(a) The Joule is the international unit of energy and is defined mechanically. It is the work done by 1 Newton of force applied for 1 meter. A watt is the international unit of power or the rate of doing work, and it is defined as doing 1 Joule of work for 1 second.
A Joule is not much energy and a watt is not much power. For this reason we often use kilowatts and kilojoules as a larger unit. A kilowatt is a convenient unit of electrical power (a typical space heater is 1.5 kw) and the power company bills us for electrical energy in units of kwh, or 1 kw used for 1 hour. The hour is a convenient unit of time.
1 kwh = 1000 Joules/second times 3600 seconds = 3.6 million Joules.
(b) The Btu is a unit of energy and is equal to 1055 Joules. It is defined as the amount of energy required to heat 1 pound of water by 1 degree F. The Btu is a convenient unit for cooking and heating because we can relate it to everyday experience.
The Btu/hour is power, or rate of supplying energy, although it is a small unit! Most cookstoves are rated in thousands of Btu/hour (e.g. 8000 Btu/hour is supplied by a typical stove top element). A Btu/hour is smaller than 1 watt. 1 watt = 3.41 Btu/hour.
(c). The answer is above, but to summarize, a Btu is a unit of energy that we can relate to. Watts are easy for the electric company to measure.
A house has 2000 sq.ft x 9 ft = 18,000 cubic
feet of air volume.
(a)To heat the air once requires (0.018 Btu/cu ft/1 deg F) x (18000
cu
ft)x 20
deg. F = 6480 Btu.
(b). If there is 1 air/change per hour, then 6480 Btu must be supplied every hour to heat the house. In a year, this is 6480 Btu/hour x 8760 hours = 5.6 x 10^7 Btu. (56.7 MBtu).
56.7 MBtu x $5.00 per MBtu is $283.50 per year, just to heat the air resulting from the required ventilation. To compensate for the heat loss through the walls, more energy would be required.
(c) A drafty house with 4 air/changes per hour (not uncommon), would cost 4 times as much to heat or about $1130 / year.
12. As the temperature of the outside air decreases, a solar collector loses more heat to the environment hence its efficiency decreases. If you want to heat water to a certain temperature, the rate of water flow has to decrease to give the water more time to heat.
15. One major problem with a direct gain solar home is that the sun shines only part of the day. Thermal mass is essentail and has two functions: the first to keep the house cool while the sun is baking down and the second to supply heat at night or on cloudy days.
16. Rocks are great for storing solar energy and their advantages were mentioned in the last problem set. They don't move in the bin so convection is not a problem. With water, convection rapidly moves warm water into contact with the walls of the storage container (or the surface), increasing the rate of heat loss. Finally, the air spaces between rocks allow air to pass through, which is great for transferring heat energy to and from the rock pile.
1. Deforestation increases the carbon dioxide
content of the atmosphere and this traps more heat, increasing global
warming. There are two major reasons why deforestation increases carbon
dioxide
levels:
(a) The trees and brush are usually burned, directly converting
plant material into carbon
dioxide in the atmosphere.
(b) There is less vegetation to
convert atmospheric carbon dioxide into plant material.
3. If the earth warms up enough to cause significant melting of the polar ice caps, there would be effects which would both increase and decrease further global warming. For example, the smaller area of ice would be less effective at reflecting the sun's rays back into space, increasing global warming. On the other hand, the warmer oceans and increasing ocean surface area (as coastal land areas are flooded) will increase the humidity of the atmosphere, cooling the earth. Other effects include releasing carbon dioxide trapped in the ice (causing heating), etc. Figure 9.6 of the text (p. 295) summarizes of some of these effects.
4. The major causes leading to global warming today are a result of human activity. Deforestation of tropical forests and burning of fossil fuels are two of the most serious problems and will have the most serious consequences. These are global problems and every human being in existence contributes to the effect. Assuming that we do not want to actively eliminate large numbers of people, only a planet-wide human movement will be able to stop or reverse the global warming trend. The worst offenders (industrialized nations) could make a significant difference if they would agree to limit carbon dioxide emissions. Currently, the U.S. administration refuses to participate in such efforts.
8. Ozone in the upper atmosphere is essential to most life on earth as it blocks a significant fraction of the sun's ultraviolet radiation. Ozone in the lower atmosphere is a major source of pollution and health problems (see Chapter 8, p 257-258.). Naturally occuring ozone is primarily located in the upper atmosphere, where it is beneficial. Humans are the primary cause of ozone in the lower atmosphere.
14. In the U.S. burning of fossil fuels is the primary source of CO2.
Two ways that this could be reduced, for example, might be by
a. promoting solar or other forms of energy in the home and industry
(to heat water, etc.) and
b. by promoting public transportation, which
uses much less fuel to move people around.
Some of the difficulties with either approach are obvious. The extra initial expense of converting to solar energy (for example), even if small, is unattractive to most people and industry. It would take a national educational program to encourage people to switch. As for public transportation, most SUV owners are not impressed, to say the least!