CHAPTER 1 The Birth of Modern Physics

Transcription

1 CHAPTER 1 The Birth of Modern Physics 1.1 Classical Physics of the 1890s 1.2 The Kinetic Theory of Gases 1.3 Waves and Particles 1.4 Conservation Laws and Fundamental Forces 1.5 The Atomic Theory of Matter 1.6 Outstanding Problems of 1895 and New Horizons James Clerk Maxwell The more important fundamental laws and facts of physical science have all been 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, 1894 There is nothing new to be discovered in physics now. All that remains is more and more precise measurement. - Lord Kelvin, 1900

2 1.1: Classical Physics of the 1890s Mechanics Electromagnetism Thermodynamics

3 Mechanics began with Galileo ( ) The first great experimentalist: he established experimental foundations. He described the Principle of Inertia.

4 Mechanics achieved maturity with Isaac Newton Three laws describing the relationship between mass and acceleration. Newton s first law (Law of inertia): An object with a constant velocity will continue in motion unless acted upon by some net external force. Isaac Newton ( ) Newton s second law: Introduces force (F) as responsible for the change in linear momentum (p = mv): Newton s 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:

5 Electromagnetism culminated with Maxwell s Equations Gauss s law: (electric field) E q/ 0 Gauss s law: (magnetic field) B 0 James Clerk Maxwell ( ) Faraday s law: Ampère s law: B E t E B 0 0 t in the presence of only stationary charges.

6 The Laws of Thermodynamics First law: The change in the internal energy ΔU of a system is equal to the heat Q added to a system plus the work W done by the system: ΔU = Q + W Second law: It s impossible to convert heat completely into work without some other change taking place. Added later: Lord Kelvin The zeroth law: Two systems in thermal equilibrium with a third system are in thermal equilibrium with each other. Third law: It s impossible to achieve absolute zero temperature.

7 Primary results of 19 th -century Thermodynamics Established the atomic theory of matter Introduced thermal equilibrium Established heat as energy Introduced the concept of internal energy Created temperature as a measure of internal energy Realized limitations: some energy processes cannot take place

8 1.2: The Kinetic Theory of Gases The ideal gas equation for n moles of a simple gas: PV = nrt where R is the ideal gas constant, 8.31 J/mol K

9 Primary Results of the Kinetic Theory Internal energy U is directly related 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: f = 3 for simple translations in 3D space where N A = Avogadro s Number

10 More Results of the Kinetic Theory speed Maxwell derived a relation for the molecular speed distribution f(v): 3/ 2 m 2 2 (v) 4 v exp( v / 2 ) f N m kt 2 kt Boltzmann determined the root-mean-square molecular speed: v rms v 2 3 kt m thus relating energy to temperature for an ideal gas.

11 Other successes for Kinetic Theory It predicted: Diffusion Mean free path Collision frequencies The speed of sound

12 1.3: Particles and Waves Two 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.

13 The Nature of Light Newton promoted the corpuscular (particle) theory Particles of light travel in straight lines or rays Explained sharp shadows Explained reflection and refraction Newton in action "I procured me a triangular glass prism to try therewith the celebrated phenomena of colours." (Newton, 1665)

14 The Nature of Light Huygens promoted the wave theory. He realized that light propagates as a wave from the point of origin. He realized that light slowed down on entering dense media. Christiaan Huygens ( ) He explained polarization, reflection, refraction, and double refraction. Double refraction

15 Diffraction confirmed light to be a wave. While scientists of Newton s time thought shadows were sharp, Young s two-slit experiment could only be explained by light behaving as a wave. Fresnel developed an accurate theory of diffraction in the early 19 th century. Diffraction patterns One slit Two slits Augustin Fresnel

16 visible Light waves were found to be solutions to Maxwell s Equations. The electromagnetic spectrum is vast. microwave infrared UV X-ray radio wavelength (nm) gamma-ray All electromagnetic waves travel in a vacuum with a speed c given by: where μ 0 and ε 0 are the permeability and permittivity of free space

17 Triumph of Classical Physics: The Conservation Laws Conservation 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 interactions. 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.

18 1.5: The Atomic Theory of Matter Initiated by Democritus and Leucippus (~450 B.C.), who were the first to use the Greek atomos, meaning indivisible. Proust ( ) proposed the Law of definite proportions (combining of chemicals always occurred with the same proportions by weight). Dalton advanced the atomic theory 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): atoms. Cannizzaro ( ) made the distinction between atoms and molecules advancing the ideas of Avogadro.

19 Opposition to atomic theory Ernst Mach was an extreme logical positivist, and he opposed the theory on the basis of logical positivism, i.e., atoms being unseen place into question their reality. Wilhelm Ostwald ( ) supported Mach, but did so based on unexplained experimental results of radioactivity, 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 it s said that he did so because so many people rejected his theory. Ernst Mach ( )

20 Unresolved questions for atomic theory at the end of the 19 th century The atomic-theory controversy raised fundamental questions. The constituents of atoms became a significant question. The structure of matter remained unknown. Scanning Tunneling Microscope image of 76 individually placed iron atoms on a copper surface. This image (taken almost 100 years later) nicely proves the atomic theory! The atomic theory wasn t actually universally accepted.

21 1.6: Problems in 19 th -century physics In a speech to the Royal Institution in 1900, Lord Kelvin himself described two dark clouds on the horizon of physics: The question of the existence of an electromagnetic medium referred to as ether or aether. The failure of classical physics to explain blackbody radiation.

22 More problems: discrete spectral lines For reasons then unknown, atomic gases emitted only certain narrow frequencies, unique to each atomic species. Absorption Emission spectra from a cold atomic gases of gas in hot front of a hot atoms. source. Wavelength

23 More problems for 19 th -century physics There were observed differences in the electric and magnetic fields between stationary and moving reference systems. When applying a simple Galilean transformation, Maxwell s 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 solid

24 Additional discoveries in contributed to the complications. X-rays (Roentgen) Radioactivity (Becquerel) Electron (Thomson) Zeeman effect Roentgen s x-ray image of his wife s hand (with her wedding ring)

25 Overwhelming evidence for the existence of atoms didn t arrive until the 20th century. Max Planck advanced the atom concept to explain blackbody radiation by use of submicroscopic quanta. Boltzmann required the existence of atoms for his advances in statistical mechanics. Max Karl Ernst Ludwig Planck ( ) Einstein used molecules to explain Brownian motion (microscopic random motion of suspended grains of pollen in water) and determined the approximate value of their size and mass. Jean Perrin ( ) later experimentally verified Einstein s predictions.

26 Speed Quantum mechanics General relativity The Beginnings of Modern Physics These new discoveries and the many resulting complications required a massive revision of fundamental physical assumptions. c Special relativity The introduction (~1900) of the modern theories of special relativity and quantum mechanics became the starting point of this most fascinating revision. General relativity (~1915) continued it th -century physics Log (size)