Physics of scientist ha

Potentials add as scalars (Fields add as vectors) To determine the electric field surrounding a collection of two or more point charges requires adding up the electric fields due to each charge Since the electric field is a vector, this can often be a chore To find the electric potential due to a collection of point charges is far easier, since the electric potential is a scalar, and lience you only need to add numbers together without concern for direction This is a major advantage in using electric potential We have to include the signs of charges, however 600 CHAPTER 23 Electric Potential Potential energy, point charges Potential energy, point charges Electron volt (energy unit) 604 CHAPTER 23 An important device th~t makes use of voltage, and that allows us to "visualize" voltages in the sense of displaying graphically how a voltage changes in time, is the cathode ray tube (CRT) A CRT used in this way is an oscilloscope-but an even more common use of a CRT is as the picture tube of television sets and computer monitors The operation of a CRT depends first of all on the phenomenon of thermionic emission, discovered by Thomas Edison (1847-1931) in the course of experiments on developing the electric light bulb To understand how thermionic emission occurs, consider two small plates telectrodes) inside an evacuated "bulb" or "tube" as shown in Fig 23-18, to which is applied a potential difference (by a battery, say) The negative electrode is called the cathode, the positive one the anode If the negative cathode is heated (usually by an electric current, as in a lightbulb) so that it becomes hot and glowing, it is found that negative charge leaves the cathode and flows to the positive anode These negative charges are ~ now called electrons, but originally they: were called cathode rays since they seemed to come from the cathode We can understand how electrons might be "boiled off" a hot metal plate if we treat electrons like molecules in VI)' Thus the wave theory predicts that the speed of light in water, for example, is' less than in air; and Newton's particle theory predicts the ~reverse An experiment to actually measure the speed of light in water was performed in 1850 by the French physicist Jean Foucault, and it confirmed the wave,theory prediction By then, however, the wave theory was already fully accepted, as we shall see in the next Section It is easy to show that Snell's law of refraction follows directly from Huygens' principle, given that the speed of light V in any medium is related to the speed in a vacuum, c, and the index of refraction, n, by Eq 33-1, V = c/n From the Huygens' construction of Fig 35-3, angle ADC is equal to 82 and angle BAD is ,equal to 81 • Then for the two triangles that have the common side AD, we have 868 CHAPTER 35 The Wave Nature of Light; Interference 872 CHAPTER 35 The Wave Nature of Light; Interference l35-41 Coherence The two slits in Fig 35- act as if they were two sources of radiation They are called coherent sources because the waves leaving them bear the same phase relationship to each other at all times This happt;ns because the waves come from a single source to the left of the two slits in Fig 35-7 An interference pattern is observed only when the sources are coherent If two tiny lightbulbs replaced the two slits, an interference pattern would not be seen The light emitted by one lightbulb would have a random phase with respect to the second bulb, and the screen would be more or less uniformly illuminated Two such sources, whose output waves have phases that bear no fixed relationship to each other over time, are called incoherent sources The subject of coherence is,rather complicated, and we discuss it only briefly Two light beams not have to be in phas~ to be coherent and produce an interference pattern For example, suppose a piece of glass were placed in front of the lower slit in Fig 35-7 and suppose the glass is just thick enough to slow down the light so that it enters the lower slit a half wavelength behind the light entering the upper slit The two beams would be a constant 180 out of phase, but there would still be an interference pattern on the screen (Can you guess what it would look like? Hint: The central point would be dark instead of bright.) Two beams can be coherent whether they are in phase or out of phase; the important thing is that they have a constant phase relation to each other over time Coherent sources of water or sound waves are easier to obtain than are coherent sources of light-two loudspeakers receiving the same pure frequency signal from an amplifier will be coherent sources And two antennas connected to the same LC oscillator can be coherent sources of low-frequency electromagnetic 15 waves But LC oscillators at the high frequencies of visible light (10 Hz) don't exist since Land C can't be made small enough For sources of visible light we have to rely on the oscillations (or acceleration) of electric charge within atoms In an incandescent light bulb, for example, the atoms in the filament are excited by heating, and give off "wave trains" of light, each of which lasts only about 10- s The light we see is the sum of a great many such wave trains that bear a random phase relation to each other Two light bulbs are thus not coherent, and an interference pattern would not be seen It wasn't until the 1950s that a really coherent source of light was developed, the laser Because of its coherence, laser light on a double slit produces a very "clean" interference pattern Coherent and incoherent SOli rces " SECTION 35-4 Coherence 873 ... gravitational equipotentialline What shape must the orbit be? 12 Suppose the charged ring of Example 23-8 was not uniformly charged, so that the density of charge was twice as great near the... potential We have to include the signs of charges, however 600 CHAPTER 23 Electric Potential Potential energy, point charges Potential energy, point charges Electron volt (energy unit) 604 CHAPTER... about E? 19 Is the electric potential energy of two unlike charges positive or negative? What about two like charges? What is the significance of the sign of the potential energy in each case? Questions