The energy we sense all around us seems to have many forms. Fires burn, rubber bands snap back, flashlights turn on and shine, objects fall, and things appear to be hot, cold, or just the right temperature. With so much going on, how can we figure out what forms of energy these things and events have? After hundreds of years of observation and experimentation, science has classified energy into two main forms: kinetic energy and potential energy. In addition, potential energy takes several forms of its own.
Kinetic energy is defined as the energy of a moving object. A thrown football, a speeding automobile, a marathon runner, or a rock falling from a cliff, are examples of objects that have kinetic energy.
Potential energy is defined as the energy associated with the arrangement of a system of objects that exert forces on one another. Potential energy is stored or released when the arrangement of the objects and/or the forces they exert on each other changes in some way. Systems of objects ranging from atoms to planets can be arranged in many ways, resulting in many forms of potential energy: chemical, elastic, electrical (electromagnetic), gravitational, nuclear, and thermal energy.
Although potential energy is often referred to as “stored” energy, two misconceptions may arise when referred to in this way. First, saying that energy is stored in something may imply that energy is some sort of invisible substance, which it isn’t. Second, consider a textbook held above a classroom floor. Many would say that the textbook has potential energy stored within it, and it is often convenient to think that only the textbook has potential energy. However, the textbook has potential energy because it is part of a system that includes the Earth, one in which both exert gravitational forces on each other. In other words, the textbook would not have potential energy if it were not for the Earth.
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. 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.
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.
Electrical potential energy is present when positive and negative electric charges are separated from each other, as in a battery. 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 windmill. A solar cell stores electrical potential energy similar to a battery as long as the sun is shining on it.
When something is lifted, 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 falls towards Earth like an apple from a tree, gravitational potential energy is converted into kinetic energy.
Nuclear potential energy, or nuclear energy, is the energy found in the nuclei of atoms. The protons in a nucleus exert very strong repulsive forces on each other due to their positive charge. In turn, the neutrons that make up a nucleus exert very strong forces on the protons to keep them from flying apart. Splitting a large nucleus (nuclear fission) or combining small nuclei into a larger nucleus (nuclear fusion) overcomes these strong forces and releases energy as a result. The energy released by an individual nucleus is very small, but huge numbers of nuclei can release huge amounts of energy, as evidenced by the energy produced by nuclear reactors (fission) and by the sun (nuclear fusion).
Thermal energy is due to the movement and the kinetic energy associated with this movement of molecules that make up an object or substance. 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. In strict scientific terms, there is a distinct difference between the two. 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.
Sound, mechanical motion, heat, and light are not easily classified as kinetic and potential energy. They are evidence of energy.
Movement, sound, heat, and light provide evidence that energy is present and being used. Sound is produced when we strike something. But does sound do work? Yes, sound can move things. Sound waves move the tiny bones in your ear and shake windows when a loud truck passes by. Sound waves are also evident in the vibrations from playing a radio.
Our body is working even when it appears to be still. Breathing, blinking and digesting food all require energy. For us to do these activities, our bodies burn the energy in food. We know this is happening because we feel warm (burning generates thermal energy [heat]). Therefore, heat is evidence that energy is being used.
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 the air or in an object, and mechanical energy is the combination of the 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.
Light is another observable form of energy. Light can change things. When light shines on your arm, it makes it feel warm. When light shines on a green plant, the plant can make food.