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1998 | Buch | 2. Auflage

Foundation Science for Engineers

verfasst von: Keith L. Watson

Verlag: Macmillan Education UK

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SUCHEN

Inhaltsverzeichnis

Frontmatter

Force, Matter and Motion

Frontmatter
Topic 1. Quantities

Engineering quantities (pressure, temperature, power, and so on) need to be expressed in terms of an agreed system of units. SI units Système International d’Unités) have been adopted in the UK and in many other countries, so we shall use them in this book. The system is founded on seven base units and two supplementary units from which all the others are derived.

Keith L. Watson
Topic 2. Forces and Matter

Isaac Newton (1642–1727) put forward three propositions concerning the relationship between the motion of a body and the forces acting upon it. These are known as Newton’s laws of motion. For the moment we shall confine ourselves to the first. (We shall deal with the others in Topic 6.)

Keith L. Watson
Topic 3. Equilibrium

A body is in equilibrium when the forces acting on it balance each other so that it either remains at rest or, if it is moving, remains in a state of uniform motion (i.e. constant speed in a straight line). To understand equilibrium we need to recognise that forces can influence the motion of a body in two ways: they can affect its translational motion from one place to another (with all its parts moving in the same direction) and they can affect its rotation.

Keith L. Watson
Topic 4. Pressure and Upthrust

This topic is mainly concerned with forces that operate in liquids at rest, although some of it refers to gases as well.

Keith L. Watson
Topic 5. Displacement, Velocity and Acceleration

So far our discussion has been about systems that are either at rest or in a state of uniform motion. This topic introduces the idea of accelerated motion but, for the moment, without any reference to the forces that cause it.

Keith L. Watson
Topic 6. Force and Motion

Newton’s first law tells us that an object will remain at rest or in a state of uniform motion unless a force acts on it. In effect, this defines force as an influence which tends to change the velocity of an object. (Remember that the force in question might be the resultant of two or more others.)

Keith L. Watson
Topic 7. Momentum and Impulse

Linear momentum is a physical quantity that provides another approach to the behaviour of objects in motion. It is a vector quantity obtained by multiplying the mass of an object by its velocity. It therefore has the unit kg m s−1. Its direction is the same as that of the velocity of the object. The word linear is used to distinguish it from the angular momentum of a rotating body, which we shall meet later. The use of the word ‘momentum’ alone implies linear momentum, and that is the convention we shall adopt here.

Keith L. Watson
Topic 8. Work, Energy and Power

When an object moves under the influence of a force, then work is done according to the equation 8.1$$W = F \times s$$ where W is the work done, F is the force (N) and s is the displacement (m) in the direction of the force. The SI unit of work is the joule (J), which is a scalar quantity defined as the work done when the point of application of a force of 1 N moves 1 m in the direction along which it is being applied. If the force is applied at an angle to the displacement, as in Figure 8.1, then we must use the magnitude of its component in the displacement direction, in which case Equation (8.1) becomes 8.2$$W = F\cos \theta \times s$$ where θ is the angle which the force makes with the displacement. If θ = 0°, then cos θ = 1 and we have Equation (8.1). If θ = 90°, cos θ = 0, so the force has no component and can do no work in the displacement direction.

Keith L. Watson
Topic 9. Motion in a Circle

So far we have tended to think about objects that are either at rest or moving in straight lines. Now we need to consider circular motion and find angular equivalents of the linear parameters that we have already met.

Keith L. Watson
Topic 10. Rotation of Solids

In the previous topic we considered the translational motion of an object, treating it as a particle moving round a circular path. Now we shall consider a solid object, such as a shaft or a flywheel, rotating about an axis without necessarily moving from one place to another.

Keith L. Watson
Topic 11. Simple Harmonic Motion

Having considered linear, circular and rotational motion, we now move on to vibrational motion, or oscillation, such as that of a pendulum, where an object is displaced from some central equilibrium position, then released so that it oscillates backwards and forwards about it. Such behaviour can often be described in terms of simple harmonic motion, which is characterised by an acceleration towards the equilibrium position that has a magnitude proportional to the displacement from it.

Keith L. Watson
Topic 12. Mechanical Waves

In the previous topic we considered the continuous interchange of potential and kinetic energy in oscillating systems where the total energy remains trapped or would remain trapped in the absence of damping. Figure 11.2 (page 87) shows a wave-like relationship between displacement and time for an isolated oscillating system of this kind.

