4.4 Summary of Section 4 Thermal energy is a random thing, so any group of particles possessing it will have a distribution of kinetic energies. The fraction of particles with energy greater than an amount E1 is proportional to exp(−E1/kT). Thermally activated rates follow Arrhenius's law and are characterised by an activation energy. Diffusion in solids and electrical conduction i
4.3.3 Getting at the activation energy The final trick I want to show you with Arrhenius's law is how to extract the constants r0 and Ea from experimental data. If the Arrhenius equation (Section 4.3.1) is ‘turned inside out’ by taking natural logarithms of both sides it becomes: 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 2.3 Types of teams Different organisations or organisational settings lead to different types of team. The type of team affects how that team is managed, what the communication needs of the team are and, where appropriate, what aspects of the project the project manager needs to emphasise. A work group or team may be permanent, forming part of the organisation's structure, such as a top management team, or temporary, such as a task force assembled to see through a particular project. Members may work as a group 6.1 A fresh approach? Polymeric materials offer substantial benefits over conventional materials in terms of their low density, relative freedom from corrosion, transparency or translucency, and a range of physical properties which cannot be achieved with metals, glasses or ceramics. Such unique properties include low coefficients of friction (e.g. PTFE), resistance to extreme environments (e.g. PTFE, silicones) as well as the ability to absorb and modulate damaging vibrations (e.g. most rubbery polymers). It is t 4.3.2 Propagation Once a small number of chains have been started, propagation involves successive addition of monomer units to achieve chain growth. At each step the free radical is regenerated as it reacts with the double bond. So in the case of styrene the propagation step is
The free radical can also add on in a 4.3.1 Initiation Initiation is the mechanism which starts the polymerization process. Vinyl monomers are quite easily polymerized by a variety of activating methods. Styrene, for example, can be converted to solid polymer simply by heating, and ultraviolet light can have exactly the same effect. Usually, however, an activating agent is used. This is an unstable chemical which produces active species that attack the monomer. A good example is benzoyl peroxide which splits up when heated:
Author(s): 6 Summary This unit has covered the background to systems engineering. It began by addressing the question ‘Why is systems engineering important?’ Two reasons were discussed: projects go wrong, and the increasing incorporation of software means that they go wrong more often now than in the past complication, complexity and risk are all increasing and need to be managed. In the second section I examined the development of en 5.2 The aims and principles of system engineering The aims of systems engineering can be divided into those to do with its outputs and those associated with the process itself. As far as its outputs are concerned, systems engineering aims to ensure that: the requirements of all the stakeholders are taken into account in engineering the system the system, as engineered and realised, meets the requirements of stakeholders the system, while meeting the req Stage 4: Conceptual model The conceptual (or activity) model contains all the activities that the relevant system would have to perform. The model is usually drawn as a block diagram. 2.4.1 Multimode distortion With multimode fibre, the main cause of pulses spreading is the multiple paths that signals can traverse as they travel along the fibre. This phenomenon of multimode distortion is illustrated in Figure 5. 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 Learning outcomes By the end of this unit you should be able to: discuss what ethics is and what constitutes an ethical issue; identify and discuss ethical issues that arise in the media, in routine conversations and, in particular, in your own everyday professional practice; discuss the role of emotions in ethical deliberations; discuss how negotiation might resolve apparent ethical differences; identify and discuss the ethical issues p 7.2.3 Chemical composition As outlined in Table 2, some deposition techniques are best suited to conducting materials, whereas others come into their own only for chemical compounds. In either case, chemical composition may be an important consideration. Impurities can interfere with the conduction properties of the material (notably in t 6.4 Driven oscillations and resonance Finally I need to consider the situation where the oscillator is driven, as in the case of the AFM cantilever. The driving force will depend on the application, but for my mass on a spring it might be a small motor driving what was the fixed end of the spring up and down. The simplest expression for an oscillating driving force FD will be something like: 5.1.2 Dipole-dipole forces In the case of dipole-dipole interactions, the molecules that bond together have a fixed asymmetry in their charge distributions (as is the case in Figure 22); if their orientations are favourable the two will bond together. All molecules produce London forces. The dipole-dipole interactions are in addition to t 3.6.1 Stiffness Just how compliant does an AFM cantilever have to be to enable it to follow the undulations in a surface on an atomic scale? How can we find out? It turns out that this is easier than at first it seems. A simple assumption we can make is that the compliance of the cantilever should be appreciably greater than that of a typical bond that holds atoms to one another. Here's one way in which a rough estimate of the stiffness (the force required to cause a given deflection) of the bonds in a 2.5 Review The materials from which this simple sensor is made have been carefully chosen. They have had to be compatible with one another during the manufacturing process – so that for example, etching the material in one layer did not affect another material laid down previously. It had to be possible to shape them into the desired form, though some compromises also had to be struck. For instance, the V-groove trench is not the ideal geometry for the pit behind the beam but it is very easily made in 2.4 Thermal and electrical conductance Thermal conductance, Gt, is analogous to electrical conductance, Ge. The longer a conductor is and the smaller its cross section, the lower its conductance will be. Thermal conductance is given by: and electrical conductance by: 2.3 The fabrication process for a MEMS Pirani sensor This section is fairly long, but is best read in one go. If you run out of time, reschedule your study to allow you to start again from here. Thin layers of material are added to the surface by a variety of means, depending on the material to be deposited, and what is already on the wafer. The sensor starts off, as so many microsensors do, with a silicon wafer, shown in cross section in Author(s):
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