1. Introductory Astronomy
Week 3: Classical Physics

2. Matter
By early 1900s: a unified understanding through atomic theory
All matter made of a hundred or so elements – types of atoms labeled by
These bind to form molecular compounds
Three states: solid, liquid, gas
Bulk properties determined by microscopic dynamics
Temperature is a measure of average random motion of atoms and molecules
In ideal gas
measured in
pressure in
Fluid will flow from a region of higher pressure to a region of lower pressure until pressures equilibrate

3. Heat Transfer
An object hotter than environment will lose energy until temperatures equilibrate
Conduction: Heat can be transferred through continuous contact. Rare in astronomy
Convection: Physical motion of fluid carries energy. Works well when heating from below - Heated fluid less dense so rises
Radiation: Hot objects glow losing energy to light

4. Luminosity and Brightness
Sun is hot so radiates energy at a rate
Luminosity in
Brightness is flux in
At a distance radiation distributed uniformly on surface of sphere

5. States
At low temperature and sufficient pressure form (almost) incompressible liquid.
In equilibrium with gravity pressure increase with depth proportional to density
Density decreases with temperature: hot fluid rises
In solid state positions of atoms fixed – maintain shape under external force
Perturbations travel through matter as sound waves with a speed characteristic of the material
We hear sound as a perturbation in pressure and density in air.
Slinky
Free-falling cup

6. Waves Periodic disturbances characterized by frequency in
Traveling at speed produce periodic wave with wavelength
Amplitude is value of the perturbation at maximum
Energy flux in carried by wave is
When two waves meet disturbances add
If opposite sign - subtract!
Waves PhET
Interference PhET

7. Interference

8. Doppler Effect
Sound from a moving source heard at higher/lower when source approaching/receding
Doppler (1842)
source receding
L = l0 + vT = l0 – vl0/c

9. Another Wave? Light carries energy at a speed of
Newton 1670: white light contains all colors. A stream of particles
Young 1799: observes interference. Light is a wave
Young 1802: we observe relative intensity of RGB
Color is the frequency of light. For visible light of order Wavelength is nm
Colors demo, grating

10. Diffraction Grating Time delay between waves from slits determined by
Extra distance is
Constructive interference when D

11. Another Force Dominant force in most of physics: electromagnetism
Coulomb force
can be attractive or repulsive
Opposite charges attract so most objects neutral
Charge is conserved
A charge creates and is affected by electric field
Electric Field PhET

12. Magnets – and Light
Moving charges create and are affected by magnetic fields (Ørsted 1820)
Changing magnetic field creates electric field (Faraday 1831)
Changing electric field creates magnetic field (Maxwell 1861)
Leads to propagating waves with velocity
Coincides with speed of light (Fizeau-Foucault 1850)
Light is an electromagnetic wave!
Faraday PhET

13. Electromagnetic Spectrum
Electromagnetic waves can have any wavelength
What we see is limited by our eyes which are adapted to transparency of atmosphere
What the Universe produces is not. Observing the Universe in many bands produces additional data

14. Heat Radiation A hot object radiates
For dense dark objects radiation completely characterized by temperature – blackbody radiation
Hotter objects are blue
Wien 1893
Hotter objects radiate more. Stefan-Boltzmann 1879
flux at object
BB simulation ICLB is ~3Ks

15. Example: Our Sun Measure Solar constant Compute luminosity Sun radius
Luminosity is so
Set to find
Use Wien
This is green
L = 4pi(1.496E11)^2*1361 = 3.83E26

16. When Light Meets Matter
Dense objects absorb light energy or reflect it.
How much absorbed can depend on wavelength – dyes. Can learn composition from reflected spectrum
Light scatters off tenuous matter (Rayleigh 1871)
Scattering decreases with wavelength: blue scatters more than red
Sunset demo

17. Scattering on Earth
Atmosphere scatters blue light making sky glow blue and Sun appear yellow
When we get more scattering – when Sun low in sky – lose green to scattering leaving Sun red
Sky blue demo

18. Scattering and Refraction
Moon halo
Rainbow

19. Line Spectra Fraunhofer 1814: Sun’s spectrum has gaps
Kirchoff-Bunsen 1859: Tenuous gas emits line spectrum
Atoms and molecules emit/absorb at characteristic wavelengths when heated or ionized
Line spectrum yields chemical composition
At higher pressure and density lines broadened
Three views spectra suimulator
Helium discovered1868

20. Structure and Spectra
Atoms simplify physics. ~100 atoms explain all chemical compounds
Line spectra provide hints to the internal structure of atoms
From a broad spectrum of excitation, atom absorbs/emits at resonant frequencies determined by internal structure
Electromagnetic radiation excites internal vibrations, associated to internal dynamics of charge

21. What’s an Atom? Mass of atom is
Are Hydrogen atoms the building blocks?
Thompson 1897: negative electrons inside atoms
In fact electrons!
Atoms bind by rearranging, sharing, or deforming their electrons
Ions have missing or extra electrons
Chemistry is the science of electronic rearrangement

22. Inside the Atom Rutherford 1909: Structure of the Atom is Keplerian
Heavy nucleus of positive charge of size
Orbited by light electrons of negative charge in orbits of size
Atoms can bind by trading, sharing, or deforming their electrons.
Chemistry is the science of electronic rearrangement
Elements immutable because nucleus not affected

23. Problems?
Electrons in an atom are accelerating so should radiate losing energy. How are atoms stable?
Why are line spectra discrete?
Light exhibits particle behavior (Planck 1900, Einstein 1905)
Electrons exhibit interference (Davisson-Germer 1927)

24. Quantum Mechanics
All resolved by a revolution in our understanding of Nature
A physical system (say, a particle) described by a (complex) wavefunction on space of configurations
Space of such functions is a vector space
Physical observables associated to Hermitian linear operators
Results of a measurement are eigenvalues
In a state given by measure with probability

25. Energy eigenvalues for electron in atom
Dominant electromagnetic interaction is emission/absorption of a single photon of frequency and transition between levels with
Pauli (1925): Exclusion principle. At most two electrons can occupy a given state. Explains periodic table and chemistry
Hydrogen Atom Phet

26. Nuclear Structure
Atomic nucleus contains positive protons (Hydrogen nuclei, Rutherford 1917) and neutral neutrons (Chadwick 1932)
Nuclei with same but different are chemically identical isotopes
New simplification: Three particles

27. Three Particles, Two ForcesQ M
Spin 1
1/2 -1
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