6.1: What is Energy?

Learning Objectives

By the end of this chapter, students will be able to

• List different types of energy
• Explain the connection between force and energy
• Calculate transformations of energy from one type to another

Watts Bar Nuclear Power Plant in East Tennessee. Image from Tennessee Valley Authority (Public Domain).

Energy is the ability of a system to do work. A system has done work if it has exerted a force on another system over some distance. When this happens, energy is transferred from one system to another. At least some of the energy is also transformed from one type to another during this process. One can keep track of how much energy transfers into or out of a system. There are two categories that all energy falls into: kinetic and potential. Kinetic energy refers to types of energy associated with motion (Figure \(\PageIndex{1}\), top). For example, a rock rolling down a hill, the wind blowing through trees, water flowing over a dam, and a cyclist riding a bicycle are just a few examples of kinetic energy. Potential energy is energy possessed by an object or system due to its position in space relative to another object or system and forces between the two (Figure \(\PageIndex{1}\), bottom). Examples include a rock poised at the top of a hill and water stored behind a dam. Some forms of energy are part kinetic and part potential energy. Chemical energy describes the potential of a chemical substance to undergo a chemical reaction and transform other chemical substances; hence it is a form of potential energy. Examples include energy stored in the food you eat and the gasoline that you put in your car. Nuclear energy is the energy stored in the nucleus of an atom. This energy can be released in radioactive decay, fission, or fusion. Radiant energy is potential energy that is in electromagnetic waves (light). There are many different categories of electromagnetic waves, each with a particular range of values for their energies. Visible light is the most common type of electromagnetic wave that we experience. Heat (thermal) energy is energy that moves between objects that have different temperatures. The so-called “useful” types of energy are generally potential energy and kinetic energy. Heat can be utilized to do useful work, but is also where a lot of useful energy is wasted and cannot be used again. For example, when two objects rub together, the force of friction generates heat. We consider that heat to be wasted energy, as it cannot be utilized as useful work. The discussion of thermal energy and its transformations into other forms will be discussed later in this textbook.

Some of the types of energy described above are not unique forms of energy. In reality, the only kinds of energy that exist come from the four fundamental forces in nature. These forces are gravitational force, electromagnetic force, weak nuclear force, and strong nuclear force. In section 6.2. we will see the connection between force and energy. However, for the remainder of the text, we will ignore the particular types of forces that are the source of energy.

Figure \(\PageIndex{1}\): Examples kinetic (top) and potential (bottom) forms of energy. All images were obtained from Wikimedia commons (public domain).

Living organisms need energy to perform life-sustaining “work” in order to survive. For nearly all living systems on Earth, the sun is the ultimate source of that energy. Over time, we humans have developed an understanding of energy that has allowed us to harness it for uses well beyond basic survival. The development and evolution of human society is largely attributed to our relationship with energy. The first major advancement in human understanding of energy was the mastery of fire for cooking and heating. Modern civilization is especially dependent on energy and some of its most distinct characteristics such as population growth, environmental impact and climate change are all a consequence of energy use. We use energy to heat and light our homes; power our machinery; fuel our vehicles; produce plastics, pharmaceuticals, and synthetic fibers; and provide the comforts and conveniences to which we have grown accustomed in the industrial age. Societal complexity, affluence, and the gap between poor and rich peoples are all related to our level of energy consumption.