Energy Transformations
Energy is the capacity to do work or to produce heat. There are many forms of energy, each of which can be classified into three categories: radiant energy, kinetic energy, and potential energy. Energy can be changed, or transformed, from one form into another. Energy transformation is also called energy conversion. The Système International d'Unités (SI) unit for energy is the joule (J), after James Joule, who demonstrated that work can be converted into heat. Lifting a medium-sized potato a distance of 1 m (3.28 ft) would require approximately one joule of energy. Energy is often expressed as the calorie (cal), which is the amount of heat needed to raise the temperature of one gram of water by 1(C. One calorie is equal to 4.184 joules. The calorie (Cal), which is used to express the energy in food, is 1,000 calories.
Kinetic energy is the energy of an object in motion. An object in motion can cause another object to do work by colliding with it, causing it to move a particular distance. The colliding objects can be a hammer swinging down on a nail, or two atoms colliding in a chemical reaction. Examples of kinetic energy are mechanical energy (caused by motion of parts) and thermal energy (caused by the random motion of particles of matter). An object that has potential energy is not moving or doing work. At some point, that object had work performed on it that resulted in energy storage. One example of performing work on an object to give it potential energy is the action of winding a watch. As it is being tightened, the spring is gaining potential energy. Winding a watch, therefore, transforms mechanical into potential energy. Another example of potential energy is water in an elevated tank. If water is allowed to fall on a wheel and the wheel turns, the wheel can be used to produce electricity. The water in the tank has the potential work once it falls; therefore, it has gravitational potential energy (gravity is what converted the potential energy of the falling water into kinetic energy). The potential energy of the water was transformed into mechanical energy of the wheel, which was further transformed into electrical energy. Conversion between potential and kinetic energies is quite common. As the potential energy of an object increases, its kinetic energy tends to decrease. Likewise, if the kinetic energy of an object increases, its potential energy decreases. For example, a rubber ball held out of a third-story window has potential energy. If it is released, gravity causes it to fall. As it falls, it loses potential energy because its height decreases. It gains kinetic energy because its velocity is increasing. Eventually, when the ball reaches the ground, it has lost all of its potential energy (because it cannot fall any farther) and at the same time reached a maximum kinetic energy.
In 1845, Joule performed an experiment that demonstrated energy transformation both qualitatively and quantitatively. The experiment was not complicated--he placed a paddle wheel in a tank of water and measured the temperature of the water. He then cranked the wheel in the water for a period of time, and read the temperature again. He found that the temperature of the water rose as he cranked the paddle wheel. He quantified this observation and discovered that an equal amount of energy was always required to raise the temperature of the water by one degree. He also discovered that it did not have to be mechanical energy; it could be energy in any form. He obtained the same results with electrical or magnetic energy as he did with mechanical energy. Joule's experiments showed that different forms of energy are equivalent and can be converted from one form to another.
These observations led to what is now called the "Law of conservation of energy." This law states that any time energy is transferred between two objects, or converted from one form into another, no energy is created and none is destroyed. The total amount of energy involved in the process remains the same. It should be noted, however, that this does not apply to processes, such solar explosions and the generation of energy in nuclear reactors, which involve nuclear energy.
Most chemical reactions involve transformations in energy. A chemical reaction is simply the process whereby bonds are broken between atoms and new ones are made. When bonds are broken in a chemical reaction, the reactants involved are colliding with sufficient kinetic energy to break the atomic bonds apart. During the collision, the kinetic energy is transformed into potential energy as the atoms are rearranged and new bonds are formed. Most chemical reactions involve the overall absorption or release of energy. The energy flow resulting from transforming kinetic energy into potential energy is heat.
There are many everyday examples of energy transformation that can be cited. One simple example is the act of lighting a match, which transforms chemical energy stored in the match head into heat and light, two other forms of energy. Another example of energy transformation is the stored chemical energy in flashlight batteries. When the flashlight is turned on, the chemical energy is first transformed into electrical energy and ultimately into light energy. Compressing a spring gives it potential energy; releasing the spring converts the potential energy into kinetic energy. Similarly, stretching a rubber band between your fingers gives the rubber band potential energy, and when it is released, the rubber band is given the kinetic energy to fly away. Electromagnetic energy in an electric motor is converted into mechanical energy. In a heat engine, the chemical energy in the fuel is converted into heat energy as it is burned. The heat energy is then converted into mechanical energy. Some energy transformations seem to go "full circle." In a microphone-loudspeaker system, for example, sound (which is a form of mechanical energy) is converted by the microphone into electromagnetic energy (electricity). The electricity reaches the loudspeaker, which converts it back into sound.
Quite often, a whole series of energy transformations is needed to complete a particular job. For example, many transformations occur in the operation of an electric hair dryer. The dryer needs to use electromagnetic energy (electricity), which does not simply originate in the outlet on the wall. A fuel source, such as coal, is burned in a power plant, releasing the chemical energy of fuel. The chemical energy is converted to heat energy as it is burned. This heat energy is used to change water into steam, and the rising steam is used to turn a turbine. In this process, heat energy has been transformed into mechanical energy. The mechanical energy performs work on a generator, which converts the mechanical energy into electromagnetic energy. This energy reaches the hair dryer in the form of electricity. The electricity is changed to mechanical energy inside the dryer, where it is transformed again into heat and sound.
The ultimate example of energy transformation is that of the radiant energy of the Sun. All of the energy on Earth originated from the Sun. The Sun's radiant energy is converted by plants into chemical energy through the process of photosynthesis. This chemical energy is stored in the form of sugars and starches. When these plants are eaten by animals, this chemical energy is either transformed into another form of chemical energy (fats or muscle) or used for mechanical or thermal energy. Whenever an animal eats another animal, a similar process occurs. Petroleum originated from ancient plants and animals buried under many layers of sediment. Ultimately, then, even the fuels we use come from the Sun's energy. The energy we use every day is the result of the transformation of solar energy over millions of years.
Energy transformations occur every time a chemical reaction takes place, a light switch is turned on, or a person takes a breath. Joule's work enabled scientists to understand energy transformations that in turn made many technological advancements possible. Understanding the nature of energy transformation is one step toward understanding how the world works.
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