This study unit introduces you to the proposition that our mental models change through learning, and
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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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The ideas for the boiler cut-out switch can now be based on some real knowledge about temperature effects. You are now ready to tackle the next exercise.

Exercise 7

List four temperature-dependent changes in material properties t
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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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The third category of thermal effects identified in Section 2 are those associated with sudden changes. Here are some technically important examples where things change suddenly at a particular temperature:

• Pure water boils at 100 Â°C (at atmospheric pressure).<
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The final trick I want to show you with Arrhenius's law is how to extract the constants rï»¿0 and Eï»¿a from experimental data. If the Arrhenius equation (Section 4.3.1) is â€˜turned inside outâ€™ by taking natural logarithms of both sides it becomes:

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Near room temperature (300 K), the average bundle of thermal energy associated with a particle is 0.026 eV (look back at Box 2 Temperature and energy). This is not going to do much damage to something stuck together with bonds that have an energy of a few eV â€“ just as the average wage earne
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Atoms without much thermal energy will not be doing very much. Consider fifty million million million (50 Ã— 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 â€“ that is because thermal energy effectively â€˜rattles it to bitsâ€™. Having the
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This is a long section and needs to be studied carefully. Keep your eye on the overall goal of seeking useful thermal effects on which to base devices.

This section continues the discussion of heat at an atomic level. You will need this background to appreciate the characteristics of processes activated by thermal energy â€“ for example, the softening of glass in a gas flame, the diffusion of atoms through solids, the electrical conductivity of ceramics, and many chemical reactions. Suc
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• The temperature of an object is intimately linked to the average kinetic energy of the atoms from which it is made. As a result, some materials properties such as electrical resistance and mean atom spacings change gradually with temperature. These properties can be modelled with a simple linear equation like the following one that describes thermal expansion:

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When the temperature of an object increases (say, by Î”T) 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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In general, as the temperature of a piece of solid is raised the volume it occupies increases. I say â€˜in generalâ€™ 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 â€“ 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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This section provides a model for properties interpreted in terms of the average thermal energy of all the constituent atoms of a material. Since absolute temperature T is a measure of average atomic kinetic energy, we shall expect to be looking at properties that change gradually with T, roughly proportionally, over a wide range. In terms of the classification introduced in Author(s): The Open University

• Thermometers sense temperature. They are transducers providing observable and quantifiable signals in variables other than temperature. Thermometers are calibrated to give numbers in accord with an internationally agreed scale. Various attributes influence the selection of an instrument for a task.

• Temperature can determine the rate at which certain physical and chemical changes proceed, and whether some changes can occur at all.

• <
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â€˜Have you taken the temperature effects into account?â€™ (Figure 1) is nearly always a valid question in any discussion about a proposed engineering solution. Everything has a temperature, and everything behaves differently at different temperatures. It therefore deserves its own special plac
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After studying this unit you should be able to:

• relate the temperature of a solid to the mean kinetic energy of its atoms;

• use models for thermally induced effects that involve linear, exponential and step changes;

• use exponentials, logarithms and graphical methods to interpret data from a thermally activated process in terms of Arrhenius's law;

• identify the changes of phase taking place in a variety of critical phenomena;

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RSS (â€˜Really Simple Syndicationâ€™ or â€˜Rich Site Summaryâ€™) newsfeeds supply headlines, links, and article summaries from various websites. By using RSS â€˜feedreaderâ€™ software you can gather together a range of feeds and read them in one place: they come to you, rather than you having to go out and look for breaking news. The range of RSS feeds on offer is growing daily. There is probably a feed to cover all aspects of your life where you might need the latest information, and you may
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The founder of TechnoratiÂ  claims that the number of â€˜blogsâ€™ doubles every five months and that the creation rate is approaching two per second. One estimate I read in July 2010 put the number at 400 million blogs. Because these online diaries offer instant publishing opportunities, you potentially have access to a wealth of knowledge from commentators and experts (if they blog) in a wide ra
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