Is Electron a Fundamental Particle ?
Atomic physics is a branch, built from big ideas about very small things. Most of the discoveries in this branch took place during the last century, so it is called Modern Physics (physics since 1890).
1998 Nobel citation for physics prize goes like this "for their discovery of a new form of quantum fluid with frac{tionally} charged excitation." 1998 Nobel prize was shared by Robert B. Laughlins (USA) Horst L. Stormer (Germany) and Daniel C. Tsui (USA). Their discovery tells that electrons in a powerful magnetic field condense to form a kind of fluid, which consists of so-called "quasi particles" that have charges that were just a frac{tion} of the charge of an electron. The trio's research has a profound impact on future. Using this idea it will be possible to make more compact televisions, computers, mobile phones etc.
If there was Physics Nobel prize in 600 B.C, it would have gone to saint Kanaad, a great Indian philosopher, for his 'discovery' that matter consists of smallest indivisible discrete particles, called 'paramanu'. Kanaad was followed by the Greek philosopher, Democritus (460 - 370 B.C.) who theorized that not only matter but the human soul also is made of particles. According Aristotle (384 - 332 B.C.), space, matter and others were continuous and that all matter was one primordial stuff that was the habitat of four elementary principles — hotness, coldness, dryness and actness. Different materials in nature have different degree of content of these principles.
Electrical Conduction in Gases
At normal temperature and pressure, the gases are perfect insulators [Of course combustion gases, drawn from flames, are conducting at normal temperature and pressure]. We can make a gas conducting (i) by applying very high voltage or (ii) by reducing its pressure to a very low value. In either case, the gas will be split into charged particles, called ions, both positive and negative.
At atmospheric pressure, about 30,000 V/cm is required to produce discharge in a gas. But if the pressure is lowered, then smaller potential gradient is sufficient for a discharge. To study this, a discharge tube is used.
Description of Different Patterns Observed in a Discharge Tube
A discharge tube, made of pyrex glass,is about 40 cm long and 4 cm in diameter. It is connected to a vacuum pump through a side tube, as shown in Fig. 27.1. Using the pump the pressure of air can be slowly
reduced. At the ends of the tube, two circ{ular} electrodes are fixed and can be connected to the secondary of an induction coil. The electrode connected to the negative terminal of the induction coil is called cathode and the electrode. Irregular streaks of light connected to the positive is called the anode. A p.d. of 10,000 to 15,000 V can be applied.
No visible effects takes place at atmospheric pressure. As the pressure is gradually reduced, using the pump, the following series of phenomena takes place in succession.
(/) At a pressure of about 10 cm of Hg, irregular streaks of light appear in the tube followed by a crackling noise (see Fig. 27.2).
(ii) At a pressure of about 1 cm of Hg, the streaks widen out into a luminous column, with a continuous buzzing sound, which extends from the cathode to the anode. The colour of the column depends on the nature of the gas inside the tube. It is red for neon, bluish for C02 etc. The luminous column is called the positive column (Fig. 27.3).
(Hi) At a pressure of about 3 mm to 2 mm Negative g|ow Poshjve co|umn
of Hg, the positive column detaches from the cathode and a glow appears in the cathode, called the negative glow. Between this glow and the positive column there is a dark space which is called Faraday dark space (FDS) (see Fig. 27.4).
(iv) At a pressure of about 1 mm of Hg., the positive column gets shortened and the Faraday dark space extends to a greater distance.
(v) At a pressure of about 0.1 mm of Hg., the negative glow moves away from the cathode and another glow called cathode glow appears on the cathode. A second dark space called Crookes dark space appears between the two glows. The positive column decreases in length and is divided into a number of equally spaced layers, called striations (see Fig. 27.5).
(vi) At a pressure of about 0.01 mm of Hg, first striations disappear, the negative and cathode glow vanishes and the whole tube is filled with Crookes dark space. Now the walls of the tube are seen to glow with a blue or green light, depending on the nature of glass. This glow is called the fluorescence (Fig. 27.6).
The fluorescence of the glass is produced by the invisible rays emerging normally from the cathode. These are called cathode rays. Crookes believed this to be an ultra gaseous state and a fourth state of matter; later cathode rays were identified to be electrons.
(vii) If the pressure in the tube is reduced further, to maintain discharge very high voltage is required, and finally the tube stops conducting.
Explanation. Various phenomena taking place in a discharge tube is based on two facts, (i) High speed atomic particles can cause ionization by collision and (ii) Atoms and molecules emit radiations, when they are in the excited state.
Under the applied electric field, by collision, the charged particles constituting cathode rays excite the atoms of the gas in the tube. The atoms cannot remain in their excited state longer. They make a transition to the ground state, by emitting radiations, which are characteristic of the gas in the tube. But, with decrease in pressure, the gas gets rarefied more and more, the mean free path of the ions becomes very large. Now the atoms can move through the tube without collision and so atoms are not excited. At this stage, the discharge tube gets black-out. Thus discharge phenomenon cannot be observed at very low pressures.