Energy
What Is Energy?
During a storm, lightning flashes in the sky and a thunderclap shakes your house with a deafening boom (Figure 1). Strong, howling winds blow tree branches wildly back and forth and a few trees blow over, knocking down power lines in your neighborhood. You fumble around in the dark looking for matches and candles. When you strike a match and light a candle, the flickering rays of light give you a little comfort during the scary storm.
All of the changes that occurred during the storm had one thing in common--energy. Energy is defined as the ability to cause change. The lightning and thunder, the blowing winds and falling trees, and lighting a match or a candle all involve energy changes or transformations. The moving winds transfer energy to the trees causing them to move and the energy stored in the match is transformed into light and heat, both forms of energy, when it is lit.
All of the changes that occurred during the storm had one thing in common--energy. Energy is defined as the ability to cause change. The lightning and thunder, the blowing winds and falling trees, and lighting a match or a candle all involve energy changes or transformations. The moving winds transfer energy to the trees causing them to move and the energy stored in the match is transformed into light and heat, both forms of energy, when it is lit.
Energy and Work
Energy is transferred to an object when work is done on the object. Work is the transfer of energy that occurs when a force is applied to an object that causes it to move some distance. For example, if you lift a box of books off the floor, you are doing work on the box (Figure 2). Notice that work is done on the object only if the object moves; if the object does not move, no work was done. If you try to pick up the box of books, but are unable to lift it because it is too heavy, no matter how much effort you exert, you perform no work on the box and no energy is transferred.
Furthermore, work is only done on an object when it moves in the same direction as the force being applied to it. Once you have performed work on the box by lifting it, carrying the box to your closet to put it away does no additional work on the box, because the motion of the box is not in the same direction as the force you are using to hold the box. During the storm, the wind did work when it applied a force to the trees that caused them to move. People use the energy in wind to perform work when they use it to turn the blades of a wind turbine, or push the sails of a sailboat. Because of this connection between energy and work, energy is also described as the ability to do work.
Furthermore, work is only done on an object when it moves in the same direction as the force being applied to it. Once you have performed work on the box by lifting it, carrying the box to your closet to put it away does no additional work on the box, because the motion of the box is not in the same direction as the force you are using to hold the box. During the storm, the wind did work when it applied a force to the trees that caused them to move. People use the energy in wind to perform work when they use it to turn the blades of a wind turbine, or push the sails of a sailboat. Because of this connection between energy and work, energy is also described as the ability to do work.
Kinetic Energy
There are two basic types of energy: kinetic energy and potential energy. Kinetic energy is the energy of matter in motion. All moving objects have kinetic energy. An object with kinetic energy, like the wind during a storm, does work when it strikes another object, such as a tree, causing it to move some distance. The wind does work on the tree when it transfers some of its kinetic energy to the tree, and if enough energy is transferred the tree may even topple.
The kinetic energy of an object depends on its speed and mass. The faster an object travels, the greater its kinetic energy. The two cars in Figure 3 have the same mass, but because the silver car is traveling faster, it has more kinetic energy than the black car.
Kinetic energy also increases when mass increases. and the greater the mass of an object, the greater its kinetic energy. The delivery truck and the black car in Figure 3 are both traveling at the same speed, but because the truck has a much greater mass, it has much more kinetic energy than the car.
The kinetic energy of an object depends on its speed and mass. The faster an object travels, the greater its kinetic energy. The two cars in Figure 3 have the same mass, but because the silver car is traveling faster, it has more kinetic energy than the black car.
Kinetic energy also increases when mass increases. and the greater the mass of an object, the greater its kinetic energy. The delivery truck and the black car in Figure 3 are both traveling at the same speed, but because the truck has a much greater mass, it has much more kinetic energy than the car.
Potential Energy
Objects do not have to be moving to have energy. An object may have stored energy due to its position or shape. This stored energy has the potential to be used to cause change or do work. Stored energy due to the position or shape of an object is called potential energy.
Potential energy stored in objects that are lifted from Earth is called gravitational potential energy. The water behind the dam in Figure 4 is raised higher above the surface of Earth, increasing its potential energy. This stored energy can be used to produce energy in the form of electricity as the water is allowed to flow through turbines in the dam.
The amount of gravitational potential energy an object has depends on the object's height and weight. When you lift your pencil off your desk, you increase its potential energy, and if you lift it higher, you increase its potential energy even more. So the higher an object is lifted above Earth, the greater its potential energy. Also, if you lift your pencil and binder the same height, your binder will have more potential energy due to its greater weight.
The amount of gravitational potential energy an object has depends on the object's height and weight. When you lift your pencil off your desk, you increase its potential energy, and if you lift it higher, you increase its potential energy even more. So the higher an object is lifted above Earth, the greater its potential energy. Also, if you lift your pencil and binder the same height, your binder will have more potential energy due to its greater weight.
