Homework 6, due fri Nov 12, 2010
			ENVR 30: Fall 2010 

1. If you start with 60 kg of Pu-239, how much will be left after 72,000 years? 
After 1 million years? After 100 years? 
[HINT: look up half life in lecture notes;  round times to nearest number of halflives (you don't need exact answers).]

2. If you start with 60 kg of I-181, how much will you have left have left after 3 weeks?  After 1 year?  After 72,000 years?

3. Write a list of the good things and bad things about nuclear power
4. Write a list of the good things and bad things about solar energy
5. Write a list of the good things and bad things about fossil fuels
6. Write a list of the good things and bad things about hydroelectic power

7. Name the four basic types of solar energy devices and give a 2 or 3 sentence
explanation of each.  Rank the four methods in terms of efficient use of solar
energy?  State which method you think will be the most useful eventually and why.

8. Suppose you have an medium/small 1200 square ft. roof, and you cover it with 
photovoltaics.
Use information from the lecture notes to figure out how many kilowatt hours
you can get per month.  Look at your electricity bill (or someone elses)
and see whether the amount you calculated is more or less than the amount used.
Could you get all your electricity needs met with PV on your roof?
[Hint:  convert area to square meters and multiply by factor in lecture to
get Watts.  Then use efficiency of PV to reduce this.  Then multiply this
available power by time to get energy in one month.]

9. The three Gorges Dam on the Yangste river will be the largest hydroelectric
plant in the world.  
The height of the water will be 80.6 meters and the flow rate will be
about 30000 cubic meters of water per second.  Assume 90% efficiency in
the turbines. (water weighs 1000kg per cubic meter)
a. Calculate how much power it is predicted to produce. (CALCULATE IT YOURSELF;
DON't LOOK IT UP ONLINE)
b. How many 1 GW nuclear or coal fired power plants will this replace?
c. The cost is around 30 billion dollars, more than 1 million people had to 
relocate, and it may lead to the "environmental collapse" of the Yangtze river
eco-system.  Look up Three Gorges Dam online and answer the question:  
"In your opinion is the three Gorges hydroelectric system worth it"?
Please write your answer in the form of a letter for the editor, backing
up your opinion with facts and giving clear reasons for what you are saying.