Conservation Laws

1
Conservation Laws

• Energy A moving object carries kinetic energy
  EK mv2/2. 1 kg m2/sec2 1 joule. 1 calorie
  4.2 joules.
• This changes. But only by conversion to/from
  other forms. E.g. if we assign potential energy
  EP -GmM/R to masses m,M at distance R then
  total energy EK EP does not change as objects
  orbit (see Kepler 2).
• Energy comes in other forms chemical, electric,
  heat, etc. When all taken into account, the
  books balance.
• Momentum A moving object carries momentum P mv
  (a vector). This is conserved (think of a truck
  colliding with a beetle).
• Angular momentum Like momentum in a circle,
  explains how ice skaters twirl.

2
Seeing the Light

• How we know so much about things that are so far
  away

Light is a form of Energy that moves through
space at a speed of c 3.0108m/sec.
3
The speed of Light

• Make light here see it there. How fast?
• Galileo (1600) too fast.
• RØmer (1676) eclipses of Jupiters moons
  close.
• Fizeau/Foucault rotating mirror - got it very
  close.

This is in vacuum (or air) see later
4
Colors

• Newton (1670) White light contains all colors.
  Light is a stream of particles each of a given
  color.
• Huygens (1678) Light is a wave.

• Young (1801) Observes interference. Light is a
  wave.

Dont count Newton out yet. In 20th century,
quantum mechanics finds wave/particle duality.
5
Waves

• Waves are periodic disturbances in space (or in a
  medium). Wave (pattern) moves medium doesnt
• Wavelength l distance (m) between peaks (space
  period)
• Frequency n how many peaks per second (Hz) at
  any fixed point
• For visible light l is about 400-700 nm (4-710-7
  m)
• Long wavelength red short wavelength violet

ln c speed of wave. In a second see n peaks
covering distance ln
6
Superposition and Interference

• Waves add at any point but then continue
  undisturbed. Overlapping waves interfere.
• If two waves are in phase they reinforce each
  other (constructive) if they are out of phase
  they reduce each other (destructive)
• Youngs two-slit experiment

7
Diffraction Grating

• We can use interference as another technique to
  separate colors. The device you hold is a
  diffraction grating.
• Light of wavelength l interferes constructively
  at angle q such that
• Dq/57.3 l

8
What waves?

• Maxwell (1860) Electromagnetic waves propagate at
  c identifies light with these
• Electromagnetic spectrum extends beyond visible
  unifies many types of radiation
• Medium in which EM waves propagate at c (ether)?

9
Spectral Bands

• Astronomers use various parts of the spectrum to
  see different types of objects. Call these
  types bands
• Radio
• Infrared
• Visible
• Ultraviolet
• X-ray
• Gamma ray

10
The Universe in Bands
11
How to make EM radiation

• Fundamentally Jiggle electric charges at
  frequency n and they will radiate at that
  frequency.
• Your power circuits run at 50 Hz and radiate at
  50 Hz hence the annoying hum when you amplify
  an open circuit.
• How to jiggle at n c/410-7 7.51014 Hz?
  Heat stuff and it will jiggle its electrons.
• This is how incandescent bulbs work.

12
Blackbody Radiation

• Heating a sufficiently dense object produces
  radiation determined completely by the
  temperature.
• The hotter the bluer. Temperature (oK) measures
  energy of electrons in matter.
• Wiens (1893) law
• lmax(m) 0.0029/T(oK)
• The hotter the brighter. Stefan-Boltzmann (1879)
• F sT4
• s 5.6710-8 (J/S) m-2 K-4

Color of a star tells us its surface temperature