\u3ci\u3ePhysics\u3c/i\u3e, Chapter 46: Nuclear Reactions

One of the most important developments of twentieth-century physics was the formulation of the special theory of relativity. This theory was an outgrowth of the failure of all attempts to show that the motion of the source of light relative to the observer had any effect on the speed of light. It is impossible to account for these experimental findings of Michelson and Morley, and others, on the basis of classical mechanics and electromagnetic theory. In 1905, Albert Einstein put forth the suggestion that all experimental findings would be clarified if it were assumed that the speed of light is a constant and is independent of the relative motion of the source and the observer. This statement forms the first postulate of the special, or restricted, theory of relativity. The second postulate of the theory is that all systems which are in uniform motion relative to one another are equally valid frames of reference, and all fundamental physical laws must have the same mathematical forms in each of these reference frames. Einstein expressed the viewpoint that all motion was relative motion, that there was no absolute coordinate frame, and that it was impossible to distinguish between a state of rest and a state of uniform translational motion by any physical experiment whatever. Thus, if the statement that the velocity of light was 3 X 1010 cm/sec was a fundamental physical law, every observer in uniform translational motion who measures the velocity of light must obtain this value, regardless of the motion of the source of light

\u3ci\u3ePhysics\u3c/i\u3e, Chapter 25: Capacitance and Dielectrics

When an isolated charged conducting sphere bears a charge Q, the potential of the sphere may be computed from the results of Section 23-6 by considering that the electric intensity outside the sphere is as though the entire charge of the sphere were concentrated at its center

\u3ci\u3ePhysics\u3c/i\u3e, Chapter 21: Vibrations and Sound

There are two aspects of sound: one is the physical aspect which involves the physics of the production, propagation, reception, and detection of sound; the other, which is the sensation of sound as perceived by the individual, depends upon physiological and psychological effects. It is not desirable to separate the two aspects of sound completely, but the main emphasis in this book must necessarily be on the physical aspect. In this chapter we shall consider mostly musical sounds. A vocabulary has been developed to describe the sensation experienced when a musical sound is heard. Such terms as the pitch of a sound, its loudness, and its tone quality or timbre are used to describe the musical sound. The physicist, on the other hand, speaks of the frequency of the sound, its intensity, and the number and intensities of the overtones present in a musical sound. Unfortunately, there is not a one-to-one correspondence between the terms used by the physicist and the terms used by the musician. A great deal of progress has been made in recent years as a result of tests involving thousands of persons which attempt to correlate the sensation of sound with the physical properties of sound. Some of these results will be mentioned at appropriate places in this chapter

\u3ci\u3ePhysics\u3c/i\u3e, Chapter 46: Nuclear Reactions

One of the most important developments of twentieth-century physics was the formulation of the special theory of relativity. This theory was an outgrowth of the failure of all attempts to show that the motion of the source of light relative to the observer had any effect on the speed of light. It is impossible to account for these experimental findings of Michelson and Morley, and others, on the basis of classical mechanics and electromagnetic theory. In 1905, Albert Einstein put forth the suggestion that all experimental findings would be clarified if it were assumed that the speed of light is a constant and is independent of the relative motion of the source and the observer. This statement forms the first postulate of the special, or restricted, theory of relativity. The second postulate of the theory is that all systems which are in uniform motion relative to one another are equally valid frames of reference, and all fundamental physical laws must have the same mathematical forms in each of these reference frames. Einstein expressed the viewpoint that all motion was relative motion, that there was no absolute coordinate frame, and that it was impossible to distinguish between a state of rest and a state of uniform translational motion by any physical experiment whatever. Thus, if the statement that the velocity of light was 3 X 1010 cm/sec was a fundamental physical law, every observer in uniform translational motion who measures the velocity of light must obtain this value, regardless of the motion of the source of light

\u3ci\u3ePhysics\u3c/i\u3e, Chapter 30: Magnetic Fields of Currents

The first evidence for the existence of a magnetic field around an electric current was observed in 1820 by Hans Christian Oersted (1777-1851). He found that a wire carrying current caused a freely pivoted compass needle in its vicinity to be deflected. If the current in a long straight wire is directed from C to D, as shown in Figure 30-1, a compass needle below it, whose initial orientation is shown in dotted lines, will have its north pole deflected to the left and its south pole deflected to the right. If the current in the wire is reversed and directed from D to C, then the north pole will be deflected to the right, as seen from above. In terms of the forces acting on the poles, these forces are clearly perpendicular to the direction of the current and to the line from the nearest portion of the wire to the pole itself

