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4.3.3 Termination and transfer There are basically three ways in which chains terminate. The first is known as coupling and occurs when two free radicals join together. This can be represented by the general equation Such a mechanism significantly increases molecular mass, if it results in
3.2.1 Thermal cracking The bulk of the major monomer and intermediate, ethylene (C2H4), is still produced in the UK by steam cracking without the use of catalysts. Paraffinic feedstocks are best for optimising ethylene yields, and the severity of cracking is specified by the rate of disappearance of a marker compound, usually n-pentane. The severity of the reaction can then be defined as follows: 3.1 Primary sources of synthetic polymers The most important primary sources of synthetic polymers are crude oil, natural gas and, to a minor extent, coal. Because all are primarily fuels rather than sources of materials, the manufacture of polymers is susceptible to changes in price or supply. However, this is also true of other materials, since fuel costs are an important component of metal, ceramic and glass manufacture where very high reaction temperatures are needed for reduction of ore to metal and/or smelting. Where polymer ma 2.3.1 Structural isomerism In the saturated hydrocarbons, whose structural formulae are shown in Figure 16, it is not possible to form distinct isomers with just three or less carbon atoms linked together. There is only one way in which one carbon and four hydrogen atoms can be linked together, the single compound being methane, CH4. A simila 1.3.2 Human/product interaction The balance of properties needed in a particular product varies enormously, depending on the exact duty that product will perform in service, the environment in which it will operate, and the way it will interact with the user or consumer. The last factor has assumed much greater influence in product design as competition between different manufacturers sharpens the perception of quality in users’ eyes. The study of human-product interactions is variously known as human factors or 2.4 Models as part of systems work Thinking systemically involves identifying systems relevant to some situation, and models are invariably used as part of this process. An example of this forms part of Checklands' Soft Systems Methodology (SSM) (Checkland, 1981). One aspect of this methodology concerns the formulation of a root definition of some system that is relevant to the situation of interest and the construction of a conceptual model of this system. The root definition is a concise, verbal description of what a 4.1 Beginnings Systems engineering has its roots in three linked strands of thinking: the concepts of systems science, engineering and public policy problem resolution. The first of these can be traced back to the work of von Bertalanffy (1968, pp. 8–15, 96–98) and others during the 1920s and 1930s but received a significant impetus when, in 1954, the Society for General Systems Theory was established at the annual meeting of the American Association for the Advancement of Science. The society later cha Stage 2: The situation analysed The first step is to develop a picture (called in soft systems terminology a rich picture) that encapsulates all the elements that people think are involved in the problem. Once the rich picture has been drawn, the analyst will attempt to extract ‘issues’ and key tasks. Issues are areas of contention within the problem situation. Key tasks are the essential jobs that must be undertaken within the problem situation. 3.3 System concepts: holism One of the distinguishing features of the systems approach is its attempt to be holistic – to include all the elements in the picture at each level at which the system operates. The premature exclusion of important elements can be dangerous and can lead to, for example: a purchasing manager being so keen to drive down raw material and component costs that he or she causes quality and production problems in construction of the system 1.8 Increasing complication, complexity and risk: are systems becoming more complex?
Figure 17 shows the evolution of two commonly encountered applications of systems – for personal transport and for the reproduction of recorded music. In both cases the degree of complexity of the systems application has increased over time. One of the main reasons for this is technology 1.3 Example 1 The Workcenter that didn't Autodesk Inc. is the world's largest supplier of design and engineering software. It currently markets over thirty products but is most famous for its AutoCAD® two- and three-dimensional design and drafting software. The company is the market leader in this type of application, with over 4 million customers worldwide. The Autodesk story began in 1982 with a group of programmers, centred on San Francisco, writing code for design software in their spare time. The group demonstrated a cob Acknowledgements Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence Grateful acknowledgement is made to the following source for permission to reproduce material within this unit: 4.3 Optical networking DWDM improves the utilisation of optical fibre for point-to-point links, but a further step in exploiting the potential of optical fibre comes from optical networking in which routeing or switching is done optically. Optical networking is in its infancy, but the concept of the optical layer based upon wavelength channels is emerging. The optical layer effectively sits below the SDH layer in the network, and provides wavelength channels from one location to another. An analogy can 2.4.3 Polarization mode distortion Because light is an electromagnetic wave, it has a ‘state of polarization’, which, for light in single-mode fibre, is at right angles to the path of the fibre. If you've not encountered electromagnetic waves before, all you need to appreciate is that as light travels down the fibre the electromagnetic field has an orientation across the fibre (Author(s): 2.4.2 Dispersion in single-mode fibre Because there is only one mode in single-mode fibre, there is no multimode distortion but pulses are spread by dispersion.
Dispersion is the effect of different frequencies propagating at different speeds, and there are various mechanisms in optical fibre which mean that in general a fibre is dispersive. Given that dispersion takes place, a transmitted pulse will be spread because different frequency components in the pulse will take different lengths of time to propagate Module team Rosalind Armson, author Joyce Fortune, case study author Ray Ison, author Martin Reynolds, course chair Laurence Newman, course manager Mike Aiken, critical reader Mandy Anton, graphic designer Simon Bell, critical reader Victor Bignell, critical reader Chris Blackmore, critical reader Jake Chapman, critical reader Tony Duggan, project controller (Technology) Pip Harris, compositor Mike Haynes, critical Acknowledgements Except for third party materials and otherwise stated (see terms and conditions), this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 Licence Grateful acknowledgement is made to the following sources for permission to reproduce material within this unit: 6.5 Appreciating some implications for practice I think for most people, the National Health Service would be experienced as a complex situation. If so this would be a good example of perceived complexity. Remember though, if you engaged with it as if it were a difficulty you would not describe the situation as one of perceived complexity. I could not call it a complex system unless I had tried to make sense of it using systems thinking and found, or formulated, a system of interest within it. This means I would have to have a stake in the 3.2 Making sense of the metaphor The metaphor of the juggler keeping the four balls in the air is a powerful way for me to think about what I do when I try to be effective in my practice. It matches with my experience: it takes concentration and skill to do it well. But metaphors conceal features of experience, as well as calling them to attention. The juggler metaphor conceals that the four elements of effective practice often seem to be related. I cannot juggle them as if they were independent of each other. I can imagine
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