Study Guide for Chapters 4-6 and 9-10

Chapter 4: See previous study guide. For this exam,

1. Review heat capacity and in-class Lab 2!

2. Thermodynamic efficiency - the theoretical limit for extracting work from heat based on a heat source (temperature = Th) and an exhaust (temperature = Tc).
Efficiency = (1 - Tc/Th) x 100%

Chapter 5: Home heat control

The central idea is that home heating is a large fraction of household energy use (50% or so) and so steps taken here can make a large difference, especially if they are required for all new homes. One drawback of taking these steps too far without careful planning is inadequate ventilation leading to mold growth etc. Attention to proper ventilation and humidity control will eliminate this problem.

Summary:

1. Better insulation, caulking, air leak seals. Air takes energy to heat but we require 1-2 complete air changes per hour in a well sealed home.
2. Alternative heating (heat pumps, "solar consciousness" in home design)
3. Thermostat setbacks at night or, just lowering the thermostat.

Home Heat Loss Control:

1. Heat flow through walls, windows and doors can be accurately calculated from published "R-values" and can be taken into account in the design stage.

Newton's law of cooling:
Heat flow (US units Btu/hr) = (1/R) x Area x Temperature difference (F).

You should be able to work an example given an R-value, area and temperature difference, see homework problems.

2. Window insulation: double pane glass (what are the physical principles?), blinds, reflective strips. (Review the three methods of heat conduction: direct conduction, convection, radiation).

3. Air infiltration, leaks, etc.

4. Alternative heating includes a) using some or all of solar energy design principles to take advantage of the sun's energy and b) heat pumps use less energy to move heat from the outside to inside (typically by a factor of 3). The expense of heat pump installation is paid off in a few years (5?).

5. Thermometer setbacks make use of Newton's law of cooling. Heat flow through walls is proportional to the (inside-outside) temperature difference. You should know how to calculate heating energy savings from thermometer setbacks!

Chapter 6. Solar Radiation

1. Review characteristics of solar radiation: heat radiation, visible light, UV. Energy inflow depends on time of day, season and latitude. Energy is also reflected by both layers of clouds: sunlight is reflected by top side, terrestrial radiation by the bottom side.

2. How does solar radiation differ from terrestrial radiation? What is the "greenhouse effect" and why is some "greenhouse effect" absolutely necessary for life on earth?

3. Understand the difference between the two concepts:

a) solar radiation measured in terms of energy per unit area per unit time (e.g. watts/meter squared, depends on time of day and season)

b) solar radiation measured in terms of expected "insolation" for a day at some location, say Eugene. That is, insolation is the total energy you can expect to collect during a typical sunny day (at that time of year) and is used as a basis for planning solar homes and collectors.

4. Home solar collector design: very practical for domestic hot water!

5. Passive solar home design: review basic features of design (insulation, heat storage, south windows), design types.

Chapter 9. Energy Use and Global Warming

Highlights: we did not cover the material in great detail

1. The earth is in a "sea of air", about 20 miles of atmosphere. The atmosphere contains nitrogen (78%), oxygen (20%), plus other gases, including carbon dioxide (CO2) and ozone (O3). In the upper layer, ozone protects us from ultraviolet light and is essential for life on earth.

2. Hydrocarbons -> hydrogen and carbon. Burning of hydrocarbon fuels produces CO2 and H20 for clean fuels such as natural gas and light petroleum products (gasoline). However, hydrocarbon pollution is caused by evaporation of gasoline or release of natural gas into the environment and should not be discounted.

3. Important Points: CO2 is not a pollutant and is produced by most living creatures, and absorbed by trees, plants and the ocean in the "carbon cycle". Plants use photosynthesis to convert carbon dioxide (CO2) into carbohydrates (sugar, starch) that is the basis for most non-plant life. The carbon cycle is a very complex phenomenon that involves all life on earth, and we are far from a full understanding (see details in chapter 9). Although CO2 is not a pollutant, it is a greenhouse gas. We are definitely increasing the CO2 content of the earth's atmosphere by burning wood and fossil fuels, and destroying forests. Increasing the CO2 content of the atmosphere may eventually change the earth's climate by trapping terrestrial radiation and the consequent "global warming". There is some evidence that the earth's average temperature is increasing and it does not take much to have serious consequences (e.g. a mere 2 degree C rise would partially melt the polar ice caps and raise the mean sea level, flooding coastal cities). Review Figure 9.6 on page 295

4. Chlorofluorocarbons are greenhouse gases and also can destroy the ozone layer. They are long-lived in the atmosphere. Many are now banned from production.

