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PHYSICS 002
Midterm 1
Date: 11 March 2004
Time allowed: 70 minutes
Total: 70 marks


Name:

Student ID:

Seat:
A





Useful data:

Velocity of light = 300,000 km/s
1 AU = 1.5 108 km
Solar radius = 6.96 105 km


Section A: Multiple choice (12 marks)

(1) If the local time is 9 pm, and the Moon is rising from the East, the most likely phase of the Moon is:
A.
new


B.
crescent


C.
quarter


D.
gibbous


E.
full Answer: D


F.




Explanation: As seen from above the North pole,




















(2) If you heat a gas so that collisions continuously excite electrons to higher energy levels, then when the electrons fall back to lower energy levels the gas produces

A. black body radiation.
B. scattered electromagnetic radiation.
C. diffracted electromagnetic radiation.
D. an absorption line spectrum.
E. an emission line spectrum. Answer: E

A photon is emitted when an electron falls from a higher to a lower energy level.

(3) Which of the following statements about thermal radiation is always true?

A. A hot object emits more radio waves than a cool object.
B. A hot object emits less total radiation than a cool object.
C. A hot object emits more total radiation than a cool object.
D. A hot object emits more X rays than a cool object.
E. A hot object emits more total radiation per unit surface area than a cool object.
Answer: E

According to Stefan-Boltzmann law, the flux (that is, the radiation energy per unit surface area per unit time) increases with the temperature. However, the total radiation depends on both the flux and the surface area of the star.

Section B: True or False (22 marks)

(1-3) Determine whether the following statements about stellar spectra are true or false.


True
False

(1) A B5 star is cooler than a star whose peak of radiation is located at 600 nm.




(2) A star which has a very broad ionized calcium absorption line is moving away from the Sun.



(3) A K5 star is less luminous than a star of temperature 7,000 K and of the same radius.






(1)
The temperature of a B5 star is 15,000 K. Using Wiens law, the temperature of the second star is 3,000,000/600 = 5,000 K.


(2)
The breadth of an absorption line of a star is related to its density, but not its velocity relative to the Sun.


(3)
The temperature of a K5 star is 4,100 K. Using Stefan-Boltzmann law, its flux is lower than the second star. Since the surface areas of the 2 stars are the same, the luminosity of the K5 star is also lower.




(4-5) Compare the following two Earthbound astronomical telescopes: (a) a century-old refracting telescope with an objective lens of diameter 2 m; (b) a modern reflecting telescope with a diameter of 4 m. Determine whether