Keith L. Watson
Topic 13. Electromagnetic Waves

An electromagnetic wave can be considered as a progressive transverse wave that consists of a fluctuating electric field coupled with a fluctuating magnetic field at right angles to it, as shown in Figure 13.1. Don’t worry if this seems a difficult idea at this stage; it will become clearer when we discuss electric and magnetic fields in later topics. For the moment the important thing to remember is that, unlike mechanical waves, electromagnetic waves do not necessarily require a medium for their propagation. They travel through empty space (vacuum) at a speed of very nearly 3 × 108 m s−, commonly called the speed of light (symbol c), and their frequency can be obtained from their wavelength via Equation (12.1) (v = fλ). The speed of light in air is very slightly less than in vacuum but considerably less in some other materials, as we shall see later.

Keith L. Watson

Structure and Poperties of Matter

Frontmatter
Topic 14. Atomic Structure and the Elements

So far our everyday general knowledge of gases, liquids and solids has provided us with sufficient background for our discussion. Now we have reached the point where we need to concern ourselves with the internal structure of matter. We shall start with atoms and see how differences in atomic structure lead to the various chemical elements such as hydrogen, carbon, oxygen, and so on.

Keith L. Watson
Topic 15. The Nucleus

In the previous topic we treated the nucleus as a tiny speck at the centre of the atom that determines its identity, contains most of its mass and keeps its electrons under control. In this topic we shall examine the nucleus in more detail, briefly considering its stability, together with radioactivity and transmutation (transformation of one element into another) by nuclear reactions.

Keith L. Watson
Topic 16. Chemical Bonding

We need to have a basic understanding of chemical bonding, because it plays a central role in determining the behaviour of all substances, including engineering materials.

Keith L. Watson
Topic 17. Heat and Temperature

Whether a particular substance exists as a solid, a liquid or a gas at a given temperature and pressure depends upon the strength of the forces of attraction between its constituent atoms, ions or molecules. Thus, at atmospheric pressure and room temperature the van der Waals’ forces between the oxygen molecules in air are not strong enough to make them stick together, nor are the hydrogen bonds between water molecules strong enough for them to form ice. On the other hand, nearly all metallic and ionic/covalent materials are solids. Furthermore, simple solids tend to turn to liquids, and liquids to gases, if they are heated. These observations seem to point to the idea that the cohesion due to the forces of attraction between atoms, ions and molecules is opposed by the effect of heat.

Keith L. Watson
Topic 18. Heat Transfer

There are three principal mechanisms involved in heat transfer. These are conduction, convection and radiation. (Note that other processes such as evaporation and condensation can also be significant.) Heat transfer is central to many areas of engineering, from domestic refrigerators to nuclear power stations, and very important in others.

Keith L. Watson
Topic 19. Gases

In Topic 17 we noted that the constituent particles in a gas are free to move around independently of one another. The constituent particles of most ordinary gases are molecules. For instance, air consists of roughly 80% N2 and 20% O2 molecules with small quantities of CO2 and other molecular substances. (The minor constituents also include the inert gases, which exist as single atoms because of their stable electronic configurations.)

Keith L. Watson
Topic 20. Liquids

We have seen that the forces of attraction operating in a liquid are able to withstand the disruptive effect of thermal energy to the extent that the constituent particles form a coherent mass but remain capable of movement relative to one another. A liquid therefore has a more or less fixed volume contained within its boundary surface and is capable of flowing.

Keith L. Watson
Topic 21. Solids

We have already seen that the constituent atoms, ions or molecules in a solid material are trapped betwen their neighbours because they have insufficient thermal energy to escape. They are confined to fixed positions on a crystal lattice, held by a network of cohesive forces that tends to oppose any attempt to deform it, and it is this that gives crystalline solids their characteristic rigidity. We shall begin by considering elasticity, which is the property of a solid that tends to return to its original dimensions when it has been deformed.

Keith L. Watson
Topic 22. Structure of Solids

In Topics 23–25 we shall discuss the nature of ceramics, metals and polymers but, before we do, we need to consider how the internal structure of materials is influenced by the type (or types) of chemical bonding involved in holding their constituent atoms together. We shall begin with metals because they are basically very simple.

Keith L. Watson
Topic 23. The Nature of Ceramics

At one time the term ceramics was normally confined to pottery and similar fired clay products. Modern usage of the word often includes artificial non-metallic inorganic materials in general (mostly compounds of metallic and non-metallic elements) including glass, brick, cement and concrete, and many rocks and minerals. Ceramics generally rely on ionic—covalent bonding, which means that the valence electrons are localised. Such materials therefore tend to be poor conductors of heat and electricity. The bonding is relatively strong, in many cases stronger than metallic bonding, with the result that ceramics tend to be resistant to heat and chemicals. A major drawback is their brittleness.