Chapter 10. Electricity. Exam will cover sections A -E only.

1. Basic properties of electricity: electric charges (+ and -), always present in neutral matter (+ and - cancel) and can be separated from each other by doing work. "Static" electricity = accumulation of charge on objects by scuffing on rugs, rubbing objects, etc. Like charges repel and unlike attract. Since work is done to separate charges, they can do work when they recombine (sparks, shocks, forces, etc.). Charges are never created or destroyed, merely moved around. Electrons are negatively charged and form the outer part of the atom. The inner part of the atom is called the nucleus and it is positively charged. In the process of "charging", we are really stripping a few electrons off of the outer layer of some of the atoms.

2. The flow of electricity in metals is due to "free electrons", which are normal components of the atoms in the metal. In metals, each atom shares an outer electron with other atoms, so they are free to move.

3. Batteries, generators merely move charges about and in the process, increase their potential energy. This energy may be used up elsewhere in an electrical circuit. The potential energy increase per unit charge is known as just the electrical potential and is called the voltage.

4. For electricity to flow, a complete circuit is required, that is, an unbroken path for electrons to flow from a battery or generator, to the place where work is done (e.g. a light bulb) and back to the battery or generator.

Question: if you have a bicycle with a generator on the wheel, there is usually only one wire leading to the light bulb. What is acting as the "other wire" to complete the circuit?

5. OHM'S LAW.

Review definition of

Voltage  (the potential energy increase of flowing electrons caused by battery or generator), or potential difference.

    1 Volt = 1 Joule / Coulomb (1 Coulomb = a large number of charges, about 10^19)

Example: an  AA cell increases the electron potential energy by 1.5 Volts per electron (and is the same for C and D cells. Why?)

Current = number of charges (electrons) that pass any point of the circuit per second.

    1 Ampere = 1 Coulomb per second

Resistance determines the current flow. Resistance is where the current flow is "restricted".

Definition of resistance R, measured in Ohms, is by OHM'S LAW

    I = V/R or V=IR or R = V/I (equivalent ways of writing the same rule).

    This is the same rule as heat conduction!

Electric current              = Coulombs/sec =  (1/R) x V                (V = potential difference)
Heat flow per unit area  =               Btu/h =  (1/R) x delta T       (delta T = Temperature difference)

Also true: the potential energy of the electrons is dissipated mostly in the resistance as heat or light.

Examples:
For a 120 Volt light bulb circuit, 1 ampere is flowing.
The resistance of the bulb is R = V/I = (120 Volts/ 1 Amp) = 120 ohms.

Power = volts times amps (P = VI, measured in watts)

A 100 watt bulb in a 120 volt circuit requires 100/120 = 0.83 amps

Circuit breakers

Wires have resistance! It is not high, but wires can heat up significantly if they carry a large current. If the current is too large, they can cause a fire. Home circuit breakers limit current through house wiring to 20-30 amps (depending on wire size) to prevent fires in case of a malfunction.

Electric current and humans: Skin resistance (R) and voltage of source (V) determine the current. Current is what gives us a shock and 0.01 amps (10 milliamps, mA) is very painful. Current of 0.07 amps (70 mA) often results in death, especially if the path of current flow includes the heart. For a typical 120 volt house circuit, shocks are usually not fatal unless your skin resistance is low, that is, if you are wet or sweaty, or a large area of your skin comes into contact with the circuit. Generally speaking, it is nearly impossible to get a fatal shock from a voltage source that is less than 30 volts, for example a 9 volt transistor battery or a 12 volt car battery.

An excellent analogy for an electrical circuit is a motor pumping water (see this figure) through a closed pipe circuit:

Battery = Pump
Voltage = water pressure
Electrical current (amperes) = water current (gallons per minute)
Resistor = constriction
The resistor (constriction) dissipates energy as heat
Total power = flow times pressure.
In electrical case P=VI or watts = volts times amps