Energy
Energy
Sound, chemical, and nuclear energy
Energy is commonly defined as the capacity to do work. Since work is defined as the action of a force through a distance, energy can also be described as the ability to make a force act through a distance. For example, a rocket motor exerts force against a rocket, accelerating it through space. In doing so, the rocket expends energy. A rocket can move through space at any velocity without expending energy, as long as its motor is not firing. Likewise, lifting a stone requires energy, because a force equal to or greater than the weight of the stone must be exerted through the height the stone is moved; but a falling stone expends no energy.
Unlike matter, energy can not be taken hold of or placed on a laboratory bench for study. We know the nature and characteristics of 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 is described 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 into one 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 the 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
Every object has energy as a consequence of its position in space and/or its motion. For example, a baseball poised on a railing at the top of the observation deck on the Empire State Building 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 formula for potential energy is p.e. = m× g× h, where m stands for mass, h for height above the ground, and g for the gravitational constant (9.8 m per second per second).
Potential energy of position is a manifestation of the gravitational attraction of two bodies for each other. The baseball on top of the Empire State Building has potential energy because of the gravitational force that tends to bring the ball and Earth together. When the ball falls, both Earth and ball are actually moving toward each other. Since Earth is so many times more massive than the ball, however, we do not see its very minute motion.
When an object falls, at least part of its potential energy is converted to kinetic energy, the energy due to an object’s motion. The amount of kinetic energy possessed by an object is a function of two variables, its mass and its velocity. The formula for kinetic energy is k.e. = 1/2m× v2, 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 or it can be moving very fast. For that reason, a fairly light baseball falling over a very great distance and traveling at a very great speed can do as much damage as a much more massive object falling at a slower speed.
Conservation of energy
The sum total of an object’s potential and kinetic energy is known as its mechanical energy. The total amount of mechanical energy possessed by a body is a constant. The baseball described above has a maximum potential energy and minimum kinetic energy (actually a zero kinetic energy) while at rest. In the fraction of a second before the ball has struck the ground, its kinetic energy has become a maximum and its potential energy has reached almost zero.
The case of the falling baseball described above is a special interest of a more general rule known as the law of conservation of energy. According to this law, energy can never be created or destroyed. In other words, the total amount of energy available in the universe remains constant and can never increase or decrease.
Although energy can never be created or destroyed, it can be transformed into new forms. In an electric iron, for example, an electrical current flows through metallic coils within the iron. As it does so, the current experiences resistance from the metallic coils and is converted into a different form, heat. A television set is another device that operates by the transformation of energy. An electrical beam from the back of the television tube strikes a thin layer of chemicals on the television screen, causing them to glow. In this case, electrical energy is converted into light. All of the modern appliances that we use transform of energy from one form to another.
In the early 1900s, Albert Einstein announced perhaps the most surprising energy transformation of all. Einstein showed by mathematical reasoning that energy can be converted into matter and vice versa. He expressed the equivalence of matter and energy in a now famous equation, E = mc 2, where E is energy, m is mass, and c is a constant, the speed of light.
Forms of energy
The operation of a steam engine is an example of heat being used as a source of energy. Hot steam is pumped into a cylinder, forcing a piston to move within the cylinder. When the steam cools off and changes back to water, the piston returns to its original position. The cycle is then repeated. The up-and-down motion of the piston is used to turn a wheel or do some other kind of work. In this example, the heat of the hot steam is used to do work on the wheel or some other object.
The source of heat energy is the motion of molecules within a substance. In the example above, steam is said to be “hot” because the particles of which it is made are moving very rapidly. When those particles slow down, the steam has less energy. The total amount of energy contained within any body as a consequence of particle motion is called the body’s thermal energy.
One measure of the amount of particle motion within a body is temperature. Temperature is a measure of the average kinetic energy of the particles within the body. An object in which particles are moving very rapidly on average has a high temperature. One in which particles are moving slowly on average has a low temperature.
Temperature and thermal energy are different concepts, however, because temperature measures only the average kinetic energy of particles, while thermal energy measures the total amount of energy in an object. A thimbleful of water and a swimming pool of water might both have the same temperature, that is, the average kinetic energy of water molecules in both might be the same. But there is a great deal more water in the swimming pool, so the total thermal energy in it is much greater than the thermal energy of water in the thimble.
Electrical energy
Suppose that two ping pong balls, each carrying an electrical charge, are placed near to each other. If free to move, the two balls have a tendency either to roll toward each other or away from each other, depending on the charges. If the charges they carry are the same (both positive or both negative), the two balls will repel each other and roll away from each other. If they charges are opposite, the balls will attract each other and roll toward each other. The force of attraction or repulsion of the two balls is a manifestation of the electrical potential energy existing between the two balls.
