(13) Environmental Science


What is Energy?


What Is Energy and How Can It Change Its Form?

CONCEPT 2-4A - When energy is converted from one form to another in a physical or chemical change, no energy is created or destroyed (first law of thermodynamics).

CONCEPT 2-4B - Whenever energy is changed from one form to another, we always end up with lower quality or less usable energy than we started with (second law of thermodynamics).

Energy Comes in Many Forms

Energy is the capacity to do work or transfer heat. Work is what is done when something is moved. Using energy to do work means moving or lifting something such as this book, propelling a car or plane, cooking your food, and using electricity to move electrons through a wire to light your room.

There are two major types of energy: moving energy (called kinetic energy) and stored energy (called potential energy). Moving matter has kinetic energy because of its mass and its velocity. Examples are wind (a moving mass of air), flowing water, and electricity (flowing electrons).

Another form of kinetic energy is heat: the total kinetic energy of all moving atoms, ions, or molecules within a given substance, excluding the overall motion of the whole object. When two objects at different temperatures contact one another, heat flows from the warmer object to the cooler object.

In electromagnetic radiation, another form of kinetic energy, energy travels in the form of a wave as a result of the changes in electric and magnetic fields. There are many different forms of electromagnetic radiation, each having a different wavelength (distance between successive peaks or troughs in the wave), and energy content. Forms of electromagnetic radiation such as gamma rays (emitted by some radioactive isotopes, X rays, and ultraviolet (UV) radiation with short wavelengths have a higher energy content than forms such as visible light and infrared (IR) radiation with longer wavelengths. Visible light makes up most of the spectrum of electromagnetic radiation emitted by the sun.

Find out how color, wavelengths, and energy intensities of visible light are related at ThomsonNOW.

The other major type of energy is potential energy, which is stored and potentially available for use. Examples of potential energy include a rock held in your hand, an unlit match, the chemical energy stored in gasoline molecules, and the nuclear energy stored in the nuclei of atoms.

Potential energy can be changed to kinetic energy. Drop these texts on your foot, and the potential energy that the texts had when you were holding it changes into kinetic energy. When a car engine burns gasoline, the potential energy stored in the chemical bonds of gasoline molecules changes into mechanical (kinetic) energy that propels the car. Potential energy stored in various molecules such as carbohydrates that you can eat becomes kinetic energy when your body uses it.

Witness how a Martian might use kinetic and potential energy at ThomsonNOW.

Some Types of Energy Are More Useful Than Others

Energy quality is a measure of an energy source’s capacity to do useful work. High-quality energy is concentrated and can perform much useful work. Examples are very high-temperature heat, nuclear fission, concentrated sunlight, high-velocity wind, and energy released by burning natural gas, gasoline, or coal. By contrast, low-quality energy is dispersed and has little capacity to do useful work. An example is heat dispersed in the moving molecules of a large amount of matter (such as the atmosphere or an ocean) so that its temperature is low. The total amount of heat stored in the Atlantic Ocean is greater than the amount of high quality chemical energy stored in all the oil deposits of Saudi Arabia. Yet because the ocean’s heat is so widely dispersed, it cannot be used to move things or to heat things to high temperatures.

Energy Changes Are Governed by Two Scientific Laws

Thermodynamics is the study of energy transformations. Scientists have observed energy being changed from one form to another in millions of physical and chemical changes. But they have never been able to detect the creation or destruction of any energy in such changes. The results of these experiments have been summarized in the law of conservation of energy, also known as the first law of thermodynamics:

When energy is converted from one form to another in a physical or chemical change, no energy is created or destroyed (Concept 2-4A).

This scientific law tells us that when one form of energy is converted to another form in any physical or chemical change, energy input always equals energy output.

No matter how hard we try or how clever we are, we cannot get more energy out of a system than we put in. This is one of nature’s basic rules. Because the first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another, you may be tempted to think there will always be enough energy. Yet if you fill a car’s tank with gasoline and drive around or use a flashlight battery until it is dead, something has been lost. But what is it? The answer is energy quality, the amount of energy available that can perform useful work.

Countless experiments have shown that whenever energy changes from one form to another, we always end up with less usable energy than we started with. These results have been summarized in the second law of thermodynamics: When energy changes from one form to another, we always end up with lower quality or less usable energy than we started with (Concept 2-4B). This lower quality energy usually takes the form of heat given off at a low temperature to the environment.

There it is dispersed by the random motion of air or water molecules and becomes even less useful as a resource.

In other words, energy always goes from a more useful to a less useful form when it changes from one form to another. No one has ever found a violation of this fundamental scientific law. It is another one of nature’s basic rules. Consider three examples of the second law of thermodynamics in action. First, when you drive a car, only about 6% of the high-quality energy available in its gasoline fuel actually moves the car, according to energy expert Amory Lovins. (See his Guest Essay on this topic at ThomsonNOW™.) The remaining 94% is degraded to low-quality heat that is released into the environment. Thus, 94% of the money you spend for gasoline is not used to transport you anywhere.

Second, when electrical energy in the form of moving electrons flows through filament wires in an incandescent light bulb, about 5% of it changes into useful light and 95% flows into the environment as low-quality heat. In other words, the incandescent light bulb is really an energy-wasting heat bulb.

Third, in living systems, solar energy is converted into chemical energy (food molecules) and then into mechanical energy (for moving, thinking, and living). During each conversion, high-quality energy is degraded and flows into the environment as low-quality heat.

The second law of thermodynamics also means we can never recycle or reuse high-quality energy to perform useful work. Once the concentrated energy in a serving of food, a liter of gasoline, or a chunk of uranium is released, it is degraded to low-quality heat that is dispersed into the environment. Energy efficiency, or energy productivity, is a measure of how much useful work is accomplished by a particular input of energy into a system. There is plenty of room for improving energy efficiency. Scientists estimate that only 16% of the energy used in the United States ends up performing useful work. The remaining 84% is either unavoidably wasted because of the second law of thermodynamics (41%) or unnecessarily wasted (43%).

Thermodynamics teaches us an important lesson: the cheapest and quickest way to get more energy is to stop wasting almost half the energy we use. We can do so by driving fuel-efficient motor vehicles, living in well-insulated houses, and using energy-efficient lights, heating and cooling systems, and appliances. Ideally, we should get as much energy as possible for these purposes from the sun and from electricity produced indirectly from the sun by renewable flowing water (hydropower), wind, and biofuels. This involves using the first scientific principle of sustainability (Concept 1-6).

See examples of how the first and second laws of thermodynamics apply in our world at ThomsonNOW.


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