Don’t Be Afraid of Physics by Ross Barrett & Pier Paolo Delsanto

Author:Ross Barrett & Pier Paolo Delsanto
Language: eng
Format: epub
ISBN: 9783030634094
Publisher: Springer International Publishing

8.2 Particles

In Chap. 5, we have seen how the idea that matter in the universe is composed of elementary particles, called atoms, has been around at least since the time of the early Greeks. By the start of the 20th Century, it was recognised that electromagnetic radiation also exhibits corpuscular properties in some circumstances. Any doubt about the atomic structure of matter—and there had been many doubters because of the difficulty of observing atoms—had been dispelled by Einstein’s paper in 1905 explaining Brownian motion (see Chap. 5).

The 19th Century had also seen the discovery of the electron, the fundamental particle of electricity. In 1897, the British physicist J. J. Thompson reported in The Philosophical Magazine experiments on the strange cathode rays emitted from a heated metal in a vacuum. His ingeniously designed experiments used deflections of the cathode rays by electric and magnetic fields to verify that the rays were indeed comprised of electrified particles, now known as electrons, and went on to determine the ratio of the electric charge of the particles to their mass.

A number of other rays were also discovered towards the end of the 19th Century. These were X-rays, produced by Wilhelm Röntgen in 1895, and the alpha, beta and gamma rays emitted during the radioactive decay of uranium, and discovered in 1899 and 1900 by Henri Becquerel, Paul Villard and Ernest Rutherford, working independently. Further experiments to determine the electric charge-to-mass ratio of the alpha and beta particles found that the positively charged alpha particle was most probably a helium atom with its two electrons stripped away, and the negatively charged beta particles were electrons. Gamma rays were a very energetic form of electromagnetic radiation. Both X-rays and gamma rays, and also visible light, heat rays (infra-red radiation) and chemical rays (ultra-violet radiation), are now known to be made of photons of different frequency (see Fig. 5.​1).

So at the end of the 19th Century there was a widespread belief that apart from a few wayward examples (electrons, and the particles emitted in radioactive decay), the fundamental building blocks of matter are atoms. Such a belief did not survive for very long. Indulging in a pursuit that is fairly common among curious children, physicists began smashing objects together to see what lay inside.

Ernest Rutherford, together with Hans Geiger and Ernest Marsden, bombarded gold atoms in a thin foil with alpha particles, and observed that a significant number of these positively charged alpha particles were deflected at large angles, some even travelling back towards their source. Rutherford attributed the deflection to electrostatic repulsion. However, the gold atom is electrically neutral, so large deflections were not to be expected. A detailed mathematical analysis showed that the observed deflections (called scattering) could be explained if the gold atom had a small, positively charged nucleus at its core, surrounded by a diffuse cloud of negative charges (presumably electrons), to maintain its electrical neutrality. The scattering is illustrated in Fig. 8.1. This picture of the atom became known as the Rutherford Model.

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