Monthly Archives: August 2017

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But Bohr’s theory was hard to apply to more complex atoms and was superseded by Erwin Schrödinger’s wave mechanics in 1927, which is the start of modern quantum theory.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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Arts, Science

Quantum Leaps: Michael Faraday

MICHAEL FARADAY (1791–1867)

Michael Faraday FRS, an English scientist, contributed to the scientific study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic induction, electrolysis and rotation. He was also the first to have discovered liquified chlorine.

Regarded as one of the great experimental scientists of all time. Even Albert Einstein (1879 – 1955) considered him to be one of the most important influences in the history of physical science. Yet the man whose discoveries and inventions, amongst them the electric motor, electric generator and the transformer, were to have such a profound impact on modern life, might not have entered the scientific arena at all but for certain fortuitous events in his youth. The first was his apprenticeship at a bookbinder’s when he was thirteen. Here his interest in science – and, in particular electricity – was stimulated upon reading pages from the books he was asked to bind.

Another fortunate incident was his appointment as assistant to the renowned chemist Sir Humphrey Davy (1778 – 1829), who had remembered the young Faraday attending his lectures. The temporary post soon turned permanent and shortly afterwards Davy took Faraday with him on a grand European tour which gave the young man the rare opportunity to meet and learn from many of the leading physicists and chemists of the day.

Much of Faraday’s early work as a scientist in the 1820s was not in physics, the area which ultimately led to his breakthrough inventions, but in chemistry. In 1823, he became the first person to liquify chlorine, albeit accidentally, while he was conducting another experiment. He quickly deduced how the new form of chlorine had been obtained and applied the process, which made use of pressure and cooling, to other gases. By employing his talent as an outstanding analyst of his own chemical experiments, he also went on to discover benzene in 1825.

. The Electric Motor

Yet it is physical science, in particular his work involving electricity, for which Faraday is best remembered today. As early as 1821, he was able to create the first electric motor after discovering electromagnetic rotation. He had developed Hans Christian Oersted’s (1777 – 1851) 1820 discovery that electric current could deflect a magnetic compass needle. Faraday’s experiment proved that a wire carrying an electric current would rotate around a fixed magnet and that conversely, the magnet would revolve around the wire if the experiment were reversed. From this work, Faraday became convinced that electricity could be produced by some kind of magnetic movement alone but it took a further ten years before he successfully proved his hypothesis.

In 1831, by rotating a copper disk between the poles of a magnet, Faraday was able to produce a steady electric current. This discovery allowed him to go on to produce electrical generators, the transformer (also invented independently at around the same time by an American, Joseph Henry) and even the dynamo: inventions which can truly be claimed to have changed the world.

. Electrical Fields

The reason Faraday was able to make such advances was because from early in his career he had rejected the concept of electricity as a ‘fluid’, an idea that had been accepted up until that time, and instead visualised its ‘fields’ with lines of force at their edges. He believed that magnetism was also induced by fields of force and that it could interrelate with electricity because the respective fields cut across each other. Proving this to be true by producing an electric current via magnetism, Faraday had discovered electromagnetic induction. He was encouraged by this and went on to explore the idea that all natural forces were somehow ‘united’.

He then focused on how light and gravity were related to electromagnetism. In turn, this led to the discovery of the ‘Faraday effect’ in 1845 which proved that polarised light could be affected by a magnet. James Clerk Maxwell proved that light was indeed a form of electromagnetic radiation, and eventually provided the mathematical expression for Faraday’s law of induction.

. The Laws of Electrolysis

Faraday’s fascination with electricity and his background in chemistry both found a natural expression in electrolysis, in which he was able to perform ground-breaking work.

In 1833, he was the first to state the basic laws of electrolysis, namely that: (1) during electrolysis the amount of substance produced at an electrode is proportional to the quantity of electricity used, and (2) the quantities of different substances left on the cathode or anode by the same amount of electricity are proportional to their equivalent weights.

Science Book

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