Tag Archives: Questions of science

Education, Environment, Science

Questions of Science: Against the grain

SAND PARTICLE DEPOSITION

Sand particles on a beach or dunes seem to reach a certain grain size and then reduce no further. After millions of years, shouldn’t most sand have become dust?

THE GRAINS that we see in desert sand dunes have been deposited mainly by wind action. These will generally have originated in other parts of the desert where there are bare rock surfaces that are constantly being weathered by exposure to the sun, wind and water – the last of which is a surprisingly powerful weathering agent in deserts.

The result is a build-up of fragments of various sizes: boulders, pebbles, sand grains and dust. The last two, being smaller, can be removed by the wind and transported hundreds of kilometres, either in suspension high in the atmosphere, or by saltation – the process of bouncing along the ground.

The maximum grain size that can be transported by the wind is proportional to the wind speed – faster winds will move larger particles. This means that the large particles are deposited when and where the wind speed drops, which is often in low or flat terrain. So sand grains of around a certain size can accumulate in great masses in lowland basins, while the smaller fragments can be carried further; dust from the Sahara Desert quite frequently falls on the UK, for example. The result is that dunes are made up of grains mainly of the same size.

Similar principles apply on beaches, although the movement of particles is also affected by a variety of additional processes such as wave action, tides, offshore currents and long-shore drift – sand creep caused by waves approaching the beach obliquely. How effective each mechanism is at moving particles depends on its energy, so each will deposit particles in a different location. For example, wave action can sought beach material so that shingle will accumulate as a ridge high up the beach, while sand will only be exposed at low tide. Or long-shore drift may carry sand to one end of a beach, leaving shingle at the other.

Of course, all these fragments – boulders pebbles and sand – may gradually be broken down into finer particles, so that we might suppose all of the world’s rocks should by now have been reduced to a mass of dust blanketing the continents. But this does not happen because deposits of sand and dust gradually get compressed and cemented together to form new rock – the sandstones and mudstones. Nor does the planet run out of sand and dust, because bare rock surfaces are constantly exposed to weathering processes, and there will always be new rock exposed as a result of tectonic movement.

National Geographic

IF the sand in a coastal system is too fine relative to the energy of the waves then it will stay in suspension in the water and will not be deposited. So for a beach of dust to exist, the environment would have to be profoundly calm, and the dust-like sand would have to be kept wet in order to prevent the wind from claiming it. Most beaches are not like this.

Dunes are deposits of wind-blown sand, and for the sand to be deposited the size of the grains must exceed the carrying capacity of the wind. Sand dunes are innately dry places and there is no way that dust-sized particles could hope to stay put in these areas, however weak the wind may be.

Desert dunes exist in gigantic systems, whereas beach dunes form only a narrow band running along the back of some sea beaches, and are created by gusts from the sea that transport sand up from the beach. Yet both systems result from the same key processes of wind-borne matter being deposited when the wind becomes too weak to keep it aloft. Of course, even the tiniest sand grains will be deposited somewhere, but they will be highly dispersed and will not form dunes.

. You may also like  Skimming pebbles on water. But how does it happen?

 

. Appendage:

Sand

 

Science in motion

Science-in-motion: a series of short articles following topics in science.

. Newtonian gravity

Isaac Newton’s law of universal gravitation, published in 1687, was the first clear mathematical description of how bodies such as planets and stars attract each other under their mutual gravitational pull.

Newton’s inspiration for the theory came from watching an apple falling from a tree. A falling apple accelerates towards the ground, so Newton reasoned from his laws of motion that there must be a force, which he called gravity, acting on the apple. This force might have a huge range and could also be responsible for the orbit of the Moon around the Earth, if the Moon had just the right speed to remain in orbit despite constantly ‘falling’ towards the Earth.

He went on to show that the gravitational force between two massive objects is directly proportional to the product of their masses and weakens with the square of the distance between them. But troubling, the theory didn’t explain why the force was transmitted across empty space. This problem is resolved in Einstein’s general relativity theory.

