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Sources for first article summary Choose one energy/climate related article from either:

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Sources for first article summaryChoose one energy/climate related article from either:

Sept. 2006 Scientific American or July 2008 Physics Today

http://www.libraries.iub.edu/index.php?pageId=92 (Swain library website)

Chapter 5 Home Energy Cons.

- Thermal Resistance:

Q/Dt = kADT/L =: ADT/R

R:= L/k

- This is useful because it folds in both the material property (k) and the thickness of the insulating layer (L), AND if you combine layers, then the thermal resistances (R) simply add, as shown on the next slide.

R-value for a typical wall

See table 5.2 in H&K for typical values of building materials

Infiltration

Q/Dt = 0.018 Btu/ft3.Fo V K DT

Here K is the number of

“Air exchanges per hour”

and V is the interior volume

of the house/building.

Note: some exchange of air

is necessary (you need to

breath!), and this is not

readily apparent in this

figure.

Chapter 5 Degree Days

- Note that for conduction and convection the rate of heat exchange is proportional to |Tout-Tin|, and so the total heat exchanged over a season will be proportional to the average temperature difference times the number of days experiencing that temperature difference.
- The # of “degree-days” is just a simple way to perform this average:

Q/Dt = ADT/R and Q/Dt = 0.018 V KDT

- Multiply through by Dt, the product DtDT, averaged over a month or season, is the number of “degree days” (DT computed with respect to some reference temp for the home interior).

Exam I Review

- EXTRA OFFICE HOURS:
- Tuesday 11:00 to NOON AND 2:00 to 3:30
- WED 2:00 to 3:45
- THURS: 8:15 TO 9:00
- Covers Chapter 1 section A through Chapter 5 section E
- 22 questions, 5 points each
- About half involve no calculation, only about 5 or 6 involve calculations with more than 1 step.
- Only 4-6 questions from each other chapter [slightly fewer for chpt. 3 (short) and chpt. 5 (incomplete)].
- Exam cover page is on ONCOURSE. Does NOT give simple formulae that are really just definitions (e.g. Power=W/Dt; h=(useful E transferred)/(total energy input))
- CLOSED BOOK, CLOSED NOTES
- Calculators are allowed (but not cell phones!).

Key points so far

- Chapter 1
- Working with numbers, units, uncertainties
- Energy: definition, patterns of use, types, sources
- Exponential growth: annual and continuous compounding, doubling time,
- Hubbert’s curve
- Renewable/non-renewable energy resources
- Energy conservation
- Chapter 2
- Energy Conversions
- Newton’s laws
- Mechanical Energy: Kinetic, Potential
- Work
- Power
- Electrical suppliers cost structure

Key points so far (cont.)

- Chapter 3
- First law of Thermodynamics
- Efficiency of energy conversions
- Energy content of fuels and unit conversions
- Lighting (as an example of new technology giving better efficiency)
- Chapter 4
- “Zeroth” law of Thermodynamics (the real meaning of temperature)
- Second Law of Thermodynamics
- Heat Capacity and specific heats
- Latent Heats (fusion and vaporization)
- Heat transfer mechanisms: conduction, convection, radiation
- Radiant transfer (higher T, more light, shorter wavelength).
- Reversibility
- Heat Engines and Carnot Efficiency

Key points so far (cont.)

- Chapter 5 (through section E).
- Conduction as a source of inefficiency in building climate control (R values)
- Control of heat transfer as a way of improving efficiency (convection, radiation, conduction).
- Infiltration
- Degree-day (concept thereof)

Example questions

- (5 points) Provide names and brief descriptions for 4 different forms of energy and list with each one the physical quantity that is associated with it.
- (5 points) A 150 W light bulb was accidentally left on in an attic for 14 full days. If the cost of electricity is $0.078/kWh, how much did this oversight cost the homeowner?
- (5 points) Of the three forms of heat transfer we considered in class, which one does not require a physical link (i.e. one made up of atoms) between the hot and cold object? For this mechanism, how does the rate of energy transfer depend on the temperature of the hot object (for a fixed temperature of the cold object)?

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