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Energy Rules!
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Section C. Energy Conversion and the Laws of Thermodynamics
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First Law of Thermodynamics
The
amount of energy that is present before and after work is the same
(scientist say energy is conserved). For example, let's say you
drop a ball. Scientists can measure the energy before, during, and
after the fall. The amount of energy remains constant throughout the
process. Likewise when an object is thrown or a spring released or
something is burned, the energy can be measured. This is the reason
behind the first law of thermodynamics, Energy can neither be created nor destroyed,
it can only be converted from one form to another; scientists
have found that the amount of energy in a closed system remains
constant.
The following set of statements are various ways of expressing the first law of thermodynamics:
* Energy is conserved.
* The amount of energy in the universe is constant.
* Energy can be neither created nor destroyed.
* There is no free lunch.
* It is impossible to build a machine that produces more energy than it uses (This type of machine is called a perpetual motion machine of the first kind.)
| As a classroom teacher, you might want to know of an experiment that demonstrates the first law of thermodynamics to students. While such demonstrations exist, they are too advanced for the common classroom. Sometimes it helps to think of an analogy. Imagine you have a closed room that contains a jigsaw puzzle. |
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| The room represents the universe and the completed jigsaw puzzle is concentrated source of energy (or an energy resource). Let's say an agitated and angry cat is in the room and this cat jumps on the puzzle, scattering the pieces all over the room, in the corners, under couches, in drawers, etc. It might look like the puzzle pieces are gone or missing, but with careful searching all the pieces can be found. | |
Likewise, energy might appear to run out or get lost (e.g., when you run out of gas) but with careful examination it is found to still exist. It might be in different forms, such as heat or sound waves, but it is not lost.
This information pertains to how energy is not destroyed, but what about energy not being created? We often talk about energy supply, development, production, and generation. All these involve converting one form of energy into another, rather than energy being literally created. On Earth, the incoming energy from the sun is in relative balance to the amount of heat loss from our planet. This holds true for the universe as a whole. In other words, over the years the amount of energy in our universe has remained constant. While energy is constant, its form and quality are continually changing. This leads us to the second law of thermodynamics.
Second Law of Thermodynamics
During energy transfers, it might seem that energy does go away or become reduced. For example, a bouncing ball stops bouncing, a battery dies, or a car runs out of fuel. The energy still exists but it has become so spread out that it is essentially unavailable. Burning a piece of wood releases light and thermal energy (commonly called heat). The light and heat become dispersed and less useful. Another way to describe this process is to say the energy is concentrated in the wood (chemical energy), and becomes less concentrated in the forms of thermal and light energy.
Let's return to the frantic cat in the room with the puzzle. Although you might be able to find all the pieces of the puzzle after the cat's actions, you cannot put the puzzle completely back together. Some pieces have been bent, others torn, and some the cat, well, use your imagination. In other words, although the quantity of the puzzle remains the same, its quality has been compromised. This cat story is a rough analogy to the second law of thermodynamics.
The following set of statements are various ways of expressing the second law of thermodynamics:
* With each energy conversion from one form to another, some of the energy becomes unavailable for further use.
* Heat cannot flow from a cold object to a hot object on its own.
* It is impossible to convert heat energy into work with 100 percent efficiency.
* You cannot break even.
* It is impossible to build a machine that produces as much energy as it uses. (This type of machine is called a perpetual motion machine of the second kind.)
* The entropy of the universe tends to a maximum. (Rudolf Clausius, 1865)
It is much easier to illustrate examples of the second law of thermodynamics. Simply turning on a light bulb shows that in addition to light, heat is generated. Also, try recapturing the light or the heat to do additional work. Tough, isn't it?
Consider this quote by Paul and Ann Erlich:
Energy is most usable where it is most concentrated--as in highly structured chemical bonds (gasoline, sugar) or at high temperature (steam, incoming sunlight [sic]). Since the second law of thermodynamics says that the overall tendency in all processes is away from concentration, away from high temperature, it is saying that, overall, more and more energy is becoming less and less usable.
Scientists and inventors over the years have recognized this trend of energy "loss" and have strived to overcome it. They have always failed. A common attempted invention to resist the laws of thermodynamics is called the perpetual motion machine. The idea behind this machine is that the motion of the machine provides the energy to continue the motion of the machine. (Huh?) In other words, once the machine starts running, no additional energy is needed (the machine provides its own energy). Think it'll work? The next section, Energy Rules! Section D. Activities and Experiments, will provide a discussion of perpetual motion machines.
Final Thoughts about Energy Rules
Energy has often been called the currency of life. It flows through Earth's processes creating wind, providing light, and enabling plants to create food from water and air (carbon dioxide). Humans have tapped into this flow to generate electricity, fuel our cars, and heat our homes. The sun provides Earth with most of its energy. It is important for students to recognize and appreciate this source of energy and to explore the transformations that bring the sun's light into their home in the form of light, heat, food, and fuel. We are fortunate to have many "concentrated" sources of energy. Besides the sun, there is chemical energy found in fossil fuels such as coal and oil and in nuclear resources.
While the amount of energy in our world remains constant, as we use it (transfer it to one form to another), it becomes spread out and less useful. Energy also gives us the ability to work. Through education and becoming aware of what energy is and how we use it, we can learn (i.e., work) to use our concentrated resources more wisely, and ensure that they will be available for future generations.
Next reading: Energy Rules!
Section D. Activities and
Experiments.
Return
to Energy Rules! - Section C.1.
Energy Conversion and the Laws of Thermodynamics.
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Copyright 2002 KEEP