What is energy? There is general awareness of the problem of depletion of the world’s energy resources. People understand energy in terms of those re­sources, namely, the supplies of oil, gas, and coal and the electricity derived from them. All of these items have made an increasingly large demand on na­tional and personal budgets.

The engineer has, by training, a somewhat different concept of energy. This de­rives from his or her undergraduate training in the field of thermod^ynamics, which is the science of energy and energy conversion. We do not intend to try to provide a basic course in thermodynamics; however, for the rest of this ^юк to be rea­sonably intelligible, it is important that some of the basic concepts be stated.

The concept of doing work to lift objects or to move an object such as a bi­cycle along is a commonly accepted one. Thus, it is relatively easy to under­stand the concept of energy as a measure of the ability to do work. Energy can appear in different forms as follows:

1. Kinetic Energy. This is energy associated with movement, for example, that of a flywheel or a moving locomotive.

2. Potential Energy. This is energy possessed by virtue of position, typically in the earth’s gravitational field. For instance, a child sitting on the higher end of a seesaw has greater potential energy than a child sitting on the lower end. Likewise, water in a mountain lake has greater potential energy than water at sea level.

3. Chemical Energy. Matter consists of atoms that are combined together in molecules. Molecules of different substances can react to release energy,

and this releasable energy is often termed chemical energy. For example, chemical energy is released when gasoline combines with air in the cylin­ders of a car’s engine.

4. Electrical Energy. Atoms consist of a central mass, known as the nucleus, around which a cloud of electrons circulates (see Figure 1.1). If there is an excess or deficit of electrons in one part of a body, the body is said to have an electrical charge and, by virtue of this, to have electrical energy. An ex­ample of this is a thunderstorm, where the clouds are charged electrically with respect to the ground.

5. Nuclear Energy. Normally, the nucleus of an atom is stable and will re­main indefinitely in its present state. An example is the nucleus of an atom of iron; no matter how much we would like it to happen, iron will never change into another element, such as gold. However, the atoms of some el­ements are unstable and can change into another form spontaneously, by the emission of radiation. We shall discuss the forms of radiation emitted further in Section 1.2; it is sufficient here to note that the radiation emitted has kinetic energy and the disintegration process results in the release of energy associated with the nucleus, namely, the nuclear energy. If the nu­cleus could be weighed before the disintegration, and the resulting nucleus and all particulate components of the radiation weighed afterward, it would be observed that a small change in mass had occurred due to the conver-

sion of mass into energy. The relationship between the loss of mass m and the energy released E is given by Einstein’s famous equation:

E = mc2

where c is the velocity of light, namely 300,000 kilometers per second (186,000 miles per second). The amount of energy deriving from a mass loss is enormous; for example, 100 kilograms of mass fully converted into energy would supply all the energy needs of the United Kingdom (at the present rate of usage) for a year. Each kilogram of mass, fully converted, is equivalent to the energy available by burning 3 million tons of coal. In a typical nuclear re­action, however, only a tiny fraction of the mass is converted into energy, typ­ically -0. 1 %. The disintegration of an unstable nucleus, and the consequent release of nuclear energy, can be stimulated by exciting the nucleus by bom­barding it with radiation. This is at the heart of the fission reaction process, which we shall discuss further below. Nuclear energy can also be released, as we shall see, by the fusion of very light atoms into heavier ones.

6. П^^ші Entergy. The atoms of all substances are in constant motion. In a solid the atoms are held in an approximately fixed position with respect to one another. However, they all vibrate to an extent that increases with in­creasing temperature. The energy associated with this vibration is called thermal energy. In fluids (namely, liquids and gases), two or more atoms may be combined with each other chemically in the form of molecules. These molecules have vibrational energy, but in the fluid state they may also have translational energy arising from their motion in space and rota­tional energy arising from their rotation. All of these components of energy add up to the thermal energy of the fluid. It will be seen from this descrip­tion that thermal energy is of a special type. It is associated with atomic or molecular movements that are randomly directed. This makes it very much more difficult to convert thermal energy into other forms of energy, as we shall see below.

The intensity of atomic or molecular movement is a measure of the energy content of a piece of matter. A body that has a high intensity of atomic or mol­ecular movement will transfer energy to an adjacent body with a lower intensity of movement. This process of transfer of thermal energy is known as conduc­tion, and we define a quantity known as temperature as a measure of the abil­

ity of a body to transfer thermal energy to adjacent bodies by the conduction process. If the temperature of a body is higher than that of adjacent bodies, heat will be conducted from it; if it is lower, the reverse is true. We conveniently choose a scale of temperature in terms of certain transitions that occur in na­ture. Specifically, we define the melting point of ice as zero degrees centigrade (0°C) and the boiling point of water as 100 degrees centigrade. In energy con­version processes involving thermal energy, it is convenient to define an alter­native temperature scale, commonly referred to as the scale of absolute temperature. Here, the measure of temperature is the kelvin (K) rather than the degree centigrade. Zero kelvin corresponds to -273.17°C and is the condition in which all atomic and molecular motions have effectively ceased.

In a system that does not receive energy from or emit energy to the outside, the total amount of energy can be increased only by converting mass into en­ergy via nuclear processes. In the absence of these processes, the total amount of energy remains constant (this is the basis of the first law of thermodynamics). However, within the given system, the form of energy may change (e. g., chem­ical energy may be converted into thermal energy or thermal energy may be converted into mechanical energy). Before discussing these conversion processes, we shall digress briefly to discuss and explain the units by which en­ergy is measured, since these are vital in what follows in this book.