\u3ci\u3ePhysics\u3c/i\u3e, Chapter 25: Capacitance and Dielectrics

When an isolated charged conducting sphere bears a charge Q, the potential of the sphere may be computed from the results of Section 23-6 by considering that the electric intensity outside the sphere is as though the entire charge of the sphere were concentrated at its center

\u3ci\u3ePhysics\u3c/i\u3e, Chapter 43: X-Rays

The study of the electric discharge through gases led directly to the discovery of x-rays by W. C. Roentgen in 1895. While operating a gas discharge tube, Roentgen observed that a platinum-barium cyanide screen at some distance from the tube fluoresced. He shielded the tube so that no visible radiation could reach the screen, but the fluorescence could still be observed. On interposing various materials between the tube and the screen, he found that the intensity of the fluorescence could be diminished, but that it was not completely obliterated. He interpreted these observations as being due to radiation coming from the walls of the tube which penetrated the absorbing screens and caused the screen to fluoresce, and he called the new radiation x-rays to indicate their unknown character. The x-rays were produced when the cathode rays struck the glass walls of the electric-discharge tube

\u3ci\u3ePhysics\u3c/i\u3e, Chapter 17: The Phases of Matter

A substance which has a definite chemical composition can exist in one or more phases, such as the vapor phase, the liquid phase, or the solid phase. When two or more such phases are in equilibrium at any given temperature and pressure, there are always surfaces of separation between the two phases. In the solid phase a pure substance generally exhibits a well-defined crystal structure in which the atoms or molecules of the substance are arranged in a repetitive lattice. Many substances are known to exist in several different solid phases at different conditions of temperature and pressure. These solid phases differ in their crystal structure. Thus ice is known to have six different solid phases, while sulphur has four different solid phases

\u3ci\u3ePhysics\u3c/i\u3e, Chapter 15: Heat and Work

Until about 1750 the concepts of heat and temperature were not clearly distinguished. The two concepts were thought to be equivalent in the sense that bodies at equal temperatures were thought to contain equal amounts of heat. Joseph Black (1728-1799) was the first to make a clear distinction between heat and temperature. Black believed that heat was a form of matter, which subsequently came to be called caloric, and that the change in temperature of a body when caloric was added to it was associated with a property of the body which he called the capacity. Later investigators endowed caloric with additional properties. The caloric fluid was thought to embody a kind of universal repulsive force. When added to a body, the repulsive force of the caloric fluid caused the body to expand. To explain the liberation of heat when a block of metal was filed, it was postulated that small filings were less able to retain caloric, by virtue of their large surface area, than a block of solid metal. Attempts were made to measure the weight of caloric by trying to observe a change in the weight of a body when its temperature was raised, but these experiments were contradictory. Among others, Count Rumford (1753-1814), an American born Benjamin Thompson, who gained his title in the service of the Elector of Bavaria, found that the weight of a block of gold was unaltered by as much as 1 part in 1,000,000 when raised from the freezing point of water to bright-red heat

\u3ci\u3ePhysics\u3c/i\u3e, Chapter 15: Heat and Work

Until about 1750 the concepts of heat and temperature were not clearly distinguished. The two concepts were thought to be equivalent in the sense that bodies at equal temperatures were thought to contain equal amounts of heat. Joseph Black (1728-1799) was the first to make a clear distinction between heat and temperature. Black believed that heat was a form of matter, which subsequently came to be called caloric, and that the change in temperature of a body when caloric was added to it was associated with a property of the body which he called the capacity. Later investigators endowed caloric with additional properties. The caloric fluid was thought to embody a kind of universal repulsive force. When added to a body, the repulsive force of the caloric fluid caused the body to expand. To explain the liberation of heat when a block of metal was filed, it was postulated that small filings were less able to retain caloric, by virtue of their large surface area, than a block of solid metal. Attempts were made to measure the weight of caloric by trying to observe a change in the weight of a body when its temperature was raised, but these experiments were contradictory. Among others, Count Rumford (1753-1814), an American born Benjamin Thompson, who gained his title in the service of the Elector of Bavaria, found that the weight of a block of gold was unaltered by as much as 1 part in 1,000,000 when raised from the freezing point of water to bright-red heat