STATES OF MATTER
First consider the behavior of collections of atoms Matter exists in three states: In a solid atomic particles are bound to permanent positions relative to each other; in a liquid the particle bonds are weak and temporary; by contrast in a gas there is no significant bonding between atomic particles, and the particles have no permanent positions relative to each other. Most of the matter in the universe is in the form of gas. Frequently matter is in the form of plasma, a gas composed of free electrons and positive ions, which are atoms from which one or more electrons have been stripped. Such ionization is generally the result of very high temperatures.
The particles of a gas can be molecules (which consist of two or more atoms), atoms themselves, or ions and electrons. In the molecule the atoms may be of the same element, as the two atoms of the oxygen molecule we breathe, or of different elements, such as the two hydrogen atoms and one oxygen atom in the water molecule. In the plasma there is a variety of . possibilities. Each atom can be stripped of one electron-so that there are two independent particles per atom-or stripped of two electrons-three particles per atom-and so on. Only some of the atoms may lose an electron, or in the extreme case all atoms lose all their electrons.
In the atomic world (as in a gas) gravity is not the cause for changes in motion as it is in the macroscopic world. The atomic world is dominated by electromagnetic forces. As with the force of gravity the intensity of electric and magnetic fields weakens as the inverse square of the distance from their source. At first glance this suggests that Newtonian mechanics ought to describe motion in the atomic domain, gravity as a cause simply being replaced by electromagnetic forces. Such is not the case in general, and the mechanics of the atom is called quantum mechanics. Its details go beyond our needs in this book, so we only point out that motion in the atomic world has a discrete nature rather than the continuous characteristics of our everyday experience.
RANDOM THERMAL MOTION
Within a gas particles dart about rapidly, colliding millions of times each second and changing their direction of motion just as frequently. Each gas particle has a kinetic energy proportional to the product of its mass and the square of its speed. After a collision the speed can be either greater or smaller than it was before the collision; the kinetic energy of each particle changes in its repeated collisions. Collectively, however, the gas particles will have some average kinetic energy, which changes only when energy is added to the gas or removed from it. Another way of saying this is that the average kinetic energy changes when the gas is heated or cooled.
Temperature is a measure of the average kinetic energy of gas particles. The motion of the particles composing a body, like ice or water or water vapor, is called random thermal motion for a gas. It increases as the temperature goes up, and it decreases as the temperature goes down. Absolute zero is reached when the average kinetic energy is zero. Seen in terms of the motion of the particles in the gas, temperature is a measure of that motion: The greater the temperature of the gas, the greater the random thermal motion.
Temperatures in astronomy are usually measured on the absolute, Kelvin (K), scale. In this system there are 100 divisions (degrees) between the freezing point (273 K) and the boiling point (373 K) of water.
TEMPERATURE AND HEAT TRANSfER
When we heat the air in a vessel, we are increasing the kinetic energy of each particle, which means a higher average kinetic energy. As a result the gas particles move about faster, and they collide more frequently and more violently with their su rroundings. If the density of the gas particles (the number per unit volume) is quite ,large, then the hot gas can transport a great deal of heat (or thermal energy) from one place to another. The flow of energy in nature is from regions in which the energy content is high to those in which it is low; natural physical processes tend to even out the energy content, whether on a small scale or a large one, and in general large energy differences and large volumes of space over which energy is to be transported require long periods of time to smooth out the differences. This fact will help us to understand many aspects of the evolution of astronomical objects.