Potential energy stored in objects that are stretched or compressed is called elastic potential energy. When the archer in Figure 5 pulls an arrow back, the shape of the bow changes, and it is now storing elastic potential energy. When the archer releases the bowstring, the bow returns to its original shape and the stored energy changes to kinetic energy as the arrow flies away. Common objects that store elastic potential energy include rubber bands, bungee cords, hair ties, springs, and rubber balls.
The food you eat, the
gasoline you use to power a lawn mower, the match you use to light a
candle, and many other substances are made up of atoms joined together
by chemical bonds. The potential energy stored in the chemical bonds between atoms is called chemical energy. The cupcake in Figure 6 contains chemical compounds called carbohydrates that are broken down and used in your body as a source of energy. Complex carbohydrates are broken down into simple sugars such as glucose, which the cells in your body break down in organelles called mitochondria. When bonds in glucose are broken, the energy stored in the bonds is released and is converted to ATP, a form of energy used by all cells as their principal source of energy.
Other Forms of Energy
Electrical energy is the energy of electrically charged particles. Electrical energy that is stored, like in a battery, is a form of potential energy. Electrical energy that moves from one place to another, such as the electrical current flowing through the wires in your home that powers lights and appliances, is a form of kinetic energy. Static electricity results from the buildup of an electrical charge in an object. When you rub a balloon on your hair like the girl in Figure 7, electrons are transferred to the balloon and it becomes negatively charged, while your hair becomes positively charged. Due to the opposite charges, your hair and the balloon are attracted to each other. Lightning results from a build up of static charges in clouds in the atmosphere.
When a drummer strikes a drum, the drumhead vibrates creating sound energy (Figure 8). Sound energy is energy carried by sound waves. As the drumhead vibrates, or moves back and forth, it pushes nearby air molecules together, creating a series of compressions which travel through the air as waves. You usually hear sound waves through the medium of air, but sound waves can also travel through solids and liquids. Sound waves cannot travel through a vacuum, such as the empty space between the sun and Earth, because there is no medium for the sound waves to travel through.
The sunlight in Figure 9 and the light from a lamp are both forms of electromagnetic energy. Electromagnetic energy, also called radiant energy or light energy, is energy that travels through space as waves. These waves have some electrical properties and some magnetic properties. Unlike sound waves, electromagnetic waves can travel through a vacuum. The x-rays your dentist uses to find cavities in your teeth, the microwaves you use to make popcorn, and the radio waves used by your cell phone are all forms of electromagnetic energy. Other types of electromagnetic energy include ultraviolet radiation, infrared radiation, and gamma rays.
All objects are made up of small particles called atoms and molecules, which are in constant motion. Thermal energy is energy due to the motion of particles that make up an object. The faster the particles in an object are moving, the greater their thermal energy. Water in an ice cube has less thermal energy than lemonade in a pitcher, because the molecules of water in the ice cube are moving more slowly. Thermal energy always moves from warmer objects to colder objects. When you place ice cubes in a pitcher of lemonade as in Figure 10, thermal energy from the warmer lemonade will be transferred to the colder ice cubes. As the ice cubes absorb this thermal energy, the kinetic energy of their molecules will increase and they will begin to melt. Because the lemonade lost thermal energy, it will now be cooler when your drink it.
Mechanical energy is the sum of the potential energy and the kinetic energy in a system. In other words, mechanical energy is energy due to an objects position or due to its motion or both. The roller coaster in Figure 11 has gravitational potential energy due to its raised position, and it has kinetic energy due to its movement. A barbell held above a weightlifter's head has mechanical energy due to its position. A bowling ball rolling down a bowling alley has mechanical energy. And a football thrown by a quarterback has mechanical energy due to both its motion and its position raised above Earth. Mechanical energy is acquired by an object when work is done on the object. Also, recall that energy is the ability to do work, so an object with mechanical energy has the ability to do work. For example, a bowling ball does work on the bowling pins when it transfers its kinetic energy to the pins knocking them over.
The energy stored in the nucleus of an atom is called nuclear energy. Nuclear energy is a type of potential energy that is released in nuclear reactions. One type of nuclear reaction, called nuclear fusion, occurs when atomic nuclei combine releasing large amounts of energy. Nuclear fusion is the type of nuclear reaction that happens in the sun and other stars. Another type of nuclear reaction, called nuclear fission, occurs when atomic nuclei are split releasing energy. Nuclear fission is the kind of nuclear reaction that occurs in nuclear reactors like those in Figure 12. Thermal energy released during nuclear fission is used to produce electricity.
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