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
These lecture notes were prepared by modifying those prepared by:
Anthony Pitucco, Ph.D.
Pima Community College
Dept of Physics, Chair
Tucson, Arizona
Prof. Rick Trebino, Georgia Tech,

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

3. Mechanics began with Galileo.
Galileo Galilei ( )
The first great experimentalist: he established experimental foundations.
He described the Principle of Inertia: If an object is at rest, it will stay at rest unless something acts upon it to move it. Similarly, if an object is moving in a straight line at constant speed, it will continue to do so unless something acts upon it.

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.
Newton’s second law: Introduces force (F) as responsible for the change in linear momentum (p = mv):
Isaac Newton ( )
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)
(magnetic field)
Faraday’s law:
Ampère’s law:
James Clerk Maxwell ( )
These are in vacuum. In the presence of charges, there are additional “source terms.”

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.
Lord Kelvin ( )
Added later:
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 the 19th Century: Thermodynamics
Established the atomic theory of matter, although not everyone agreed on this issue
Established heat as energy
Created temperature as a measure of internal energy
Introduced the concept of internal energy: kT/2 per degree of freedom, where k is Boltzmann’s constant
Realized limitations: some energy processes cannot take place
Image from

8. 1.2: The Kinetic Theory of Gases
The ideal gas equation for n moles of a gas:
pV = nRT
where R is the ideal gas constant, 8.31 J/(mole·K)
Average molecular kinetic energy, K, is directly related to absolute temperature.

9. More Results of the Kinetic Theory
Speed
Maxwell derived the molecular speed distribution f(v):
Boltzmann determined the root-mean-squared (rms) molecular speed:
f(v)
v (m/s)
thus relating kinetic energy to temperature for an ideal gas.

10. Other Successes for Kinetic Theory
It predicted:
Diffusion
Mean free path
Collision frequencies
The speed of sound

11. 1.3: Particles and Waves Two ways in which energy is transported:
Point mass interaction, which transfers momentum and kinetic energy: particles.
Extended regions wherein energy is transferred by vibrations and rotations (collective motions of particles): waves.

12. More Properties of Particles and Waves
Particles are highly localized in space and time.
Particles have well-defined trajectories.
Particles are either there (1) or not (0).
Particles cannot cancel each other out.
Waves are extended in space and time.
Waves have poorly defined trajectories.
Waves can be kind of there (~1/2) or even the opposite of there (<0).
Waves can cancel out.

13. The Nature of Light Newton promoted the particle theory of light.
Particles of light would travel in straight lines or rays.
Explained (what were thought to be) 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. Christiaan Huygens (1629-1695)
The Nature of Light
Huygens promoted the wave theory.
He felt that light propagates as a wave from the point of origin.
He realized that light slowed down on entering dense media.
Christiaan Huygens ( )
Double refraction
He explained polarization, reflection, refraction, and 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 19th century.
Diffraction patterns
One slit
Two slits
Augustin Fresnel ( )

16. Light waves were found to be solutions to Maxwell’s Equations.
The electromagnetic spectrum is vast.
infrared
X-ray UV
visible
wavelength (nm)
microwave
radio
105
106
gamma-ray
All electromagnetic waves travel with a speed c, where:
What exactly was waving was simply called “aether,” whose existence and properties were left for future physicists to determine.

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. 19th Century Philosophy
Until the 19th century, science had been a branch of philosophy, called Natural Philosophy. But the scientific method and dedication to observable data inspired a break and its own name.
Philosophy remained concerned with that which could not be measured: Metaphysics, which means Beyond Physics. Its main concerns were, and continue to be, Being, Existence, and Reality.
Typical questions of metaphysics:
Aristotle (384 BC – 322 BC)
Does essence precede existence? (Avicenna)
Is affirmation of existence nothing but denial of the number zero? (Frege)
Is being the value of a bound variable? (Quine)

19. Logical Positivism
Positivism, a field of philosophy developed by Comte in the early 19th century, states that the only authentic knowledge is that which allows positive verification. Positivism assumes that there is valid knowledge (truth) only in scientific knowledge.
Auguste Comte (1798 – 1857)
“The traditional pursuits of philosophers (metaphysics) are as unwarranted as they have been unfruitful.”
Positivism in its original form is no longer popular. It has been modified to allow for knowledge that might someday be verified.
Even in this form, it leaves little or nothing for philosophers to do.
But metaphysics remains popular even today among philosophers.
Wikipedia
A. J. Ayer, Language, Truth, and Logic

20. 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 (1754 – 1826) proposed the Law of definite proportions (combining of chemicals always occurred with the same proportions by weight).
Avogadro proposed that all gases at the same temperature, pressure, and volume contain the same number of molecules (atoms).
Cannizzaro (1826 – 1910) made the distinction between atoms and molecules, advancing the ideas of Avogadro.
Atom image from

21. Opposition to Atomic Theory
Ernst Mach was an extreme logical positivist, and he opposed the theory on the basis of logical positivism, i.e., he couldn’t see atoms.
Wilhelm Ostwald (1853 – 1932) 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 ( )

22. Unresolved Questions for Atomic Theory at the End of the 19th Century
The atomic-theory controversy raised fundamental questions.
The constituents of atoms became a significant question.
The structure of matter remained unknown.
The atomic theory wasn’t actually universally accepted.
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!

23. 1.6: Problems in 19th 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 electro-magnetic medium—referred to as “aether.”
The failure of classical physics to explain blackbody radiation.

24. More Problems: Discrete Spectral Lines
For reasons then unknown, atomic gases emitted only certain narrow frequencies, unique to each atomic species.
Absorption spectra from a cold atomic gas in front of a hot source.
Emission spectra from gases of hot atoms.
Wavelength

25. More Problems for 19th 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?
Picture:
Bismuth crystal, an interesting solid

26. Additional discoveries in 1895-7 contrib-uted 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)

27. Overwhelming evidence for the existence of atoms didn’t arrive until the 20th century.
In 1827 biologist Robert Brown noted that, under a microscope, tiny pollen grains wandered randomly through water. Why?
Wikipedia
In 1905, Einstein explained Brownian motion by molecules bumping into the pollen grain and determined the approximate value of molecules’ size and mass.
Jean Perrin (1870 – 1942) later experimentally verified Einstein’s predictions.

28. The Beginnings of Modern Physics
These new discoveries and the many resulting complications required a massive revision of fundamental physical assumptions.
The introduction (~1905) of the modern theories of special relativity and quantum mechanics became the starting point of this most fascinating revision. General relativity (~1915) continued it.
Log (size)
Speed c
19th-century physics
General relativity
Quantum mechanics
Special relativity