Current flow
ELECTRIC CURRENT IN GASES
The molecules of a gas are known to be in a state of continuous motion, constantly colliding with one another. These collisions knock off electrons, producing free electrons and converting the atoms that have lost electrons into positive ions. Since a charged particle is called an ion, we may consider the free electrons as negative ions. Thus, the gas contains positive and negative ions, just as an electrolyte does. If positive and negative electrodes are placed in the gas, the free electrons tend to travel to the positive electrode, and the positive ions to the negative electrode, thus producing an electric current. Normally, a positive or a negative ion cannot travel very far in a gas before meeting an ion of opposite charge. This meeting would tend to produce neutralization and would result in neutral molecules. Since neutral molecules are not affected by the electric field between the two electrodes, the current would tend to stop flowing. But if the gas is placed in a sealed container (such as a glass tube or a bulb with the two electrodes sealed in) and if most of the gas is pumped out, then the ions can travel considerable distances without being impeded. The effect of the electric field is to speed up the motion of the ions, so the farther they travel, the more velocity they obtain. If a fast-moving ion collides with a neutral molecule, the ion tends to knock electrons off the neutral molecule, thus creating more ions. This process is cumulative and tends to keep a constant stream of ions moving towards the electrodes. In this manner, an electric current flows through a gas.
ELECTRIC CURRENT IN A VACUUM
If a free electron were in a vacuum within the electric field set up between positive and negative electrodes, the negatively charged electron would be attracted to the positive electrode. The movement of the electron would constitute a flow of electric current. It is upon this principle that the electron tubes used in radio and television receivers operate. We can construct an electron tube by sealing a pair of metal electrodes into opposite ends of a glass bulb and by evacuating the air from within the bulb, leaving a vacuum. Connecting the electrodes to a source of electromotive force makes them positive and negative, respectively. A question now arises: how can we get the free electron into the tube? As previously described, there is always a disorderly movement of free electrons within all substances, especially metals. If the difference of potential between the two sealed- in electrodes be made great enough, some of the free electrons of the negative electrode will be attracted so strongly to the positive electrode that they will leave the former fly through the vacuum to the latter. If a substance is heated, the movement of free electrons within that substance is increased. If the temperature is raised high enough, the movement of free electrons is increased to the point where some of the electrons actually fly off from the substance. We call this process thermionic electron emission. In most electron tubes, the negative electrode is heated to the point where it emits electrons.
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GASES, SOLIDS, LIQUIDS AND PLASMA
According to the molecular theory a gas consists of a large number of molecules which are far apart in comparison to their diameter. Each molecule moves freely and rapidly in the straight line except when it meets another molecule or collides with the walls of its container. As a result of numerous collisions, the speed of any one molecule constantly changes in direction and, generally, in amount. A gas being compressed, these collisions become more frequent. In addition, the speed a given molecule travels with is greatly increased when a gas is heated.
The law explaining gas pressure on the walls of a container due to the collision of separate molecules was first established and stated by Daniel Bernouilli, a Petersburg academician, as early as 1738. Even today the "Bernouilli effect" is considered to be one of the basic laws of thermodynamics.
The principal physical property distinguishing a gas from a liquid and a solid consists in its ability to expand and occupy all the space available to it. Gases, therefore, have neither definite volume, nor definite shape, on the contrary, they take both the volume and shape of the container into which they are placed. They readily diffuse. The ease with which the molecules of a gas diffuse shows not only that they are in rapid motion but also that the distance between them is large compared with the space occupied by the molecules themselves, in other words, compared with their own size.
Solid bodies and their behavior are the next subject we shall deal with. It is not difficult to distinguish the property of a solid from that of a gas. It may be stated as follows: in the solid phase or condition the molecules attract each other strongly and, hence, they are unable to move about freely and rapidly as in the case of a gas. Of course, they are also in motion but they are limited both in the manner and range of movement. In fact, they are limited to vibrational and sometimes rotational motions. This explains the definite size and shape of solid bodies which neither flow as do liquids, nor occupy all available space as gases do. We may consider them as vibrating about their mean positions with a motion like that of a weight vibrating at the end of a spring. It is these vibrations which produce the sensation of temperature. In a hot body the energy of the vibrations is large and the speed of the molecules, in passing through their mean positions, is found to be rather high. On the other hand in a cold body the energy of the vibrations is less, the speed of molecular motion being greatly reduced.
What feature distinguishes a liquid from the two other states of matter? We find that a liquid is not so compressible as a gas. The reason for a certain degree of compressibility of liquids is easily understood. The molecules of a liquid are much closer together than those of a gas. They are free to move but owing to the closeness of different molecules to each other they are unable to travel far without colliding with other molecules. Under such conditions on the one hand the molecules of a liquid are more free to move than the molecules of a solid but on the other hand they are less free to travel than those of a gas. In short, the molecule of a liquid is in a state of constant motion without any definite direction.
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It is quite free to travel from place to place within the liquid itself; but it cannot easily leave its surface, i. e. evaporate, because of a rather strong force of attraction observed between the molecules of any liquid. This force serves to fix the volume of a given liquid although its shape is changeable. In other words, liquids have a definite volume at a given temperature but they do not occupy all the available space and always take the shape of the container. They are found, in general, to diffuse much more slowly than gases do.
The fourth state of matter is considered below. One of the oldest fields of science is the one of electrical discharges in gases. Considering special properties of matter in discharge tubes William Crookes put forward the idea that such gases should be considered a fourth state of matter namely the plasma state.
The plasma is first of all an ionized gas in which the charges of both signs are approximately equal. The plasma is caused by heating the matter to very high temperatures. Under such conditions the so-called ionized gas is produced with a great mass of free electrons forced away from the atoms.
The production of the plasma can take place in a number of ways. Among them we find: spark discharge, arc discharge, chemical reactions of high energy and nuclear reactions. One should also mention here the bombardment of electromagnetic fields or particles of any other origin provided sufficient energy is available to provide the required work of the neutral atoms and ions.
An electrical conductivity which can be compared to that of some metals and strong electrolytes exists in the plasma.
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