現(xiàn)代物理學(xué)的誕生

1、CHAPTER 11.1 Classical Physics of the 1890s1.2 The Kinetic Theory of Gases1.3 Waves and Particles1.4 Conservation Laws and Fundamental Forces1.5 The Atomic Theory of Matter1.6 Outstanding Problems of 1895 and New Horizons The more important fundamental laws and facts of physical science have all bee

2、n discovered, and these are now so firmly established that the possibility of their ever being supplanted in consequence of new discoveries is exceedingly remote Our future discoveries must be looked for in the sixth place of decimals. - Albert A. Michelson, 1894There is nothing new to be discovered

3、 in physics now. All that remains is more and more precise measurement. - Lord Kelvin, 1900James Clerk Maxwell1.1: Classical Physics of the 1890sMechanics ThermodynamicsElectromagnetism Mechanics began with Galileo (1564-1642)The first great experimentalist: he established experimental foundations.H

4、e described the Principle of Inertia.Newtons third law (Law of action and reaction): The force exerted by body 1 on body 2 is equal in magnitude and opposite in direction to the force that body 2 exerts on body 1:Mechanics achieved maturity with Isaac NewtonIsaac Newton (1642-1727)Three laws describ

5、ing the relationship between mass and acceleration. Newtons first law (Law of inertia): An object with a constant velocity will continue in motion unless acted upon by some net external force.Newtons second law: Introduces force (F) as responsible for the change in linear momentum (p = mv):00EBt BEt

6、 0B0/EqElectromagnetism culminated with Maxwells EquationsGausss law: (electric field)Gausss law: (magnetic field)Faradays law:Ampres law:James Clerk Maxwell (1831-1879)in the presence of only stationary charges. The Laws of ThermodynamicsFirst law: The change in the internal energy U of a system is

7、 equal to the heat Q added to a system plus the work W done by the system:U = Q + W Second law: Its impossible to convert heat completely into work without some other change taking place. The “zeroth” law: Two systems in thermal equilibrium with a third system are in thermal equilibrium with each ot

8、her. Third law: Its impossible to achieve absolute zero temperature.Added later:Lord KelvinPrimary results of 19th-century ThermodynamicsEstablished the atomic theory of matterIntroduced thermal equilibriumEstablished heat as energyIntroduced the concept of internal energyCreated temperature as a me

9、asure of internal energyRealized limitations: some energy processes cannot take place1.2: The Kinetic Theory of Gases The ideal gas equation for n moles of a “simple” gas:PV = nRTwhere R is the ideal gas constant, 8.31 J/mol KPrimary Results of the Kinetic TheoryInternal energy U is directly related

10、 to the average molecular kinetic energy.Average molecular kinetic energy, K, is directly related to absolute temperature.Internal energy equally is distributed among the number of degrees of freedom (f ) of the system:where NA = Avogadros Numberf = 3 for simple translations in 3D space3/222(v)4v ex

11、p(v /2)2mfNmkTkTMore Results of the Kinetic TheoryMaxwell derived a relation for the molecular speed distribution f(v):thus relating energy to temperature for an ideal gas.Boltzmann determined the root-mean-square molecular speed:speed23vvrmskTmOther successes for Kinetic TheoryIt predicted: Diffusi

12、on Mean free path Collision frequencies The speed of sound1.3: Particles and WavesTwo ways in which energy is transported:Point mass interaction: transfers of momentum and kinetic energy: particles.Extended regions wherein energy is transferred by vibrations and rotations: waves.The Nature of LightN

13、ewton promoted the corpuscular (particle) theoryParticles of light travel in straight lines or raysExplained sharp shadowsExplained reflection and refractionI procured me a triangular glass prism to try therewith the celebrated phenomena of colours. (Newton, 1665)Newton in actionThe Nature of LightH

