Introduction
History of the Energy System
Human Energy Needs
Science Notes
Energy Transformation
Measuring Matter, Force, & Energy
Energy Accounting & Balance
Fundamental Forces of Nature
Energy and Chemical Stability
Chemical Formations
Chemistry of Fossil Fuels
Energy Use, Efficiency, and the Future
Energy Sources, Technologies, & Impacts
Exercises
Internet Links
Other Resources
Energy System PDF
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Science Notes: Energy Transformation

The definition of energy as the ability to do work came from the 19th century as steam engines and other work-producing machines were developed. The first engines converted heat (thermal energy) into motion (dynamics). The science of heat engines developed by Lord Kelvin in England and Joule and Clausius in France founded the science of thermodynamics. They showed that two rules always held when energy was used to produce motion or work. These are called the two Laws of Thermodynamics. It was noticed that when engines performed work, heat was always produced in addition to the work, and that this represented wasted energy.

The Two Laws of Thermodynamics
One of our observations about energy is that the total quantity of mass and energy combined in the universe is always the same. Energy can change forms and mass and energy may even change into each other, but the total quantity remains the same. This fact is called the principle of Conservation of Energy, or the First Law of Thermodynamics. For most reactions that are studied here the total energy remains constant.

Conservation of energy was an idea proposed by numerous people. Julius Robert Mayer, a German physician, proposed in 1844 that energy was conserved by observing physical processes involving heat and respiration, but with no quantitative measurements. The first formal statement was in a paper by a young physicist, Hermann von Helmholtz, in 1847. (Helmholtz is known as one of the greatest physicists of the 19th century). Joule's experiments from 1843 and his paper in 1849 publicizing his most precise experiments stimulated gradual acceptance of the principle. However, it met with a lot of skepticism early on because it seemed speculative. Even as late as 1858, William Barton Rogers, the founder of Massachusetts Institute of Technology (MIT), wrote to his friend that the principle of conservation of energy was "mysticism"! This is an example of both the slow pace of science and of the fact that great discoveries are often a leap of speculation, although based on observations.

The First Law of Thermodynamics states that energy cannot be created or destroyed. Simply stated: energy can change forms; mass and energy may even change into each other (this happens only in special reactions within nuclei of atoms) but the total energy in the universe remains constant.

The first law says that you can convert energy into work, and into the simultaneous production of heat. If the total energy is E to start with, and all of it is spent doing an amount of work (W), with a production of heat (Q), the First Law states:
E=W+Q

More correctly, we are talking of the change of energy, work, and heat in a system. Using the symbol "" to denote the changes, the first law is written as:

Q
+
W
=
E
non useful work
+
work done
=
change in energy of the system
heat and noise
+
mechanical energy
=
transformation from chemical energy to mechanical energy
Q1
+
W1
=
Q2+ E2
E1
=
E2

All the descriptions above are equivalent.

As energy changes forms, the energy becomes more and more spread out and inaccessible to us. For example, the energy that is stored in a compact form within a gallon of gasoline in a car tank becomes transformed into work in moving the car, and dissipated into the energy of the surrounding air and road. As the molecules are heated they spread over a much larger area. This energy, now spread out all over, is not destroyed but has become dissipated and therefore unavailable for us to do more work with it. In this process, matter (such as the heated molecules in the air surrounding the car) has also become more disordered. These two facts combined are known as the Second Law of Thermodynamics. This law states that the unavailable energy in the universe increases, or equivalently, that the disorder (also called entropy) in the universe increases (as energy is used).

The second law then simply states that within each process of producing work, we are increasing the unavailable energy and the disorder in the universe. This means that even though the total energy in the universe is constant, we are decreasing the "quality" of the energy. We are decreasing the amount of available energy -- the energy that we can use to produce work.

We cannot harness all the energy coming out of a process of energy transformation. We define efficiency as:

Efficiency N =
work done
energy spent
or

work output
energy input

 


EXAMPLES OF EFFICIENCY:
How efficient is an automobile? In other words, how much of the energy in the gasoline results in kinetic energy or energy of motion of the automobile?
  12% if well maintained, 8-10% if not maintained
What are the implications of this inefficiency? Of the 20 gallons you of gasoline you put in your car, how much actually moves the car to your destination?
1.6-2.4 gallons
(the rest is transformed to waste heat and noise)
How efficient is a coal-fired power plant?
 
transformation by steam turbine:
30-40%
How efficient is a hydroelectric plant?
 
transformation by water driven turbine:
80%
(the difference from above is because no conversion to steam is involved)
How efficient is a nuclear plant?
  The nuclear reaction is 90% efficient, however the same combustion process (steam turbines) is used to generate electricity as with the coal-fired plant, so:
the net efficiency:
30-40%
How efficient is the human body?
 
conversion of the energy in the food to muscular movement and other kinds of work:
20% efficient

Based on the First Law of Thermodynamics, we neither create nor destroy energy. Whenever we say that we are producing energy, what we really mean is that we are transforming energy from one form to another that is more usable. Energy that is the result of work usually manifests as change in position or as the motion of an object. Energy that is stored is called potential energy -- energy that has the 'potential' to do work. A second form of energy -- energy of motion -- is called kinetic (meaning "moving") energy. Both kinetic and potential energy can be transformed into work.

Later in this unit we present the physical basis of energy and work. Without going into the details that will come then, let us discuss two transformations of energy: a waterfall and a pendulum. Every gallon of water in the fall has potential energy at the top of the waterfall by virtue of its position. At the bottom of the waterfall, this gallon of water travels faster and has gained kinetic energy at the expense of potential energy. As a pendulum swings back and forth, the energy changes from potential energy at the top position to kinetic energy at the lowest position and potential energy again as it goes to the top. We will explain the deeper meanings of potential and kinetic energy later in the unit.

We begin by reviewing some fundamentals of physics and chemistry relevant to understanding the basic principles of energy transformation. In particular, we focus on concepts related to the fundamental aspects of energy: Matter, Force, and Energy and the Fundamental Forces of Nature. Then we describe the physics and chemistry of Measuring Energy, Work, and Power. Chemical reactions and energy release are then described to understand more clearly how the chemical combustion of fossil fuels produces the carbon dioxide and other products that cause environmental problems.

 

 

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  ©Copyright 2003 Carnegie Mellon University
This material is based upon work supported by the National Science Foundation under Grant Number 9653194. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.