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What is Energy? Energy is the capacity of a system to do work. That system may be a jet, carrying hundreds of passengers across the ocean. A kite, rising on the wind. Or a wave of light crossing a space. In moving or growing, each of these systems is doing work, and using energy. Every living organism does work, and needs energy from food or photosynthesis. Humans also create machines that do work for them, and that derive energy from fuels. Some of the many forms that energy takes are: v * Mechanical energy, which includes - Potential energy, stored in a system. - Kinetic energy, from the movement of matter. * Radiant or solar energy, which comes from the light and warmth of the sun. * Thermal energy, associated with the heat of an object. * Chemical energy, stored in the chemical bonds of molecules. * Electrical energy, associated with the movement of electrons. * Electromagnetic energy, associated with light waves (including radio waves, microwaves, x-rays, infrared waves). * Mass (or nuclear) energy, found in the nuclear structure of atoms. One form of energy can be converted to another form. This transfer is based on the law of conservation of energy—one of the laws of thermodynamics. Humans converted energy from one form to another when they lit the first fire. By burning wood, they released the chemical energy stored in the bonds of the wood molecules, generating thermal energy, or heat. Other examples? A battery generates electrons from chemical reactions, which are used to make electrical energy. A toaster takes electrical energy and converts it to heat. Your leg converts the chemical energy stored in your muscles into kinetic energy when you pedal a bicycle. Sound is a form of kinetic energy. Molecules of air are vibrated, causing them to move in wave patterns. When these waves hit the eardrum, they make it vibrate too. This vibration energy is turned into electrical energy impulses, which your brain interprets as sound. Many times, multiple conversions are involved. Consider nuclear power generation. Atoms in the nuclear fuel are split, releasing their nuclear (mass) energy and creating thermal energy. This heat energy is, in turn, captured in the form of steam and used to drive a turbine generator, creating kinetic energy. And, finally, this kinetic energy spins a magnetic field around a conductor, causing a current to flow—creating electrical energy. To measure energy, we use the "heating value" of the fuel, which indicates how much of a certain fuel is converted to how much heat. Common units of energy are the calorie and Btu. Two Main Forms of Energy Kinetic energy is motion energy. Potential energy is energy stored in matter. In strict scientific terms energy is classified into two main forms: kinetic and potential energy. Kinetic energy is defined as the energy of a moving object. A thrown football, a speeding automobile, a waterfall, or a rock falling from a cliff are examples of objects that have kinetic energy. Waterfall  Potential energy appears in many different forms, and is defined as the energy in matter due to its position or the arrangement of its parts. The various forms of potential energy include gravitational potential energy, elastic potential energy, chemical potential energy, and electrical potential energy. Potential Energy is often referred to as stored energy. Some scientists avoid use of the word "stored" because it inaccurately depicts energy as a substance that is contained within a substance. In other words, some scientists and energy educators believe saying energy is "stored" is a misconception. Various Forms of Potential Energy Apple Falling from an Apple Tree Gravitational Potential Energy When something is lifted or suspended in air, work is done on the object against the pull of gravity. This work is converted to a form of potential energy called gravitational potential energy. When the item succumbs to the force of gravity, falling towards Earth like an apple from a tree, it converts potential energy into kinetic energy. Wind-up Toy Elastic Potential Energy Trampoline A stretched rubber band has the potential to do work or change things. This form of energy is called elastic potential energy. It occurs when an object (such as our skin, a spring, a trampoline, or a rubber band) resists being stretched out of shape. The elastic potential energy in a rubber band can be used to do work. For example, toy airplanes fly when a rubber band untwists and spins a propeller. The elastic potential energy in the rubber band was converted into kinetic energy. Chemical Reaction Chemical Potential Energy Battery It would take millions of rubber bands to move a real airplane, so gasoline is used instead. But you don't stretch gasoline to make it work, you burn it. The chemical makeup (arrangement of molecules) of gasoline makes it a good fuel source. All nonliving and living things, from automobiles to zebras, are made up of molecules. It takes energy to make these molecules and hold them together. The energy stored in molecules is called chemical potential energy. During combustion, bonds are broken and reformed creating new products. The energy stored in gasoline is released by burning it (combustion). During combustion, chemical bonds are broken and reformed (changing gasoline into byproducts such as water and carbon dioxide) releasing energy. The airplane motor uses this released energy to turn a propeller. There are many examples of chemical potential energy being converted to kinetic energy to do work. The chemical energy in food is used by our bodies to move. In a lighted firecracker chemical energy is used to make a loud sound and to scatter pieces of the firecracker all over. Power Plant Electrical (Electromagnetic) Potential Energy Electrical Cord A battery has chemical potential energy along with electrical potential energy. When you turn on a device that is battery-operated, such as a flashlight or a toy, the electrical potential energy stored in the battery is converted into other forms of energy such as sound, mechanical motion, thermal energy, and light. For an electrical appliance you plug in, the electrical potential energy is maintained by a spinning generator of a power plant, hydroelectric dam, or a windmill. A solar cell stores electrical potential energy similar to a battery as long as the sun is shining on it. Thermal Energy When you feel a warm object, you are actually feeling thermal energy, which is the movement of molecules that make up the object. All objects possess thermal energy (even cold ones) since they have a temperature above absolute zero. Evidence of thermal energy can be detected by measuring the temperature of an object. Although technically incorrect, the word heat is often used to mean thermal energy. A way to think about this distinction is objects possess thermal energy, while heat is transferred from one object to another. Go to More about Thermal (Heat) Energy for more information. The Evidence of Energy is All Around Us! Sound, mechanical motion, thermal energy, and light are not easily classified as kinetic and potential energy. They are evidence of energy. Evidences of Energy Catching Butterflies (Movement) Radio (Sound) sound Portable Heater (Thermal Energy) thermal energy Table Lamp (Light) light Light is an example of electromagnetic radiation and has no mass, so it has neither kinetic nor potential energy. The remaining forms have qualities of both kinetic and potential energy. Sound is made up of vibrations (put your hand on a stereo speaker), thermal energy consists of moving molecules in air or in an object, and mechanical energy is the combination of kinetic and potential energy of a moving object. A pendulum has mechanical energy; it continually converts kinetic energy into gravitational potential energy and back into kinetic energy as it swings back and forth. A note about thermal energy and heat. In strict scientific terms, there is a distinct difference between heat and thermal energy. Thermal energy pertains to the kinetic energy of the molecules within an object. Heat is the transfer of energy between two objects. Wherever possible, we have tried to remain true to these distinctions. However, since heat is the more familiar term we often use that to facilitate understanding. The Scientific Concept of Energy To understand the concept of energy a little more deeply, one needs to first understand the concept of "work" as defined by the branch of science called physics. Suppose you push something, say, your couch, across your living room floor. Then the measure of the "work" you do, as defined by the branch of science called physics, is equal to the force you pushed with, multiplied by the distance over which you did the pushing: Work = Force x Distance. |