Keith L. Watson
Topic 24. The Nature of Metals

We have already discussed some aspects of the nature of metals. In Topic 16 we saw that the valence electrons in the metallic bond are free to move randomly between the positive ions, thus providing an attractive force that holds the metal together. We noted that the ions do not all have to be of the same kind, which means that different elements can be combined to form alloys. In Topic 18 we saw that the freedom of movement of the valence electrons is responsible for the high thermal conductivity of metals, and in Topic 29 we shall see that it is responsible for their electrical conductivity as well. In Topic 22 we noted that the generally non-specific and non-directional nature of the metallic bond means that, as far as any particular metal atom is concerned, one neighbour is as good as any other. As a result of this, most metals have the ability to undergo plastic deformation before they break because individual metal atoms can change their neighbours without affecting the integrity of the metal as a whole (see Figure 21.6).

Keith L. Watson
Topic 25. The Nature of Polymers

In Topic 16 we saw that the specific and directional nature of the covalent bond leads to the formation of compounds consisting of individual molecules of particular shapes and sizes. We also saw that a carbon atom, with its valency of 4, can form four covalent bonds that tend to be distributed in a tetrahedral configuration — as, for example, in methane. We noted that ethane is the first member of a series of chain-like hydrocarbon molecules. Figure 25.1 suggests that, by successively inserting —CH2— groups into a chain, we could make it as long as we like.

Keith L. Watson

Electricity and Magnetism

Frontmatter
Topic 26. Electric Charge

Electricity and magnetism both stem from electric charge.

Keith L. Watson
Topic 27. Electric Field

In the previous topic we saw that forces exist between electric charges. It follows that a charge must in some way influence the space around itself. This property can be described in terms of an electric field surrounding the charge. Where two or more charges are involved, their fields interact with one another to produce forces.

Keith L. Watson
Topic 28. Capacitance

The creation of an electric field involves separating positive and negative charge. Work has to be done which is then stored in the field as potential energy. Figure 28.1 illustrates this in terms of an electric cell or battery connected across two parallel metal plates. The figure shows the conventional symbol for an electric cell (although the signs are usually omitted). A battery is simply a number of cells connected together to form a single unit.

Keith L. Watson
Topic 29. Electric Current

Electric current provides a very convenient means of transporting energy from place to place. Metal conductors are used to carry it, and all sorts of electrical devices are available to convert it into heat, light or whatever other form of energy is required.

Keith L. Watson
Topic 30. Resistance

In the previous topic we saw how metal conductors tend to resist the flow of electric charge that constitutes an electric current. We have now reached the point where we need to be able to quantity this electrical resistance.

Keith L. Watson
Topic 31. Some Simple Circuits

The purpose of this topic is to develop some of the ideas that we have already met and to broaden our discussion of electrical circuits.

Keith L. Watson
Topic 32. Magnetic Fields

We have already seen that an electric charge gives rise to an electric field. In this topic we shall see that if an electric charge is in motion, it will produce a magnetic field as well.

Keith L. Watson
Topic 33. Electro-Magnetic Induction

In the previous topic we saw how electric current produces motion in a magnetic field. In this topic we shall see how motion in a magnetic field induces electric current.

Keith L. Watson
Topic 34. Magnetic Behaviour of Materials

As Figure 32.4 (page 318) indicates, there is a region of more or less uniform magnetic field inside a solenoid when it carries an electric current. The flux density can be varied by filling the solenoid with a core of material. So-called diamagnetic materials slightly reduce the flux density and paramagnetic materials slightly increase it. On the other hand, ferromagnetic materials increase it greatly, some by a factor of many thousands.

Keith L. Watson
Topic 35. Alternating Current

The transmission of electrical power is more efficient when high voltages are used. To take an example, 100 kW of power is carried by a 1000 A current at 100 V, and by a 100 A current at 1000 V (remembering that 1 W = 1 A × 1 V). Assuming that identical cables of resistance R are used for both, the power loss P will be 100 times greater in the first case than the second because the current I is ten times greater and P = I2R (Equation 30.12 on page 298). Since transformers provide a very efficient means of stepping the voltage up or down, it makes good sense to use alternating current to transmit electrical power over long distances. In practice, enormous voltages are used for this purpose, sometimes as high as 400 kV.

Keith L. Watson
Backmatter
Metadaten
Titel
Foundation Science for Engineers
verfasst von
Keith L. Watson
Copyright-Jahr
1998
Verlag
Macmillan Education UK
Electronic ISBN
978-1-349-14714-4
Print ISBN
978-0-333-72545-0
DOI
https://doi.org/10.1007/978-1-349-14714-4