Introduction

This unit will facilitate your own exploration of key environmental, social and economic threats that will converge to challenge communities in the near future. You will be required to develop this exploration according to three modes of modelling and communication: verbal, visual, and numeri
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6.1 Review

Let's see if we have made any progress in studying thermal effects. The following SAQ is based on Exercise 3, although this time I have a higher expectation of how much you should be able to do.

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4.3.1 Arrhenius's law

In 1889 Arrhenius, a Swedish chemist, put forward a model to describe the way in which the rates of many chemical reactions could be accelerated by increasing temperature. His model is based on the idea that the rate at which such chemical reactions happen is proportional to the number of particles with enough thermal energy to overcome some sort of energy barrier. In other words, it relates the rate at which things happen to the fraction of particles having energies beyond some threshold ene
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4.3 Thermally activated processes

Thermally activated processes are those that get going not because of average effects, but because the fraction of particles in the tail of the distribution increases with temperature. This is a basic property of the thermal distribution we have been discussing. For instance, what would take 30 000 years at room temperature may happen in under one second at 1000 K if it depends on how many particles have an energy in excess of 1 eV.

The next step in the study of energy distribu
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4.2 Energy distribution

Atoms without much thermal energy will not be doing very much. Consider fifty million million million (50 × 1018) silicon atoms, bonded into a single massive network; I've chosen silicon, but any elemental solid would do. It will be a speck just large enough to be seen without a microscope. You know that if it is heated it will expand, at some stage it will melt and then eventually it will vaporise – that is because thermal energy effectively ‘rattles it to bits’. Having the
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3.3 Thermal stresses

When the temperature of an object increases (say, by ΔT) it expands. According to the linear model of thermal expansion the length increase is described by

What if there is a temperature change, but some constraint prevents the proper thermal size changes? The constraint
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3.2 Room to rattle: modelling thermal expansion

In general, as the temperature of a piece of solid is raised the volume it occupies increases. I say ‘in general’ because as we shall see it is not always the case, and we ought to investigate whether we can exert any control over the phenomenon – which could be useful. Evidently, if a solid expands, the average spacing between its constituent parts must have increased. Since matter is made up of atoms, the issue is really about the volume occupied by the arrangements of atoms that make
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2.2 Thermal effects in outline

Temperature is, of course, the measure of ‘thermal’ conditions. Nowadays it is measured by thermometers and expressed as a number on an agreed scale. Some features of thermometers and of their use are discussed in Thermometers and process control

The theoretical construct of temperature relates it to the kinetic energies of atoms. This gives clear insights into the way temperature affects the behaviour of materials. Energy is given to things to make them hot and taken
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Module team

Dr Peter Lewis (Chair)

Dr George Weidmann (Lecturer in Materials)

Dr Bob Dyson (Senior Lecturer, University of North London)

Richard Black (Microphotographer)

Dr Keith Cavanagh (Editor)

Dr Clive Fetter (Editor)

Sarah Hofton (Designer)

Caryl Hunter-Brown (Technology Librarian)

Gordon Imlach (Technician)

Mike Levers (Photographer)

Laurence Newman (Course Manager)

Jennifer Seabrook (Secretary)

Ian Spratley (BBC)<
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Acknowledgements

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Grateful acknowledgement is made to the following sources for permission to reproduce material in this block:

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6.4.1 Materials selection

Among the common thermoplastics available in the mid-1970s, polypropylene appeared as a front runner on grounds of toughness, density and cost Table 9). However, it is subject to creep (being uncrosslinked) and possesses a low tensile modulus of ca. 1500 MN m−2. Its merit index is 12.7 due to the low density of 0.
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6.4 Case history: the Topper boat

Replacement of one polymeric material by another may be undertaken entirely for manufacturing reasons, and this is what happened in the redesign of the Topper dinghy for thermoplastic polymer. The dinghy was originally designed for hand lay-up GRP in 1969 by Ian Proctor, a well known designer of small boats and yachts (Figure 61
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3.5 Wavelength multiplexing and demultiplexing

Wavelength multiplexers and demultiplexers are needed in order to be able to use wavelength division multiplexing. With just two wavelengths, the multiplexers and demultiplexers can be based on directional couplers because, as mentioned earlier in Section 3.2, couplers are naturally wavelength-de
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2.7 Cabling

A distinction must be made between the optical fibre – a single strand of glass fibre – and the optical-fibre cable consisting of one or more strands of fibre and various protective coverings.

Bare optical fibre is fragile and vulnerable, and the cabling must provide the properties given below.

  • Tensile strength: The cable should prevent the fibre being strained when the cable is under tension. When the cable is being laid, for exampl
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2.6.1 Connectors

Many techniques have been used to design connectors that align the fibre ends accurately with high reliability and a long lifetime. The development of such components, at a low enough price, has been an important part of the overall development which has made fibre a feasible proposition for commercial transmission systems.

With fibre attenuation down to 0.2 dB km−1 (for single-mode fibre), the losses resulting from connectors and splices can be very significant over a whol
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2.5 Non-Linearity

A linear system can be defined in two ways: (1) one which obeys the principle of superposition, and (2) one possessing the frequency-preservation property.

If we consider an optical fibre with electromagnetic field as the input and output, then provided that the power level of the input signal is not too great (less than 1 mW, which is 0 dBm), the fibre may be well modelled by a linear system for most purposes.

When fibre is used for a single point-to-point link to convey a digita
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2.4 Pulse spreading and bandwidth

Activity 3

Calculate the maximum signalling rate given by the Nyquist rate for the 1550 nm window, assuming that it runs from 1450 nm to 1610 nm.

Answer

Using the
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2.3 Attenuation

Activity 2

At approximately what wavelength is the attenuation of optical fibre lowest? What, approximately, is the attenuation at that wavelength? What other wavelengths are used and why?


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1.1 Uses of optical fibre in communication

Using optical fibres, very high data rates (gigabits per second and higher) can be transmitted over long distances (tens of kilometres) without amplifiers or regenerators. As a consequence, optical fibre has completely superseded copper wires as the primary medium for cabled transmission over long distances. Until recently, however, optical fibre has been used less in LANs, where twisted-pair copper cable has been dominant. Similarly, fibre has been slow to penetrate the access network, from
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Introduction

This unit is from our archive and it is an adapted extract from Digital Communications (T305) which is no longer in presentation. If you wish to study formally at The Open University, you may wish to explore the courses we offer in this curriculum area.

By using optical fibre, very high data rates (gigabits per second and higher) can be transmitted over long d
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