Astro 109 Lecture 11: Energy, Density, and Pressure October 8, 2014

Transcription

1 Astro 109 Lecture 11: Energy, Density, and Pressure October 8, 2014

2 of the day: Scorpius Zodiac visible in the sky at night in northern summer; Antares is the 15th brightest star in the sky. Located close to the plane of the Milky Way. In Greek mythology, represented the scorpion sent to kill Orion (now a northern winter constella@on!) by Artemis or Apollo. Image credit: IAU/Sky & Telescope

3 Why astronomers like Scorpius The globular cluster M80 lies 28,000 light- years from Earth, is about 95 light- years across, and contains several hundred thousand stars. Globular clusters are very old (> 10 Gyr) and thought to have formed in burst events when Milky Way was young. Image credit: HST/NASA/ESA

4 reminders Midterm will be in class on Wednesday, October 15 th. Four (!) more sets of in- person office hours before the midterm: Friday 10/10 (3:00-4:30pm in Scoa 102), Saturday 10/11 (3:00-4:30pm in Busch Student Center food court), Sunday 10/12 (3:00-4:30pm in Livingston Student Center food court), and next Tuesday 10/14 (3:00-4:30pm in Serin 401). Homework # 5 due this Friday in Sakai before class. Review session on Friday (no clicker scores recorded): Focus will be on last year s midterm: I will go over the ten hardest ques@ons in detail, then open the floor to ques@ons about any topics, then invite you to work through the other 20 ques@ons with your neighbors while I float between groups. I ll post an answer key tomorrow evening in Sakai, to help you iden@fy any points you re confused about.

5 Preparing for the midterm Midterm will have 30 choice based on reading and lectures slides from first 11 classes (including today s!), resembling the homework and clicker Tips for studying: explaining ideas, answers to example etc. to a classmate, or a friend, or a stranger, or a brick wall. Don t just listen; if you can explain something, you ve understood it! Prac@ce drawing pictures, not just visualizing in your head. Take advantage of the Sakai chat room to discuss points on which you re confused (and chime in if you know the answer). If you d like to be matched up with others to study with, send me an with your name and preferred campus (Busch, College Ave, Cook/Douglass, Livingston). I ll do my best.

6 Sign up for study groups in Sakai Wednesday/Thursday this week and Monday/Tuesday next week are now available for signup via the Sign- up tab in Sakai (note: 4 for today is already full). Any topics from lectures 1-11 can be discussed. Wednesday/Thursday study groups next week will meet, but will mainly discuss midterm (s@ll an opportunity for extra credit). from last year s midterm you should be able to answer fully amer today s lecture: #7 (21%), #11 (71), #15 (52%), #25 (52%). At this point, you should be able to answer all ques@ons (but skip #21, which is flawed).

7 Doppler effect and Aside from orienta.on: faster rota.on produces a wider line. Note: figure is a bit misleading because rightmost panels implicitly have wavelength increasing to the le>!

8 Group clicker # 1 You have used the Southern African Large Telescope (SALT) to measure emission lines from two stars. The emission line from star Alpha spans a narrower range in wavelength than the emission line from star Beta. What statement can you make about the proper@es of these two stars? A. Alpha is rota@ng faster than Beta. B. Beta is rota@ng faster than Alpha. C. Alpha and Beta are rota@ng at different rates, but it is not possible to say which one is rota@ng faster. D. Alpha and Beta may be rota@ng at the same rate or different rates, depending on how close their rota@on axes are to the line of sight.

9 Two final thoughts about light

10 Imaging vs. spectroscopy Imaging size shape brightness color quick/inexpensive: don t have to disperse light less informa@ve Spectroscopy chemical composi@on temperature line- of- sight mo@on rota@on speed slow/expensive: have to disperse light into its cons@tuent colors more informa@ve

11 Why is the sky blue? Why is sunset red? SCATTERING! Oct. 4

12 Energy

13 Energy energy = energy of mo@on poten@al energy = stored energy (gravita@onal, chemical, etc.) radia@ve energy = energy of light Oct. 16

14 of energy Energy can change form e.g., - - > poten@al - - > kine@c Total amount of energy remains the same ( energy is conserved )

15 Clicker # 2 I will hold the pendulum at rest and then let go. Ignoring the effects of fric@on, how hard will it hit me when it swings away and back? A. It will collide with me at high speed. B. It will collide with me at low speed. C. It will just barely touch me. D. It will not touch me at all. E. It will not swing back at all, due to my telekine@c powers.

16

17 Oct. 16

18 Clicker # 3 How does a standard roller coaster work? At the start, A. the train is given a lot of chemical energy. B. the train is given a lot of kine@c energy. C. the train is given a lot of poten@al energy. D. the train is given a lot of radia@ve energy. E. the riders cross their fingers and hope for the best.

19 Clicker # 4 What can you say about the kine@c and poten@al energy of a planet when it is closest to the Sun (compared to the rest of its orbit)? kine@c energy poten@al energy A highest highest B highest lowest C lowest highest D lowest lowest E constant constant

20 Heat is a form of energy Thermal (heat) energy is the kine@c energy of par@cles moving randomly although usually we reserve the term kine@c for bulk mo@ons.

21 Temperature scales T F = 9 5 T C + 32 T C = 5 9 (T F 32) T K = T C Best way to measure thermal energy

22 of mass/energy Einstein: mass is a form of energy: E = mc 2 Can convert energy - - > mass or mass - - > energy

23 Density

24 Float or sink? What determines whether something floats or sinks? density = mass volume kg/m 3 g/cm 3 rock water air

25 Example: density of the Earth Mass = kg, diameter = 12,576 km, radius = 6,378 km volume = 4 3 R3 = m 3 = m 3 density = kg m 3 = 5, 500 kg m 3

26 Example: density of Saturn Mass = kg and radius = 54,364 60,268 km (larger at equator), so mean density = g/cm 3. Self- consistent defini@on of mean density a bit tricky, but the boaom line is: Saturn would float in a big enough bathtub! Image credit: Thomas Wilk

27

28 Clicker # 5 What will happen when we add salt to the water? Pepsi Diet Pepsi A float float B float sink C sink float D sink sink E explode explode

29 Pressure

30 Pressure = force per unit area Same force spread over larger area means lower pressure. Greater force applied to same area means higher pressure.

31

32 Atmospheric pressure Pressure = force / area At sea level, approx. 15 psi What happens to the air pressure when you go up a mountain or in an airplane?

33 Clicker # 6 What will happen to me when I lie on the bed of nails? A. I will be in extraordinary pain for a few seconds, but then my body will adjust. B. I will feel like I weigh less than usual, because there is a lot of empty space between the nails. C. I won t feel any pain because the normal weight of my body will be supported by so many nails. D. I will feel like I weigh more than usual, because the nails will be pressing upward with greater force than the floor. E. Next week s midterm will be cancelled.

34