance of one meter, as to move 5 kilograms the same distance. It would also be true that the same amount of work would be done in moving 5 kilograms over 10 meters, as in moving 10 kilograms over 5 meters. The push or pull required to move the object is called a
force
.
Whenever work is done, energy is from one form to another. If I lift a baseball over my head, I use chemical energy in my body to lift the ball.
Chemical energy
is energy stored in chemical bonds. The energy is transferred from my body to the ball and is stored in the ball as
potential energ
y. Potential energy is stored energy. If I drop the ball, that stored energy is changed to
kinetic energy
(the energy of action) as the ball falls back down. Several examples, such as rolling a rock up a hill, can be used to demonstrate this concept to the students.
When energy is transformed in the form of heat, changes in composition or state will occur. If we transfer the potential energy stored in wood to heat by burning the wood, the wood is transformed into gases and ashes and the temperature of the surrounding air will rise. If we add heat to ice, it becomes a liquid.
Energy is measured in units called
calories
. A calorie is the amount of heat required to raise the temperature of 1 gram of water by 1 degree Celsius. This calorie of heat supplies the energy to do 4
joules
of work. One joule of work is used to move a 1 kilogram mass for a distance of 1 meter.
The term
watt
refers to doing 1 joule of work
in
each second. A 100 watt light bulb uses the energy required to do 100 joules per second.
When
we buy electric power to do our work, we buy it in units called kilowatt hours. A kilowatt hour is 1000 watts used for a period of one hour. The term horsepower, used to describe motors, is equal to 746 watts. It originated from noting about how much work a horse could accomplish
on a
treadmill.
As in all other aspects of energy follows certain natural laws. The energy laws The first law of thermodynamics states that in any ordinary physical or chemical process, energy is nor destroyed, but merely changed from one form to another—the energy lost by one system must be gained by another. The second law of thermodynamics, known as the “law of energy degradation. states that in any energy transformation, some energy is transformed into low quality heat energy which is lost to the environment. An excellent example is the automobile engine. In its operation, 20% of the energy contained in the gasoline is used to move the car, 80% is lost in heat. This great loss is typical of the efficiency of an energy transformation. 10% efficiency is very common.
The same type of loss occurs as energy moves through biological systems. The energy is transformed by photosynthesis into potential chemical energy stored in food. In the case of corn, for example, if we eat the corn ourselves, we lose some energy, but we make the most efficient use of it. However, it we eat a cow that has eaten the corn, the energy has been transformed twice and each time 80 or 90 percent of the energy content is lost. Transforming the corn into gasohol would not be as efficient a use as eating the corn directly.
It should become obvious to the student that the more directly a source of energy is used the more efficiently it is used, and the smaller the heat loss. This should be remembered when we compare the efficiency of a water wheel to generating electricity with water and then transferring the energy over long distances for the sake of comenience, or of heating our homes with coal as compared to heating with electricity generated
by burning
coal.
