2.1 Some basic concepts Energy is defined as the capacity to do work, and Author(s):
7.5 Outstanding issues Evidence from rotation studies shows that some AGNs do indeed contain compact, supermassive objects within them, though there is no direct evidence that these are black holes. Quasars were most abundant at redshifts of 2–3 and have been declining in number for the last 10 billion years. It seems probable that AGNs fade with time as the supply of accreting material is used up. There is speculation that AGNs may be rejuvenat
7.3 The central engine An object that fluctuates in brightness on a timescale Δt can have a radius no greater than R ∼ cΔt. The point-like nature of AGNs and their rapid variability imply that the emitting region is smaller than the size of the Solar System. The central engine of a typical AGN is believed to contain a supermassive black hole of mass ∼108M ⊙ and Schwarzschild radius
Radio-loud AGNs The second model (Figure 36b) is similar to the first, but now the engine is producing a pair of jets that will eventually end in a pair of lobes, as seen in radio galaxies and some quasars. Looking at the model from the side, one expects to see narrow lines in the spectrum (but not broad lines) and two
5.3 The broad- and narrow-line regions In our model, the engine is surrounded by gas clouds (Figure 32). You have already seen how common these are in our own and other galaxies, so it is reasonable to expect them to be present in at least the spiral galaxies that contain AGNs. If these gas clouds are illuminated by ultraviolet or X-rays from the e
4.2 The size of AGNs AGNs appear point-like on optical images. It is instructive to work out how small a region these imaging observations indicate. Optical observations from the Earth suffer from ‘seeing’, the blurring of the image by atmospheric turbulence. The result is that star-like images are generally smeared by about 0.5 arcsec or more. One can do much better with the Hubble Space Telescope where, thanks to the lack of atmosphere, resolved images can be as small as 0.05 arcsec. What does this mean in
1 How did the notion of public dialogue arise? There is a good case to be made that the emphasis on ‘dialogue’ in relation to science and the public in the UK coincided with the publication in 2000 of the House of Lords report on Science and Society. But the impact of that report has to be seen in the context of what was happening under the ‘public understanding of science’ (PUS) banner in the years between the publication of the Bodmer report (1985) and the House of Lords report 15 years later. In the UK, this period
7.3 Disposal requirements When carrying out a risk assessment, you must consider disposal requirements. For example, any chemical d
6.1 Control of Substances Hazardous to Health (COSHH) Regulations For those companies involved in the handling of chemicals and/or biological material there are a number of additional regulations. These are known as the Control of Substances Hazardous to Health (COSHH) Regulations. These were introduced in 1988, and the last update was in 1999. These regulations apply to chemicals, biological hazards and dusts. The essential requirements are that the employer must: make an assessment of the health risk to empl
References A strategy for ridding the world of VAD? In July 2000, Time magazine announced that a potential solution to VAD had been found – ‘Golden Rice’ (Figure 8). This was a variety of rice that had been genetically modified to introduce β-carotene into the endosperm (part of the grain of the rice). The name arises from the fact that the otherwise white grains of rice are given a golden colour by the presence of carotenoid compounds. The announcement came at the height of the global controversy over genetically modified 2.2 Using A. tumefaciens to genetically modify plant cells Genetic engineers have capitalised on the fact that part of the DNA from the Ti plasmid of A. tumefaciens is integrated into the plant genome during the infection process. Ti plasmids can be isolated and a foreign gene spliced in at an appropriate point, making it possible to transfer the novel gene into the plant. 1 Hydropower Hydroelectric energy is ultimately solar energy converted through evaporation of water, movement of air masses and precipitation to gravitational potential energy and then to the kinetic energy of water flowing down a slope. That energy was harnessed for centuries through the use of water wheels to drive mills, forges and textile works, before being supplanted by coal-fired steam energy. Electricity generation using water turbines, although first centred on constricted streams, has increasing 2.1 The problems of iron uptake Iron has a high natural abundance. It is the second most abundant metallic element by mass in the Earth's crust (7.1 per cent). What are the main oxidation states of iron? 3.8 Review In this unit we have looked at some of the factors which infuence a couple's decision to have a child. For people who choose not to do so, at least for some part of their lives, a range of contraceptive practices is available. We have looked at the biological processes involved in producing gametes, and at the precise sequence of events necessary to produce a healthy, fertilized egg. We have seen how this egg develops, first as a pre-programmed, free-living embryo, then becomes buried within 3.6.1 Identifying the pregnacy Our description of the developing embryo has, so to speak, detached it from its mother. But we should remember that on the other end of the placenta is a woman whose reaction to her pregnancy may lie almost anywhere in the scale of human emotion, and whose behaviour during her pregnancy will have an enormous effect on its outcome. This section attempts to look at the pregnancy from the mother's point of view. Of course, it cannot possibly be applicable to all women in all pregna 3.1 How fertilization happens Now that we have dealt with the basic biology, we can resume and give more detail to our story, and return to where we left it: fully mature, strongly swimming sperm have been deposited in the vagina, and will begin their race to the newly ovulated egg. 4.6 Hormonal control of egg production As you can see from the preceeding section, hormones play a crucial role in the maturation of the oocyte. Figure 3 showed you how levels of oestogen and progestogen vary throughout the menstrual cycle, and suggested that hormone balance is important for a woman's fertility, but you can now see how subtle the control really is. Cells have to develop sensitivity to hormones at the times when the hormones are likely to be present, otherwise the entire operation will fail. 8 Summary The rate at which water infiltrates into the ground depends on the permeability of the rocks and the state of the ground surface. Below the ground surface there is an unsaturated zone which has air in the pore spaces, and a saturated zone which has all the pores filled with water. The water table is the boundary between the unsaturated zone and the saturated zone, and is the level at which water stands in wells. Water below the water table is called groundwa 6 Permeability It is important to distinguish clearly between porosity and permeability. Porosity is a measure of how much water can be stored in a rock, whereas permeability is a measure of the properties of a rock which determine how easily water and other fluids can flow through it (see Section 4). Permeability depends on the exte
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