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2.1 Crown gall disease: genetic engineering in nature

A. tumefaciens causes crown gall disease in a wide range of dicotyledonous plants. (Dicotyledonous plants, are also known as dicots, have broad leaves with branching veins. An example would be a broad leaved tree like an oak. Narrow leaved plants with parallel grains such as grasses are known as monocotyledonous plant or monocots.) The infection normally occurs at the site of a wound in the plant. The disease gains its name from the large tumour-like swellings, or galls, that o
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Learning outcomes

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

  • explain the principles that underlie the ability of hydropower to deliver useable energy;

  • outline the technologies that are used to harness hydropower;

  • discuss the positive and negative aspects of hydropower in relation to natural and human aspects of the environment.


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1 How do organisms acquire iron?

Metals are an essential part of biological chemistry. Of all the trace elements, iron is the most important, especially as it is present in many essential enzymes and proteins. But how do organisms acquire the iron from their surroundings? Clearly, organisms need to absorb iron biochemically before it can be used in proteins. Also, some method of replacing lost iron quickly is needed: for instance, how is blood replaced once it has been lost through a cut? This prompts the question: what bioc
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2 How should we think of monotremes?

This section contains the first of the activities, Activity 1. If possible, you should do each activity as you come to it; the text that follows
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Acknowledgements

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Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:

The content acknowl
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7.1 Introduction

So far we have presented two pieces of evidence pointing to the occurrence of a big bang: the redshift of the galaxies (indicating the continuing expansion of the Universe), and the 3 K radiation (the remnant of the primordial radiation). But there is a third imprint such a big bang ought to have left on our present-day world. We cannot at this juncture trace out the full sequence of events following the instant of the big bang (that can only be done after we have worked through the next two
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4 Maxwell's equations

We have reached a major milestone. All four of Maxwell's equations are now in place. This is an appropriate place to review their meaning and significance. We concentrate here on the differential versions, which are as follows:

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3.3.2 A capacitor with time-varying charges on its plates

Figure 4 shows a parallel plate capacitor with circular plates, which is being charged by steady currents flowing along straight wires. We know that there is a circular pattern of magnetic field lines around the wires, but what happens inside the capacitor, between the plates?

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3.3.1 An expanding sphere of charge

First consider an expanding spherically-symmetric ball of positive charge. This is not an implausible state of affairs. If the charges in the distribution are not held in place, their mutual repulsion leads to a spherically-symmetric expansion and a spherically-symmetric outward flow of current. Any spherically-symmetric distribution of current is magnetically silent – that is, it produces no magnetic field. This is true both outside and inside the current distribution. We will now show tha
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3.3 The Ampère–Maxwell law in action

To give some further insight into the Ampère–Maxwell law, we will now consider two situations where it plays a significant role.


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3.2 Generalising Ampère's law

We need to generalise Ampère's law beyond the confines of static charge densities. Let's try adding an extra (and at this stage unknown) vector field, K to the right-hand side of the differential form of Ampère's law. The modified equation then reads

What can b
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3.1 Limitations of Ampère's law

In order to analyse the limitations of Ampère's law, and suggest ways of overcoming them, we need to use some properties of divergence. For ease of reference, these properties are given below:

Some properties of divergence

  1. The divergence of any curl is equal
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2 The equation of continuity

The conservation of charge is a basic tenet of electromagnetism. It can be simply expressed by the equation

where Qtot is the total charge in the Universe. However, such an equation does not really help us very much, because we are not usually concerne
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1 Maxwell's greatest triumph

This unit presents Maxwell's greatest triumph – the prediction that electromagnetic waves can propagate vast distances through empty space and the realisation that light is itself an electromagnetic wave. Visible light has a very narrow range of wavelengths, but this tells us more about the sensitivity of our eyes than about the nature of electromagnetic radiation. A few years after Maxwell's death other types of electromagnetic radiation, including radio waves, X-rays and gamma rays, were
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Learning outcomes

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

  • explain the meaning of the emboldened terms and symbols, and use them appropriately;

  • state the equation of continuity and use it in simple problems;

  • state the conditions under which Ampère's law is true and explain why it does not apply more generally;

  • state the Ampère–Maxwell law and explain why it has a greater domain of validity than Ampère's law;

  • state and name the
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Introduction

James Clerk Maxwell produced a unified theory of the electromagnetic field and used it to show that light is a type of electromagnetic wave. This prediction dates from the early 1860s when Maxwell was at King's College, London. Shortly afterwards Maxwell decided to retire to his family estate in Galloway in order to concentrate on research, unhindered by other duties. He was lured out of retirement in 1871, when he became the first professor of experimental physics in the Cavendish Laboratory
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Acknowledgements

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Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit:

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References

Altmann, P., Cunningham, J., Dhanesha, U., Ballard, M., Thompson, J. and Marsh, F. (1999) ‘Disturbance of cerebral function in people exposed to drinking water contaminated with aluminium sulphate: retrospective study of the Camelford water incident’, British Medical Journal, vol. 319, pp. 807–811.
Bank, M. S., Crocker, J. B., Davis, S., Brotherton, D. K., Cook, R., Behler, J. and Connery, B. (2006) ‘P
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4.3.3 Stage 3: Passive continental margin formation

Eventually, movement along faults initiated during the continental rifting stage ceases, and the entire continental margin starts to subside. Subsidence at this stage occurs because of lithospheric cooling as the distance between the margin and spreading ridge-axis increases, rather than as a result of tectonic movement of the fault blocks (as during the continental rifting stage). By now, all tectonic activity is focused at the new oceanic spreading axis and the continental lithosphere can b
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2.2 Going up: using scientific notation for large numbers

Think again about the value for the total volume of water stored on Earth: 1460 000 000 km3.

When dealing with large numbers such as one thousand four hundred and sixty million (1460 000 000), it is tedious to write the number in words or to keep writing all of those zeros. Worse still, it is very easy to lose some of the zeros or add extra ones by mistake. Fortunately, large numbers can be referred to without having to write out all of the zeros. The powers of ten not
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