Die Welt anderer Kulturen (1)
Die Welt anderer Kulturen (1).
Why Learn Chinese?
Why Learn Chinese?
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.
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
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 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. Anthropogenic eutrophication has caused a widespread loss of biodiversity in many systems. Recent attempts to reverse the process are proving difficult and expensive. Symptoms of eutrophication are most readily seen in aquatic sys
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
Summary of Section 7 Packaging of DNA serves to protect against damage, to compact the DNA helix into a suitable size within the cell, and to act as both a platform for and an intrinsic part of the structural and regulatory machinery involved in DNA metabolism. DNA compaction in prokaryotes achieved through a combination of supercoiling and interactions with proteins that aid DNA bending. Compaction of the eubacterial chromosome is facilitated
Acknowledgements Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit: The content acknowledged below is Proprietary (see terms and conditions) and is used under licence (not subject to Creative Commons licence ).
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4.1 Natural stores of carbon The major natural stores of carbon (called ‘reservoirs’) are shown below in Figure 1.9. 2 Energy, work, power and efficiency In everyday speech we often refer colloquially to the powerful politician, the energetic child, the working mother and the efficient administrator. We use these terms imprecisely, and often wrongly, compared with their scientific definitions. Narrow-line region The model places the narrow-line region much further out from the central engine where orbital speeds are lower; 200–900 km s−1 is typical for the NLR. An important consequence of the NLR being outside the dust torus is that it is always in view, so narrow lines will be seen even if the broad-line emitting gas is obscured. Broad-line region In the model, the clouds of the broad-line region surround the central engine within the opening in the middle of the dust torus. The radius of the BLR is of the order of 1014 m, placing it well inside the torus. At this distance from the black hole orbital speeds are several thousand kilometres per second, which is consistent with the typical speed of 5000 km s−1 that is measured from Doppler broadening. The clouds are fully exposed to the intense radiation from the e 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.
4.6 Accretion power Calculations based on the above accretion disc hypothesis show that if a mass m falls into the black hole, then the amount of energy it can radiate before it finally disappears is about 0.1 mc2, or about 10% of its rest energy. Other than matter-antimatter annihilation, this is the most efficient process for converting mass into energy ever conceived. A comparable figure for the nuclear fusion of hydrogen in stars is only 0.7% of the rest energy of the four hydrogen n 4.4 A supermassive black hole A black hole is a body so massive and so small that even electromagnetic radiation, such as visible light, cannot escape from it. It is its combination of small size and very strong gravitational field that makes it attractive as a key component of the engine that powers an AGN. There is good evidence of a black hole of mass 2.6 × 106M⊙ at the centre of the Milky Way. As you will see, it turns out that much more massive black holes are needed to explai 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 6.2 How DEMOCS works DEMOCS games involve groups of around six to eight participants and take a couple of hours to play. They come as self-contained kits, which can be downloaded from nef's website. To register and log in for access to DEMOCS games, see http://www.neweconomics.org/gen/z_sys_DemocsRegister.aspx?destination=/gen/democsdownload.aspx, accessed 13 March 2007. Topics covered so far include stem cell research, over-the-counter genetic testing kits, xenotransplantation, pre-implantation genetic diagnosis 3.4.2 Compaction and adhesion Around the time of the 8- to 16-cell division, the conceptus undergoes a morphological (shape) change, called compaction, in which the cells fatten on each other, and the outlines of individual cells become hard to distinguish. This stage, sometimes referred to as a morula, from the Greek word for mulberry, is shown in Figure 17i. At this stage it is hard to see individual cells; in fact, unless the cells are separated by various laboratory treatments, it is not possible to see the two 3.4.1 Pre-implantation and assymetric division Let us now return to the Fallopian tube, where a fertilized egg is assembling its chromosomes prior to commencing a series of mitotic divisions which will eventually give rise to the millions of cells that make up the human body. Obviously these millions of cells do not just exist as an amorphous mass: they are differentiated into many different types of cell, and they are organized into recognizable, discrete structures: tissues and organs. This is accomplished by a coordinated sequen
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