14、uygens promoted the wave theory.He explained polarization, reflection, refraction, and double refraction. Double refractionChristiaan Huygens (1629-1695)He realized that light propagates as a wave from the point of origin.He realized that light slowed down on entering dense media. Diffraction confir

15、med light to be a wave.Diffraction patternsOne slitTwo slitsWhile scientists of Newtons time thought shadows were sharp, Youngs two-slit experiment could only be explained by light behaving as a wave. Fresnel developed an accurate theory of diffraction in the early 19th century.Augustin FresnelLight

16、 waves were found to be solutions to Maxwells Equations.All electromagnetic waves travel in a vacuum with a speed c given by:10-1100101102103104infraredX-rayUVvisiblewavelength (nm)microwaveradio10-1100101102103104105106gamma-rayThe electromagnetic spectrum is vast.where 0 and 0 are the permeability

17、 and permittivity of free spaceTriumph of Classical Physics: The Conservation LawsConservation of energy: The sum of energy (in all its forms) is conserved (does not change) in all interactions. Conservation of linear momentum: In the absence of external forces, linear momentum is conserved in all i

18、nteractions.Conservation of angular momentum: In the absence of external torque, angular momentum is conserved in all interactions.Conservation of charge: Electric charge is conserved in all interactions.These laws remain the key to interpreting even particle physics experiments today.1.5: The Atomi

19、c Theory of MatterInitiated by Democritus and Leucippus (450 B.C.), who were the first to use the Greek atomos, meaning “indivisible.”Proust (1754 1826) proposed the Law of definite proportions (combining of chemicals always occurred with the same proportions by weight).Dalton advanced the atomic th

20、eory to explain the law of definite proportions.Avogadro proposed that all gases at the same temperature, pressure, and volume contain the same number of molecules (atoms): 6.02 1023 atoms.Cannizzaro (1826 1910) made the distinction between atoms and molecules advancing the ideas of Avogadro.Opposit

21、ion to atomic theoryErnst Mach was an extreme “l(fā)ogical positivist,” and he opposed the theory on the basis of logical positivism, i.e., atoms being “unseen” place into question their reality.Wilhelm Ostwald (1853 1932) supported Mach, but did so based on unexplained experimental results of radioacti

22、vity, discrete spectral lines, and the formation of molecular structures. (These are good points, but not against atomic theory, as it turned out.)Boltzmann committed suicide in 1905, and its said that he did so because so many people rejected his theory.Ernst Mach (1838-1916)Unresolved questions fo

23、r atomic theory at the end of the 19th centuryThe constituents of atoms became a significant question.The structure of matter remained unknown. The atomic theory wasnt actually universally accepted.The atomic-theory controversy raised fundamental questions.Scanning Tunneling Microscope image of 76 i

24、ndividually placed iron atoms on a copper surface. This image (taken almost 100 years later) nicely proves the atomic theory!1.6: Problems in 19th-century physicsIn a speech to the Royal Institution in 1900, Lord Kelvin himself described two “dark clouds on the horizon” of physics:The question of th

25、e existence of an electro-magnetic mediumreferred to as “ether” or “aether.” The failure of classical physics to explain blackbody radiation.More problems: discrete spectral linesWavelengthEmission spectra from gases of hot atoms.For reasons then unknown, atomic gases emitted only certain narrow fre

26、quencies, unique to each atomic species.Absorption spectra from a cold atomic gas in front of a hot source.More problems for 19th-century physicsThere were observed differences in the electric and magnetic fields between stationary and moving reference systems. When applying a simple Galilean transf

27、ormation, Maxwells Equations changed form. The kinetic theory failed to predict specific heats for real (non-ideal) gases.How did atoms form solids?Bismuth crystal, an interesting solidAdditional discoveries in 1895-7 contributed to the complications.X-rays (Roentgen)Radioactivity (Becquerel)Electron (Thomson)Zeeman effectRoentgens x-ray image of his wifes hand (with her wedding ring)Overwhelming evidence for the existence of atoms didnt arrive until the 20th century