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Energy Transformations
Changes Between Forms of Energy
As you probably noticed in many of the examples in the previous section, energy can change from one form to another. The change in energy from one form to another is called an energy transformation. Most forms of energy can be transformed into other forms of energy. You may never have really thought about it, but you use energy transformations to do many different things in your daily life. When the boy in Figure 13 speaks to a friend on his cell phone, the sound energy of his voice is transformed into electrical energy by the phone, which is in turn transformed into electromagnetic energy in the form of radio waves. His friend's phone will receive a signal in the form of electromagnetic energy, which will be transformed into electrical energy, which will be changed back to sound energy by the phone's speaker.
Transformations Between Kinetic and Potential Energy
A common energy transformation is the change between kinetic energy and potential energy. Any object that rises or falls, such as the volleyball in Figure 14, goes through changes in its kinetic and gravitational potential energy. When the volleyball player hits the ball into the air, it is moving the fastest and has the greatest amount of kinetic energy. As the ball rises it slows down and its kinetic energy decreases. However, as the ball rises, its potential energy increases because its height increases. Kinetic energy is being transformed into potential energy. At its highest point, the ball is not moving and has no kinetic energy, but it has the greatest amount of potential energy.
As the ball begins to fall, it speeds up and its kinetic energy increases. Because the ball is now losing height, its potential energy decreases. Potential energy is being transformed into kinetic energy. When the ball reaches the second volleyball player, it is again moving the fastest and has the greatest amount of kinetic energy, but it has lost all of the potential energy it gained during the course of its flight.
Conservation of Energy
The lawn mower shown in Figure 15 uses chemical energy in gasoline as its source of energy. As the gasoline is used up, you may think the energy has also been used up. But this is not the case. All of the energy in the gasoline is transformed into other types of energy, such as thermal energy, sound energy, and the mechanical energy that is used by the lawn mower's blade to cut the grass.
The law of conservation of energy states that when energy is transformed from one form to another, or transferred from one object to another, no energy is created or destroyed in the process. All of the energy that was in the gasoline still exists, it has just been changed into different forms of energy.
The law of conservation of energy states that when energy is transformed from one form to another, or transferred from one object to another, no energy is created or destroyed in the process. All of the energy that was in the gasoline still exists, it has just been changed into different forms of energy.
Friction and Waste Energy
Have you ever rubbed your hands together on a cold day in an attempt to warm them like the young man in Figure 16 . Why does this make your hands warmer? The answer is friction. Friction is the force that resists the sliding of two surfaces that are touching. Anytime two objects are in contact with each other and moving, there will be friction between them. Because of friction, the mechanical energy of your moving hands is transformed into thermal energy that warms them.
A car's brakes use friction to stop the car, and as the car slows, its mechanical energy is transformed into thermal energy. Friction causes mechanical energy to be transformed into thermal energy, but other types of energy may also be transformed into thermal energy, which is released into the environment and no longer available to perform work. For example, a light bulb transforms electrical energy into electromagnetic energy, but some of the energy is also transformed into thermal energy. If you've ever tried to change a light bulb that is still hot, you are probably well aware of this. The thermal energy that is released in an energy transformation is called waste energy. Whenever there is an energy transformation, some of the energy changes into useful energy and some changes into waste energy.
A car's brakes use friction to stop the car, and as the car slows, its mechanical energy is transformed into thermal energy. Friction causes mechanical energy to be transformed into thermal energy, but other types of energy may also be transformed into thermal energy, which is released into the environment and no longer available to perform work. For example, a light bulb transforms electrical energy into electromagnetic energy, but some of the energy is also transformed into thermal energy. If you've ever tried to change a light bulb that is still hot, you are probably well aware of this. The thermal energy that is released in an energy transformation is called waste energy. Whenever there is an energy transformation, some of the energy changes into useful energy and some changes into waste energy.
Other Energy Transformations
It's Friday night and you make some popcorn in the microwave and watch a movie on TV (Figure 17). You are having a good time... thanks to energy transformations! The microwave oven transforms electrical energy into microwaves, a type of electromagnetic energy. The microwaves are transformed into thermal energy as they are absorbed by the popcorn kernels. When the kernels become hot enough they pop. The television also uses electrical energy, which it transforms into electromagnetic energy and sound energy so you can see and hear the movie.
An apple tree, such as the one in Figure 18, transforms electromagnetic energy from the sun into chemical energy during photosynthesis. The tree uses this energy to produce chemical compounds that it needs to grow and reproduce. When you eat an apple from the tree, your body transforms the chemical energy stored in the apple into the kinetic energy you use to move. Some of the apple's chemical energy is also transformed into thermal energy, which keeps your body warm.
Fossil fuels, such as oil, coal, and natural gas, also contain chemical energy. A car's engine uses gasoline made from oil. The engine transforms the chemical energy in the gasoline into thermal energy as it is burned within the engine. The thermal energy is then transformed into mechanical energy that moves the car. The gas stove in Figure 19 transforms the chemical energy in natural gas into thermal energy that is used to heat food. Heating oil and natural gas are also both frequently used in hot water heaters and in furnaces used to heat homes.
We use electrical energy to power many common devices. Cellular phones, MP3 players, tablets, flashlights, and many toys are powered by batteries. An MP3 player, such as an iPod, transforms chemical energy stored in a battery into electrical energy, which is then transformed into sound energy by the speakers in the earbuds. A flashlight transforms chemical energy stored in batteries into electrical energy, which is then transformed into electromagnetic energy by a light bulb. Electrical energy from power plants runs many other common devices and appliances that you may have in your home, such as a toaster that transforms electrical energy into thermal energy. The hair dryer in Figure 20 transforms electrical energy into mechanical energy and thermal energy. The mechanical energy is transformed into the kinetic energy of the moving air.
The electrical energy that reaches your home is produced in many different ways. Coal power plants transform chemical energy stored in coal into thermal energy. The thermal energy is used to heat water producing steam, which is transformed into mechanical energy when it is directed through turbines. Finally, the mechanical energy of the turbines is transformed into electrical energy. Nuclear power plants transform nuclear energy stored in atoms into thermal energy as a result of nuclear fission reactions. This thermal energy is also used to produce steam, which is then used to operate turbines that produce electricity. Hydroelectric power is generated from water held behind dams. The gravitational potential energy of the water transforms into kinetic energy as the water is allowed to flow through the dam. The kinetic energy of the moving water transforms into mechanical energy as it is directed through large turbines like those shown in Figure 21. Finally the mechanical energy of the turbines is transformed into electrical energy.
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