Homework 4, due Mon April 25, 2011
		Physics 12: Spring 2011

1. Why does your hand not feel the radiant energy from a fireplace if you 
place it behind a sheet of glass, even though you can see the fire easily?

2. Describe the pros and cons of photovoltaic electricity.

3. A car wash needs 1400 gallons of warm water a day heated from 50F to 100F.
How large a solar collector would be needed to do this?  Assume an incident
solar energy of 1100 Btu/ft^2, and assume the collector operates at 50%
efficiency.

4. You build a passive solar home and want to store
the solar energy in a concrete floor of 800 square feet area.  
How thick should you make this floor to store 200,000 Btu with
a temperature difference of 20 degrees F.

5. How many kilowatt hours per year could you expect from a photovoltaic
system consisting of 20 1 square meter solar panels? 
(Yes include estimates of efficiencies, etc.)

6. Compare the energy used by electric vs. gasoline cars as follows.
Suppose the gasoline car has a total efficiency of 15-20% (it can
be higher if it is a hybred, runs its engine at maximum effiency
and does regenerative braking, or lower if it is not a hybred and
spends time idling or running around in town at slow speeds).
Compare this to an all electric car which gets its electricity
from a coal fired power plant.  Take into account Carnot-type efficiency
of coal plant (look it up), transmission of power (8% loss of energy), 
battery charging efficiency (around 90%), and power train 
inefficiency (around 5% loss).
Which is better from an energy use point of view?  Next consider the
CO2 produced by both (coal generated electricity vs. gasoline).
Which is better from a global warming point of view?


Ristinen and Kraushaar, Chap 4: Multiple Choice: 3,6,7,16,17