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Activation and information in working memory and attention

Bradley R. Postle, recorded 12/5/12


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Excitonics

An exciton is an excitation that mediates the absorption and emission of light, especially in low-cost disordered solar cell and LED materials. In the Center for Excitonics, we seek to supersede traditional electronics with devices that use excitons to mediate the flow of energy. In my presentation, I’ll describe two applications of excitonics in devices: high-brightness quantum dot-based LEDs, and exciton fission in high efficiency solar cells.


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Liquid Biofuels: An International Solution to an International Challenge

Concerns about national security issues and global warming have kindled interest in production of liquid biofuels. In order to minimize the conflict between food and fuel use, it is increasingly important to establish systems for the efficient production of biofuels from non-food polysaccharides such as renewable lignocellulosic biomass or directly from carbon dioxide and other gases. Ethanol, typically used as “biofuel” in cars— alcohol refined from grain or sugar cane &mda
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How Far Can a Shirt See: The Birth of a Revolution in Fibers and Fabrics

Fibers and fabrics are among the earliest forms of human expression; these materials shield us from the environment and play an important role in defining who we are. Surprisingly, in sharp contrast to other areas of our existence, fibers have remained practically unchanged for thousands of years.

Can fibers become highly functional objects similar to computers and smartphones? Can they see, hear, sense, and communicate? Our research focuses on extending the f
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Acknowledgements

The content acknowledged below is Proprietary (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 in this unit:

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5 Summary

  1. Eutrophication is a process in which an ecosystem accumulates mineral nutrients. It can occur naturally, but is usually associated with human activity that releases nutrients into the environment.

  2. Anthropogenic eutrophication has caused a widespread loss of biodiversity in many systems. Recent attempts to reverse the process are proving difficult and expensive.

  3. Symptoms of eutrophication are most readily seen in aquatic sys
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4.4 Reducing nutrient availability

Once nutrients are in an ecosystem, it is usually much harder and more expensive to remove them than tackle the eutrophication at source. The main methods available are:

  • precipitation (e.g. treatment with a solution of aluminium or ferrous salt to precipitate phosphates);

  • removal of nutrient-enriched sediments, for example by mud pumping; and

  • removal of biomass (e.g. harvesting of common reed) and using it for thatchi
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3.2 Anthropogenic sources of nutrients

In addition to the natural sources of nutrients referred to above, nitrogen and phosphorus enter the environment from a number of anthropogenic sources. These are considered below.


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3.1.1 Phosphorus

Phosphorus has a number of indispensable biochemical roles and is an essential element for growth in all organisms, being a component of nucleic acids such as DNA, which hold the code for life. However, phosphorus is a scarce element in the Earth's crust and natural mobilization of phosphorus from rocks is slow. Its compounds are relatively insoluble, there is no reservoir of gaseous phosphorus compounds available in the atmosphere (as there is for carbon and nitrogen), and phosphorus is also
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2.7 A gene ‘for’ obesity?

So far we have mostly emphasized the way in which different environmental factors may affect body weight and provide a partial explanation of both individual cases of obesity and the increase in average body weight that has been so clearly documented in both North America and Western Europe during the last two decades. There is also marked individual variability in body weight. For example, any weight between about 58 and 78 kg would be regarded as ‘desirable’ for a person of height 1.77
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Acknowledgements

The material acknowledged below is contained in Chapter 3 of An Introduction to Galaxies and Cosmology (eds Mark H Jones and Robert J Lambourne), published by the Press Syndicate of The University of Cambridge  in association with The Open University. Copyright © The Open University, 2003, 2004.

This publication forms part of an Open University course S282 Astronomy.


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5.1 Introduction

So far we have seen how the properties of the central engine of the AGN can be accounted for by an accreting supermassive black hole. Though there are many questions still to be resolved, this model does seem to be the best available explanation of what is going on in the heart of an AGN. But of course all AGNs are not the same. We have identified four main classes and in this section we will attempt to construct models that reproduce the distinguishing features of these four classes.


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4.7 Jets

You have seen that two kinds of active galaxies – quasars and radio galaxies – are often seen to possess narrow features called jets projecting up to several hundred kiloparsecs from their nuclei. If these are indeed streams of energetic particles flowing from the central engine, how do they fit with the accretion disc model? How could the jets be produced?

The answers to these questions are not fully resolved, but there are some aspects of the model of the central engine which prob
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3.1 Introduction

Active galaxies have occupied the attention of an increasing number of astronomers since the first example was identified in the 1940s. By one recent estimate, a fifth of all research astronomers are working on active galaxies, which indicates how important this field is. In this section you will learn about the observational characteristics of the four main classes of active galaxies: Seyfert galaxies, quasars, radio galaxies and blazars. This will set the scene for subsequent sections in wh
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Normal galaxies

Figure 8 shows schematically the broadband spectrum of a normal spiral galaxy. It resembles that of the Sun, although the peak occurs at a slightly longer wavelength and there are relatively greater spectral flux densities at X-ray, infrared and radio wavelengths.

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2.3 Broadband spectra

The broadband spectrum is the spectrum over all the observed wavelength ranges. To plot the broadband spectrum of any object it is necessary to choose logarithmic axes.

  • Why is it necessary to use logarithmic axes?

  • Because both the spectral flux density, Fλ, and the wavelength vary by many powers of 10.

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Learning outcomes

By the end of this unit you should be able to:

  • explain how and why the optical spectrum of an active or starburst galaxy differs from that of a normal galaxy;

  • explain how and why the broadband spectrum of an active or starburst galaxy differs from that of a normal galaxy;

  • describe briefly the observed features of starburst galaxies and the four main classes of active galaxies (quasars, radio galaxies, Seyfert galaxies and blazars);


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