Review for Test #2 October 12

Topics:• Radiation and the Electromagnetic Spectrum - Black bodies• Atoms and Spectroscopy - Doppler Effect, Bohr model• The Solar System and its Formation

Methods • Conceptual Review and Practice Problems • Review lectures (on-line) and know answers to clicker & HW questions• Try practice quizzes on-line (in MA)•Bring:• Two Number 2 pencils• Simple calculator (no electronic notes)

Reminder: There are NO make-up tests for this class

Test #2 Review

How to take a multiple choice test1) Before the Test:• Study hard• Get plenty of rest the night before2) During the Test:• Draw simple sketches to help visualize problems• Solve numerical problems in the margin• Come up with your answer first, then look for it in the choices• If you can’t find the answer, try process of elimination• If you don’t know the answer, Go on to the next problem and

come back to this one later• TAKE YOUR TIME, don’t hurry• If you don’t understand something, ask me.

Test #1 Useful Equations

P2 a3 Kepler’s laws, including:

Newton’s laws, including: F = ma

F =G m

1 m

2

R2

Gravitation:

Speed of electromagnetic waves: c = Energy = h

max energy

1T

Wien’s Law:

Stefan’s Law: L = A T4 where the area A = 4r2 for a sphere

Radiation travels as waves.Waves carry information and energy.

Review: Properties of a wave

wavelength ()

crest

amplitude (A)

velocity (v)trough

is a distance, so its units are m, cm, or mm, etc.

Period (T): time between crest (or trough) passages

Frequency (): rate of passage of crests (or troughs),

Also, v = 1T

(units: Hertz or cycles/sec)

c =

1 nm = 10 -9 m , 1 Angstrom = 10 -10 m

The Electromagnetic Spectrum

The frequency or wavelength of a wave depends on the relative motion of the source and the observer.

1. Refraction

Waves bend when they pass through material of different densities.

swimming pool

air

water

prism

airairglass

Things that waves do

Kirchhoff's Laws

1. A hot, opaque solid, liquid or dense gas produces a continuous spectrum.

2. A transparent hot gas produces an emission line spectrum.

3. A transparent, cool gas absorbs wavelengths from a continuous spectrum, producing an absorption line spectrum.

Example: Blackbody - the microwavebackground

Emits mostly in the radio, with a peak around 200 GHz

cold dust hotter star (Sun)

“cool" star

frequency increases, wavelength decreases

Approximate black-body spectra of astronomical objects demonstrate Wien's Law and Stefan's Law

very hot stars

Star wobbling due to gravity of planet causes small Doppler shift of its absorption lines.

Amount of shift depends on velocity of wobble. Also know period of wobble. This is enough to constrain the mass and orbit of the planet.

initial gas and dust nebula

dust grains grow by accreting gas, colliding and sticking

continued growth of clumps of matter, producing planetesimals

planetesimals collide and stick, enhanced by their gravity

result is a few large planets

Hubble observation of disk around young star with ring structure. Unseen planet sweeping out gap?

The Structure of the Solar System

~ 45 AU~ 5 AU

L4

L5

L3

Oort Cloud is a postulated huge, roughly spherical reservoir of comets surrounding the Solar System. ~108 objects? Ejected planetesimals.

A passing star may dislodge Oort cloud objects, plunging them into Solar System, where they become comets.

If a Kuiper Belt object's orbit takes it close to, e.g., Neptune, its orbit may be changed and it may plunge towards the inner Solar System and become a comet.

Study hard and do well!