Standard
Science

Questions of Science: Wave mechanics and stable atoms…

GRAVITATIONAL PUSH AND PULL

In an atom, why doesn’t the negatively charged electron collapse into the positively charged nucleus?

Is this in any way similar to the reason why large systems like stars and planets do not collapse into each other under the pull of gravity?

WHEN Ernest Rutherford, the New Zealand-born founder of nuclear physics, first discovered the atomic nucleus he did suggest that electrons did not fall toward the nucleus of the atom. This was because, he said, that the attractive forces of the nucleus were being balanced by the orbital velocity of the electron, in much the same way as a planet orbits a star.

However, the Danish physicist Niels Bohr modified this theory after Albert Einstein and Max Planck found that energy could only exist in certain discrete amounts, or quanta. This meant that electrons could be seen to have both wave and particle properties, and required that the circumference of the orbit of an electron could not be zero. This means, of course, it could never reach the nucleus.

Science has since adopted the model by the Austrian and Nobel Prize-winning theoretical physicist Erwin Schrödinger. Instead of orbiting the nucleus like planets, his model has electrons occupying ‘clouds’ where it is statistically probable that they will exist, although it has to be appreciated we may never determine an electron’s position and velocity at the same time.

Niels Bohr’s insight in 1913 is worth further explanation. The atom was known to have a small heavy nucleus, and the much lighter electrons were thought to orbit it like planets around the sun. As long as a planet does not lose energy, it can continue its orbit indefinitely.

According to the laws of electromagnetism, charged particles moving in a circle ought to radiate energy as waves. Bohr calculated that a hydrogen atom should collapse with a flash of light in a matter of femtoseconds. Because this does not happen, he proposed what has become known as the ‘old’ quantum mechanics. It asserted that the electron’s angular momentum had to be a multiple of Planck’s constant.

The rule meant that electrons could only occupy particular orbits, and there was a minimum size of orbit. Using this, Bohr was able to predict the entire spectrum of excited states of hydrogen, which, of right, was quite an astounding achievement.

But Bohr’s theory was difficult to apply to more complex atoms and was superseded by Erwin Schrödinger’s wave mechanics in 1927. This became the start of modern quantum theory.

Schrödinger’s formulation shows that an electron has a wave character, and a stable atom can be thought of as a box confining the wave. An electron has a wavelength equal to Planck’s constant divided by its momentum, so the faster an electron moves, the shorter its wavelength. To confine the electron near the nucleus the electron must move very quickly.

Conversely, a fast-moving electron can escape the pull of the nucleus. So you can think, too, of the size of an atom as resulting from a compromise between the electrons having enough kinetic energy for their waves to fit in the box, but so much that they can escape.

Standard
Health, Medical, Science

Questions of Science: ‘Rubbing salt in to the wound’…

Health

Rubbing salt in the wound was a way of preventing infection. But how did it work?

Applying salt to a wound creates a highly saline environment, one in which it is difficult for microbes to grow. The high concentration gradient between the salt solution and the fluid inside bacterial cells makes it far more difficult for the microbes to extract water from the solution without using a lot more energy. As a result, the bacteria become placid and dehydrated and cannot function normally or proliferate.

Concentrated sugar solutions also have a dehydrating effect. This accounts for the extended shelf life of chutneys and preserves, and explains why honey can be used on wound dressings and, ironically, on bee stings as an antiseptic.

Blood is 83 per cent water. Because salt is hygroscopic, it absorbs water, accelerating the tendency for blood to clot and drying the wound. This helps deny microorganisms a favourable habitat. Saline solutions do generate osmotic pressure – it forces water out of microbes to equalise the salt concentration across their cell membranes. This can kill them, so salt acts as a disinfectant.

The stinging of the wound signals that salt does cause injury to the body. But in the absence of a better option at the time, killing a few healthy skin cells was deemed acceptable collateral damage when the alternative may have been a serious infection or possible death.

Standard