Energy in Perspective (eBook)

Jerry B. Marion (Autor)

eBook Download: EPUB

2013 | 1. Auflage
220 Seiten
Elsevier Science (Verlag)
978-1-4832-6021-1 (ISBN)

Infos

Energy in Perspective attempts to place the 1973 ''energy crisis'' in perspective. It discusses sources of energy, its uses, and the projections for the future. It is concerned primarily with the patterns of energy consumption, the fuels required to produce this energy, and the effect that energy usage is having on the environment. It examines the overall situation and discusses both the short-term problems and the long-term outlook. Emphasis is given to questions of fuel supplies and new energy technologies rather than crisis remedies such as gasoline rationing, reduced speed limits, and fuel oil allocations. The book also engages in an exercise of ''futurism'': How much energy will be needed in the year 2000? In the year 2050? How much fuel can be extracted from the Earth by these dates? What new technologies will be available in 25 or 75 years? This book is intended for use in classroom courses as a text or supplementary text and for individual reading. It is not intended as a sourcebook of new and authoritative data. The figures, estimates, and projections given here are not original; instead, they represent what the author believes to be the most reliable information and the most reasonable projections available at present.

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Energy in Perspective attempts to place the 1973 "e;"e;energy crisis"e;"e; in perspective. It discusses sources of energy, its uses, and the projections for the future. It is concerned primarily with the patterns of energy consumption, the fuels required to produce this energy, and the effect that energy usage is having on the environment. It examines the overall situation and discusses both the short-term problems and the long-term outlook. Emphasis is given to questions of fuel supplies and new energy technologies rather than crisis remedies such as gasoline rationing, reduced speed limits, and fuel oil allocations. The book also engages in an exercise of "e;"e;futurism"e;"e;: How much energy will be needed in the year 2000? In the year 2050? How much fuel can be extracted from the Earth by these dates? What new technologies will be available in 25 or 75 years? This book is intended for use in classroom courses as a text or supplementary text and for individual reading. It is not intended as a sourcebook of new and authoritative data. The figures, estimates, and projections given here are not original; instead, they represent what the author believes to be the most reliable information and the most reasonable projections available at present.

Textauszug

Chapter 2

WORK, ENERGY, AND POWER

Publisher Summary

This chapter discusses the physical principles that govern situations involving energy. Only a few of the large number of terms used to describe situations involving energy apply to physical quantities—work, energy, and power. In its physical meaning, work always involves overcoming some opposing force. The amount of work done in any situation depends on how much force was exerted and on how far the object moved. Increasing either the applied force or the distance through which the object is moved increases the amount of work done. That is, the work done is proportional to both the applied force and the distance through which the force acts. When an object is moved against a force, work is done and energy is expended in the process by the agency responsible for the movement. The energy that an object possesses by virtue of its motion is called kinetic energy. The more massive the object is and the faster it moves, the greater is its kinetic energy. Power is the rate at which work is done or the rate at which energy is used.

We all have some intuitive notions about the quantity that is the central topic of this book–energy. We know that we must buy gasoline to supply the energy that runs our automobiles, and we pay a monthly bill to the electric company for the electrical energy that is delivered to our homes. We understand that coal, oil, and gas play important roles in supplying the energy that is necessary for our everyday living. But to pursue our topic in detail we need more than these qualitative ideas. We need to understand some of the basic physical principles that govern situations involving energy.

Before we can begin a meaningful discussion of energy problems, we must establish the language we will use. That is, we must define the terms and the units that are necessary to describe various situations involving energy. We will require only a few of the large number of the terms that apply to physical quantities–primarily, work, energy, and power. The units we will use are metric units–meters, kilograms, and seconds, as well as a few derived units such as watts and kilowatt-hours. Thus, we will employ only a limited vocabulary, one designed to cover only the situations of immediate interest.

THE DEFINITION OF WORK

We frequently use the term work in ordinary conversation. We might say, for example, “That job requires a great deal of work.” What does “work” really mean here? If you lift a number of heavy boxes from floor level and place them on a high shelf, you will feel tired after the job is completed–you will know that you have done work. This is exactly right. Gravity pulls the boxes downward and when you lift the boxes, you are doing work against the gravitational force.

In its physical meaning, work always involves overcoming some opposing force. Suppose that instead of lifting one of the boxes, you push it across a rough floor. In this case, you are not working against the gravitational force–the box is at the same height throughout the movement. Instead, you are now working against the frictional force that exists between the moving box and the floor.

How do we measure work? The amount of work done in any situation depends on how much force was exerted and on how far the object moved. Increasing either the applied force or the distance through which the object is moved increases the amount of work done. That is, the work done is proportional to both the applied force and the distance through which the force acts (Fig. 2.1). The equation which expresses this statement is

Figure 2.1 The work done by the force F is W = Fd.

