Energy is the capacity to do work. In science, the term work has a very special meaning. It means that an object has been moved through a distance. Thus, pushing a brick across the top of a table is an example of doing work. By applying this definition of work, then, energy can also be defined as the ability to move an object through a distance. Imagine that a bar magnet is placed next to a pile of iron filings (thin slivers of iron metal). The iron filings begin to move toward the iron bar. We say that magnetic energy pulls on the iron filings and causes them to move.
Energy can be a difficult concept to understand. Unlike matter, energy cannot be held or placed on a laboratory bench for study. We know about energy best because of the effect it has on objects around it, as in the case of the bar magnet and iron filings mentioned above.
Energy can exist in many forms, including mechanical, heat, electrical, magnetic, sound, chemical, and nuclear. Although these forms appear to be very different from each other, they often have much in common and can generally be transformed from one to another.
Over time, a number of different units have been used to measure energy. In the British system, for example, the fundamental unit of energy is the foot-pound. One foot-pound is the amount of energy that can move a weight of one pound a distance of one foot. In the metric system, the fundamental unit of energy is the joule (abbreviation: J), named after English scientist James Prescott Joule (1818–1889). A joule is the amount of energy that can move a weight of one newton a distance of one meter.
Potential and kinetic energy
Objects possess energy for one of two reasons: because of their position or because of their motion. The first type of energy is defined as potential energy; the second type of energy is defined as kinetic energy. Think of a baseball sitting on a railing at the top of the Empire State Building. That ball has potential energy because of its ability to fall off the railing and come crashing down onto the street. The potential energy of the baseball—as well as that of any other object—is dependent on two factors: its mass and its height above the ground. The baseball has a relatively small mass, but in this example it still has a large potential energy because of its distance above the ground.
Words to Know
Conservation of energy: A law of physics that says that energy can be transformed from one form to another, but can be neither created nor destroyed.
Joule: The unit of measurement for energy in the metric system.
Kinetic energy: The energy possessed by a body as a result of its motion.
Mass: Measure of the total amount of matter in an object.
Potential energy: The energy possessed by a body as a result of its position.
Velocity: The rate at which the position of an object changes with time, including both the speed and the direction.
The second type of energy, kinetic energy, is a result of an object's motion. The amount of kinetic energy possessed by an object is a function of two variables, its mass and velocity. The formula for kinetic energy is E = ½mv 2 , where m is the mass of the object and v is its velocity. This formula shows that an object can have a lot of kinetic energy for two reasons: it can either be very heavy (large m) or it can be moving very fast (large v).
Imagine that the baseball mentioned previously falls off the Empire State Building. The ball can do a great deal of damage because it has a great deal of kinetic energy. The kinetic energy comes from the very high speed with which the ball is traveling by the time it hits the ground. The baseball may not weigh very much, but its high speed still gives it a great deal of kinetic energy.
Conservation of energy
In science, the term conservation means that the amount of some property is not altered during a chemical or physical change. At one time, physicists believed in the law of conservation of energy. That law states that the amount of energy present at the end of any physical or chemical change is exactly the same as the amount present at the beginning of the change. The form in which the energy appears may be different, but the total amount is constant. Another way to state the law of conservation of energy is that energy is neither created nor destroyed in a chemical or physical change.
As an example, suppose that you turn on an electric heater. A certain amount of electrical energy travels into the heater and is converted to heat. If you measure the amount of electricity entering the heater and the amount of heat given off, the amounts will be the same.
The law of conservation of energy is valid for the vast majority of situations that we encounter in our everyday lives. In the early 1900s, however, German-born American physicist Albert Einstein (1879–1955) made a fascinating discovery. Under certain circumstances, Einstein said, energy can be transformed into matter, and matter can be transformed into energy. Those circumstances are seldom encountered in daily life. When they are, a modified form of the law of conservation of energy applies. That modified form is known as the law of conservation of energy and matter. It says that the total amount of matter and energy is always conserved in any kind of change.
Forms of energy
We know of the existence of energy because of the various forms in which it occurs. When an explosion occurs, air is heated up to very high
Energy can be converted from one form to another, but the process is often very wasteful. An incandescent lightbulb is an example. When a lightbulb is turned on, electrical current flows into the wire filament in the bulb. The filament begins to glow, giving off light. That's what the bulb is designed to do. But most of the electrical energy entering the bulb is used to heat the wire first. That electrical energy is "wasted" since it is lost as heat; the lightbulb is not designed to be a source of heat.
The amount of useful energy obtained from some machine or some process compared to the amount of energy provided to the machine or process is called the energy efficiency of the machine or process. For example, a typical incandescent lightbulb converts about 90 percent of the electrical energy it receives to heat and 10 percent to light. Therefore, the energy efficiency of the lightbulb is said to be 10 percent.
Energy efficiency has come to have a new meaning in recent decades. The term also refers to any method by which the amount of useful energy can be increased in any machine or process. For example, some automobiles can travel 40 miles by burning a single gallon of gasoline, while others can travel only 20 miles per gallon. The energy efficiency achieved by the first car is twice that achieved by the second car.
Until the middle of the twentieth century, most developed nations did not worry very much about energy efficiency. Coal, oil, and natural gas—the fuels from which we get most of our energy—were cheap. It didn't make much difference to Americans and other people around the world if a lot of energy was wasted. We just dug up more coal or found more oil and gas to make more energy.
By the third quarter of the twentieth century, though, that attitude was much less common as people realized that natural resources won't last forever. Architects, automobile and airplane designers, plant managers, and the average home owner were all looking for ways to use energy more efficiently.
temperatures. The hot air expands quickly, knocking down objects in its path. Heat is a form of energy also known as thermal energy. Temperature is a measure of the amount of heat energy contained in an object.
Other forms of energy include electrical energy, magnetism, sound, chemical, and nuclear energy. Although these forms of energy appear to be very different from each other, they are all closely related: one form of energy can be changed into another, different form of energy.
An example of this principle is an electric power generating plant. In such a plant, coal or oil may be burned to boil water. Chemical energy stored in the coal or oil is converted to heat energy in steam. The steam can then be used to operate a turbine, a large fan mounted on a central rod. The steam strikes the fan and causes the rod to turn. Heat energy from the steam is converted to the kinetic energy of the rotating fan. Finally, the turbine runs an electric generator. In the generator, the kinetic energy of the rotating turbine is converted into electrical energy.