Electrical potential energy is analogous to gravitational energy. In the case of the latter, any two bodies in the universe exert a force of attraction on each other that depends on the masses of the two bodies and the distance between them. Any two charged bodies in the universe, on the other hand, experience a force of attraction or repulsion (depending on their signs) that depends on the magnitude of their charges and the distance separating them. A lightning bolt traveling from the ground to a cloud is an example of electrical potential energy that has suddenly been converted to it “kinetic” form, an electrical current.
An electrical current consists of moving electrical charges and flows any time three conditions are met. First, there must be a source of electrical charges. A battery is a familiar source of electrical charges. Second, there must be a pathway through which the electric charges can flow. The pathway is known as a circuit. Third, there must usually be an electric field to cause the charges to move.
An electric current is useful, however, only if a third condition is met—the presence of some kind of device that can be operated by it. For example, one might insert a radio into the circuit through which electrical charges are flowing. When that happens, the electrical charges flow through the radio and make it produce sounds. That is, electrical energy is transformed into sound energy within the radio.
Magnetic energy
A magnet is a piece of metal that has the ability to attract iron, nickel, cobalt, or certain specific other kinds of metal. Every magnet contains two distinct regions, one known as the north pole and one, the south pole. As with electrical charges, unlike poles attract each other and like poles repel each other.
Masses set up gravitational fields, charges set up electric fields, and magnets set up magnetic fields. A field is physical influence in a region in space that can cause a magnetic, electrical, or other kind of force to be experienced. For example, imagine that a piece of iron is placed at a distance of 2 in (5 cm) from a bar magnet. If the magnet is strong enough, it may pull on the iron strongly enough to cause it to move. The piece of iron is said to be within the magnetic field of the bar magnet.
The concept of a field was, at one time, a difficult one for scientists to accept. How could one object exert a force on another object if the two were not in contact with each other?
One of the great discoveries in the history of physics was made by the English physicist James Clerk Maxwell (1831–1879) in the late nineteenth century. Maxwell found that the two major forms of energy known as electricity and magnetism are not really different from each other, but are instead closely associated with each other. That is, every electrical current has associated with it a magnetic field and every changing magnetic field creates its own electrical current.
As a result of Maxwell’s work, it is often more correct to speak of electromagnetic energy, a form of energy that has both electrical and magnetic components. Scientists now know that a number of seemingly different types of energy are all actually forms of electromagnetic energy. These include x rays, gamma rays, ultraviolet light, visible light, infrared radiation, radio waves, and microwaves. These forms of electromagnetic energy differ from each other in terms of the wavelength and frequency of the energy wave on which they travel. The waves associated with x rays, for example, have very short wavelengths and very high frequencies, while the waves associated with microwaves have much longer wavelengths and much lower frequencies.
Sound, chemical, and nuclear energy
When sound is created, sound waves travel through space, creating compressions (areas of higher pressure) in some regions and rarefactions (areas of lower pressure) in other regions. When these sound waves strike the human eardrum, they cause the drum to vibrate, creating the sensation of sound in the brain. Similar kinds of sound waves are responsible for the destruction caused by explosions. The sound waves collide with building, trees, people, and other objects, causing damage to them.
Chemical energy is a form of energy that results from the forces of attraction that hold atoms and other particles together in molecules. In water, for example, hydrogen atoms are joined to oxygen atoms by means of strong forces known as chemical bonds. If those are broken, the forces are released in the form of chemical energy. When a substance is burned, chemical energy is released. Burning (combustion or oxidation) is the process by which chemical bonds in a fuel and in oxygen molecules are broken and new chemical bonds are formed. The total energy in the new chemical bonds is less than it was in the original chemical bonds, and the difference is released in the form of chemical energy.
Nuclear energy is similar to chemical energy except that the bonds involved are those that hold together the particles of a nucleus, protons and neutrons. The fact that most atomic nuclei are stable is proof that some very strong nuclear forces exist. Protons are positively charged and one would expect that they would repel each other, blowing apart a nucleus. Since that does not happen, some kinds of force must exist to hold the nucleus together.
One such force is known as the strong force. If something happens to cause a nucleus to break apart, the strong force holding two protons together is released in the form of nuclear energy. That is what happens in an atomic (fission) bomb. A uranium nucleus breaks apart into two roughly equal pieces,
KEY TERMS
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.
Field— A region in space in which a magnetic, electrical, or some other kind of force can be experienced.
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.
Magnetic pole— The region of a magnetic in which magnetic force appears to be concentrated.
Potential energy— The energy possessed by a body as a result of its position.
Temperature— A measure of the average kinetic energy of all the elementary particles in a sample of matter.
Thermal energy— The total amount of energy contained within any body as a consequence of the motion of its particles.
and some of the strong force holding protons together is released as nuclear energy. The resulting fragments weigh slightly less than the original atom, and the mass lost is equal to the energy released.
David E. Newton