(2.1)

In this equation, d stands for the distance of movement, measured in meters (m), and F stands for the applied force. According to Newton’s law of dynamics, F = Ma, the force F necessary to impart an acceleration a of 1 meter per second per second (1 m/s2) to a mass M of 1 kilogram (1 kg), is 1 kg-m/s2. To this unit we give the special name, 1 newton (1 N). Therefore, in Eq. 2.1, we have

F = force (in newtons)

d = distance (in meters)

W = work done (in newton-meters)

We give to the unit of work the special name joule:

(2.2)

How much work must be done to lift a block of mass M through a vertical height h? In this case, work is done against the gravitational force. The magnitude of this force is the weight of the object and is given by Newton’s equation F = Ma, when we identify a as the acceleration due to gravity. We usually indicate the gravitational acceleration by the symbol g, so that the expression for the weight of an object (the gravitational force acting on the object) is

(2.3)

The value of g on or near the surface of the Earth is 9.8 m/s2.

Now, we can use Eq. 2.1 to write the work required to lift a block of mass M through a vertical height h:

That is,

(2.4)

If the mass is M = 10 kg and the height is h = 3 m, the work done is

A mass of 1 kg corresponds to 2.2 pounds (lb) and 1 m is a bit more than 3 feet (ft). Therefore, the amount of work done in this example corresponds approximately to that required to lift a 22-lb mass to the height of a basketball basket (10 ft).

ENERGY

When an object is moved against a force, work is done and energy is expended in the process by the agency responsible for the movement. Thus, we say, “A person must have a lot of energy to do a hard day’s work.” In fact, one way to define energy is

Suppose that a cart is rolling at constant velocity v across a floor and strikes a block at rest on the floor (Fig. 2.2). As a result of the collision, the block will slide a certain distance d across the floor before coming to rest because of friction. The sliding block has moved against the frictional force and has therefore done a certain amount of work.

Figure 2.2 The kinetic energy of the moving cart is transferred to the block in a collision and the block slides across the floor. The sliding block does work against the frictional force.

The block moved and did work because energy was supplied to it by the moving cart. The energy that an object possesses by virtue of its motion is called kinetic energy. The more massive the object is and the faster it moves, the greater is its kinetic energy. The expression for the kinetic energy of an object with a mass M moving with a velocity v is

(2.5)

Notice that the kinetic energy depends on the square of the velocity. A block moving with a velocity of 16 m/s has a kinetic energy 4 times greater than when it is moving with a velocity of 8 m/s.

What is the kinetic energy of a 12-kg object when it moves with a velocity of 7 m/s? Using Eq. 2.5,

which turns out to be exactly enough energy to raise a 10-kg object to a height of 3 m, as we found in the preceding section.

Notice that kinetic energy and work have the same units, namely, joules. We can see this more clearly by writing the units for the various physical quantities in the expressions for work and kinetic energy:

In the preceding section we considered lifting a mass M to a height h. We found that the work done in such a case is W = Mgh. The object was originally at rest and in its final position the velocity is again zero. Thus, no kinetic energy was imparted to the object. But the object has a capability to do work that it did not have in its original position. For example, if we drop the object and allow it to fall through the height h, work can be done in driving a stake into the ground (Fig. 2.3). That is, the raised block has the potential to do work and we call this capability the potential energy of the object:

Figure 2.3 The potential energy of a raised block can be converted into work.

(2.6)

In falling toward the stake, the block loses potential energy (because h decreases), but it gains kinetic energy (because v increases). During the fall, the energy of the block is partly potential energy and partly kinetic energy. Just before striking the stake, all of the potential energy, Mgh, has been converted into kinetic energy so that 1/2 Mv2 = Mgh. The potential energy is first converted into kinetic energy and then the kinetic energy is converted into work in driving the stake into the ground.

An appreciation for the amount of energy that is involved in various physical processes can be obtained by referring to Table 2.1. Notice that we use here (and throughout the remainder of this book) the exponential or...

Produkt-Details

• Erscheint lt. Verlag: 3.9.2013
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Physik / Astronomie
• Themenwelt: Recht / Steuern ► Wirtschaftsrecht
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• Themenwelt: Wirtschaft ► Betriebswirtschaft / Management ► Rechnungswesen / Bilanzen
• Themenwelt: Betriebswirtschaft / Management ► Spezielle Betriebswirtschaftslehre ► Immobilienwirtschaft
• ISBN-10: 1-4832-6021-6 / 1483260216
• ISBN-13: 978-1-4832-6021-1 / 9781483260211