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Education, Science

Questions of Science: Push and pull

Questions of Science

Intro: Why, in an atom, does the negatively charged electron not 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 indeed propose that electrons did not fall toward the nucleus of the atom because the attractive forces of the nucleus were being balanced by the orbital velocity of the electron in much the same way as a planet orbiting 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.

We have since adopted the model proposed by the Austrian 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 we may never determine an electron’s position and velocity at the same time.

Niels Bohr’s questioning in 1913 deserves 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 was a quite astounding achievement.

But Bohr’s theory was hard to apply to more complex atoms and was superseded by Erwin Schrödinger’s wave mechanics in 1927, which is the start of modern quantum theory.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 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 not so much that they can escape.

Large solar systems don’t collapse for quantum-mechanical reasons. They don’t collapse because the planets’ velocities keep them in freefall.

Science in motion

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

. Atomic structure

Atoms consist of a tiny dense nucleus containing positively charged protons and uncharged neutrons, surrounded by clouds of electrons. Because protons and neutrons are much heavier than electrons, most of an atom’s mass resides in the central nucleus.

Each chemical element has a unique number of protons in its nucleus, its ‘atomic number’. For instance, the element carbon with six protons has the atomic number six. However, single elements can have different numbers of neutrons in the nucleus. For example, carbon has three naturally occurring ‘isotopes’ with six, seven or eight neutrons. The sum of protons and neutrons in an atom’s nucleus is called the atomic mass number.

Normally, the net electric charge of an atom is zero because the number of electrons is the same as the number of protons, and their equal and opposite electric charges cancel out. However, it’s possible to knock electrons out of atoms or add extra ones to create positively or negatively charged particles known as ‘ions’.

Atomic Structure

. Negatively charged electrons in orbit around the nucleus . Nucleus containing positively charged protons and neutrons

 

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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.

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Education, Government, Scotland, Society

The importance of libraries and our need to rely on them

LIFELONG LEARNING

Library

Public libraries are places for educational and lifelong learning.

Reading is one of life’s great pleasures. Not much comes near the feeling of being engrossed in the pages of a book, growing to love and loathe the characters and becoming familiar with fictional worlds. Often, the difference between fiction and reality is wafer thin.

For some, reading for pleasure involves non-fiction to learn about people, places and subjects, whether it’s sport, hobbies, history, travel, science or study and business-related material.

There is a unique and idiosyncratic joy that comes from reaching the closing chapter and pages of a great book – and sometimes a momentary sadness that our journey into the world of imagination and learning created by the book’s author has come to an end.

Reading for pleasure is one of our nation’s favourite pastimes, but it also has a very important role to play in educational development and the health and wellbeing of individuals and society.

There is a growing body of evidence that demonstrates the positive impact of reading for pleasure on literacy and attainment, as well as on our emotional wellbeing. Reading also manifests itself by allowing people to experience other cultures and faiths and to develop an understanding, empathy and appreciation of different beliefs and values.

It should go without saying, but literacy is a fundamental and lifelong skill. The nation always needs a population that is ably literate and educated. The ability to read improves chances of employment and leads to better health and higher levels of wellbeing. It also helps people to comprehend and ask questions that otherwise would be missing.

But, for reading to become a lifelong habit, it must be encouraged from a young age and promoted as an enjoyable activity.

Libraries, of course, are not alone in promoting literacy, but they do have a distinct role as a local space where people of all ages and backgrounds can come to explore, learn and discover.

In Scotland, the Scottish Government is currently developing a cultural strategy, with core themes emerging around access, equality and excellence. The library sector will have a significant contribution to make to this discussion for the role libraries play in supporting lifelong learning, skills and appreciation of culture. Libraries across Scotland promote a range of reading displays – with often new books on offer – and the importance of reading for pleasure is recognised at a national level too.

Libraries are also running the Tesco Bank Summer Reading Challenge Scotland. Coordinated by The Reading Agency, the initiative encourages children aged 4-11 to read at least six books over the summer holiday period. Book Week Scotland, an annual celebration of books and reading for readers of all ages, encourages adults and children alike to enjoy the pleasure of reading through library, school, community and workplace events.

In 2016, more than 17 million books were loaned from Scotland’s public libraries, more than six million of these borrowed by children.

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