Physics 5
Introduction to Astronomy

H. E. Smith                                                                                                       Spring 2007

Quiz 3 Monday 4/23/07

Physics 5 – Spring Quarter 2007

Prof. Smith

Answer Key

 

1. Which of the following transitions is not possible?

A. An electron begins in the ground state and then gains enough energy to become ionized.

B. An electron begins in an excited state and then gains enough energy to become ionized.

C. An electron begins in the ground state and then gains enough energy to jump to an excited state.

D. An electron begins in an excited state and then gains enough energy to jump to the ground state.

 

2. Does the uncertainty principle affect our ability to follow the path of a baseball? Why or why not?

A. No, because the exclusion principle says that large objects are excluded from the consequences of the uncertainty principle

B. Yes, because we cannot know both where the baseball is and which way it is going at the same time.

C. No, because the uncertainties in the position and momentum of the baseball are so small in comparison to its size and total momentum that they are unnoticeable.

D. No, because the uncertainty principle applies only to electrons.

 

3. Which of the following conditions lead you to see an absorption line spectrum from a cloud of gas in interstellar space?

A. The cloud is cool and very dense, so that you cannot see any objects that lie behind it.

B. The cloud is cool and lies between you and a hot star.

C. The cloud is extremely hot.

D. The cloud is visible primarily because it reflects light from nearby stars.

 

4. Which of the following statements about thermal radiation is  always true?           

A. All the light emitted by hot object has higher energy than the light emitted by a cooler object.

B. A hot object produces more total infrared emission than a cooler object.

C. A cold object produces more total infrared and radio emission per unit surface area than a hot object.

D. A hot object emits more radiation per unit surface area than a cool object.

 

5. Betelgeuse is the bright red star representing the left shoulder of the constellation Orion. All the following statements about Betelgeuse are true. Which one can you infer from its red color? 

A. Its surface is cooler than the surface of the Sun.

B. It is much more massive than the Sun.

C. It is much brighter than the Sun.

D. It is moving away from us.

 

6. Why do sunspots appear dark in pictures of the Sun?        

A. They are too cold to emit any visible light.

B. They are holes in the solar surface through which we can see through to deeper, darker layers of the Sun.

C. They are extremely hot and emit all their radiation as X-rays rather than visible light.

D. They actually are fairly bright, but appear dark against the even brighter background of the surrounding photosphere.

 

7. Suppose the surface temperature of the Sun were about 18, 000 K, rather than 6,000 K. How much more thermal radiation would the Sun emit? How would the thermal radiation spectrum of the Sun be different?

 

From the first rule of thermal radiation from your book (or from the second equation under thermal radiation on the web), we know that tripling the temperature of an object increases the amount of thermal radiation it emits per unit area by a factor of 3^4=81 (since L~T^4). Thus, increasing the surface temperature of the Sun from 6,000K to 18,000 K would increase its thermal radiation by a factor of 81. The higher temperature of the Sun would shift the peak of its thermal radiation spectrum from its current place in the visible light region into the ultraviolet. The hotter Sun would emit more energy at all wavelengths, with the greatest output coming in the ultraviolet.