Narciso Garcia

Crystal Structures and Bonding in Solids

Matter comes in three forms or phases: gases, liquids, and solids; the last two are called condensed systems. Semiconductors are crystalline solids. To understand their electrical properties we need a basic knowledge of their crystal structure.

Interference of Waves

In the preceding chapter we introduced the concept of waves as a periodic disturbance of a medium. We used familiar concepts such as waves on a string or in water to illustrate the phenomena. The general equation for a traveling wave was derived as was the concept of phase shift. In this chapter we will start with the behavior of two waves when they come together and the effect produced by their relative phase. We will first discuss this phenomenon with waves in water and then extend it to light waves. At this point we will assume, as did early investigators, that air could be the substance in which light-wave motion occurs. However, we will show in a later chapter that light waves do not require some substance or medium to support them.

Kinetic Theory of Gases and the Concept of Temperature

Heat and temperature proved to be very elusive concepts to early scientists. Most historic theories—for examples, the phlogiston and the caloric-assumed that heat was a substance that could flow, much as a gas or a fluid. In fact, the mathematics of heat flow were correctly worked out before scientists learned the true nature of heat and its associated property, temperature. For indeed “heat” does flow; but what is heat?

The Beginning of the Quantum Story

By the end of the nineteenth century, most physicists felt rather good about the state of their art. In fact, some felt that their successors would spend their time simply taking measurements to the next decimal place. There were reasons for this complacent attitude. Most of the astronomical data about the motion of the planets, as well as the behavior of ordinary mechanical systems, could be explained using Newton’s laws of motion and his law of universal gravitation. The empirical laws concerning electric and magnetic fields had been discovered and fused together by Maxwell, and his prediction of the existence of electromagnetic waves had been experimentally verified by Heinrich Hertz: The nature of light was no longer a mystery. More important, the same laws used to explain the behavior of macroscopic systems were also able to explain the behavior of submicroscopic objects (atoms and molecules). This came about with the development of the techniques of statistical mechanics. By applying Newton’s laws statistically the ideal gas law, PV = nRT could be derived. Similarly, the specific heat of gases could be predicted in agreement with the available experimental data.

Magnetic Fields and Electromagnetic Waves

Almost everyone has performed elementary experiments with bar magnets. If the bar magnet is suspended by a thread or supported by a pivot, one of the ends will point in a northerly direction. This end of the magnet is called the north pole of the magnet, with symbol N. The opposite end of the magnet is called the south pole, with symbol S. Elementary experiments also show that like poles repel and unlike poles attract. This suggests that there is something that we call a magnetic field by which poles can exert forces on each other. This field is similar to the two other fields we have already considered, the gravitational field and the electric field. There is one important difference, however: If we break a bar magnet in half, we cannot make single poles, but instead we will have two bar magnets. The broken end becomes the south pole of the half that has the north pole, and the other broken end becomes the north pole of the half that has the south pole.