C1210 lecture 17

C1210 lecture 17 - We have a problem. Maxwells equations...

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We have a problem. Maxwell’s equations tell us that a moving charged particle (like an electron) must radiate energy when it is slowed or when its path is bent. When an electron radiates energy it loses its energy. When an electron circles an nucleus, it must be accelerated in a closed path, so it must radiate energy and lose energy. When the electron loses its energy, it must spiral into the nucleus and the atom should be destroyed. This (classical) description of the atom must be wrong, since we’re still here. How do we find an explanation out of this dilemma?
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Electromagnetic Radiation • In addition to transfer as heat, energy can be transferred as light or radiation. • Some chemical reactions emit light as they proceed: fire, “light sticks”, fireflies. Radiation is propagated through space as waves : when a charged particle is accelerated, it produces a pulsating electric field. The electric field creates a pulsating magnetic field that gives rise to another electric field farther away, and so on, making a train of pulses called an electromagnetic wave. Characteristics of electromagnetic waves: The “strength” of the electromagnetic wave is called its amplitude . The amplitude of an electromagnetic wave oscillates like a sine wave:
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Amplitude min max time One cycle
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The wave repeats at regular intervals. 1/the repeat time is the frequency of the wave. The frequency of the wave is the number of cycles of the wave in one s. Units of frequency: s -1 (hertz, Hz). If the wave oscillates twice in one second, its frequency is two Hz. The symbol for frequency is the lowercase Greek nu ( ν ) As the wave moves away from its source, the distance between successive peaks remains constant. The distance between peaks is the wavelength of the wave. Since it is literally a distance the units are those of distance (meters, cm, etc . Symbol: lowercase Greek lambda, λ . The product of the frequency and the wavelength is the velocity at which the wave moves: 1 - ms s 1 m : units SI in velocity s meters meters s 1 = × = = ×
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An electromagnetic wave in a vacuum moves at c , the (constant) speed of light: 3.00 × 10 8 ms -1 (to three significant digits). Therefore,
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This note was uploaded on 12/10/2009 for the course CHE 1220 taught by Professor Jespersen during the Spring '09 term at St. John's.

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C1210 lecture 17 - We have a problem. Maxwells equations...

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