Chapter 4: Matter and Energy in the Universe

The scientific process of photosynthesis, which is a way that plants receive energy.

The next time you drive your car, consider the source of its energy. Millions of years ago, radiation flowing through space from the Sun was intercepted by the Earth. That sunlight allowed plants to grow, and after the plants died and were buried deep underground, the stored energy was turned into petroleum. Humans pumped up the petroleum and refined it into the gasoline that fills your tank. The stored chemical energy is being transformed into forward motion by your engine. As you stop to ponder this, your hot engine cools. The heat energy it releases rises through the atmosphere and eventually escapes and continues its journey through space.

Scientists define energy as the ability to do work. Perhaps a more intuitive way to express this is to say that energy is anything that can cause a change. This sounds vague but in fact, scientists have found concrete and quantitative ways to define energy. Energy comes in various forms, many of which are familiar to us in the everyday world. Scientists in the 19th century had the great challenge of showing how these forms are related. Scientists try to look for patterns and relationships in nature and describe them with simple physical laws. Energy is a simplifying concept that allows us to relate phenomena that are on the face of it very different.

An example of potential energy.

One broad type of energy is called potential energy. We can think of this as stored energy or energy waiting to be released. We have already discussed the basis of chemical energy. When paper or wood burns, the oxygen in the atmosphere reacts with the carbon to release energy. In the case of gasoline, the combustion in your engine releases the energy stored in long molecules of hydrogen and carbon. Chemical energy is released when electrons in atoms change their configuration. This type of stored energy can be substantial. In addition to being the source of power in gasoline and dynamite, chemical energy powers all the rockets of the world's space program.

Another major type of potential energy is the ability of gravity to cause a change. Gravitational energy is a form of energy that depends on an object's response to gravity. If we pick up a brick up off the floor and put it on the edge of a table, we have used our own energy by moving the brick upward in the Earth's gravity. That energy is stored in the brick as gravitational energy. If the brick is knocked off the table, it will release its gravitational energy as motion. A falling brick clearly has the ability to cause a change! The higher a body is raised against gravity, the more gravitational energy it has because it has more potential to release energy as it crashes downward. Hydroelectric power plants, for example, convert the energy of water falling over a dam into electrical energy.

A very familiar type of potential energy is mechanical energy, like the energy stored in a coiled spring or a stretched rubber band. There are two other types of potential energy that we will encounter often in astronomy. Changing electric and magnetic forces create radiation that can travel through space or a thin gas. Light is one example of electromagnetic energy. Humans have learned to create light in various ways, but the Sun is by far the most bountiful source of this type of electromagnetic energy. We will also discover that energy is stored in the form of mass. When the "frozen" energy stored as mass is released, it is the most efficient energy source known.

The cars of a roller coaster reach their maximum kinetic energy when at the bottom of their path. When they start rising, the kinetic energy begins to be converted to gravitational potential energy. The sum of kinetic and potential energy in the system remains constant, ignoring losses to friction.

The second broad type of energy is kinetic energy, which is the energy of motion. Think of a flying bird, a falling tree, a speeding car, and a running child. All of these have kinetic energy. The more massive or faster the object is, the more kinetic energy it has. For example, a bowling ball rolling across the floor has a greater kinetic energy than a billiard ball moving at the same speed. A car moving at 60 mph has more kinetic energy than the same car moving at 20 mph.

Energy is only a useful concept in science if it can be quantified. In ordinary speech, we use the word "energy" in many ways. For example, the term "chi" from Chinese medicine is translated as "energy," but means something closer to bodily vitality and wellbeing. People speak of "focusing your energy" or "putting mental energy" into some effort, and a mystic might speak of "psychic energy." These concepts may or may be meaningful but they are definitely not quantitative. The scientific concept of energy is very specifically defined. Kinetic energy is given by a specific formula, and another formula describes gravitational energy. Energy is measured in a single set of units, called calories in the English system of units, or Joules in the International System of Units (abbreviated SI after the French term for this system).