# Calculate the rate of heat conduction through a layer of still air that is 1 mm thick, with an…

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Calculate the rate of heat conduction through a layer of still air that is 1 mm thick, with an area of 1 m^{2 }, for a temperature difference of *20° *C.

Q57:

Home owners and builders discuss thermal conductivities in terms of the *R *value *(R *for *resistance) *of a material, defined as the thickness divided by the thermal conductivity:

(a) Calculate the *R *value of a 1/8-inch (3.2 mm) piece of plate glass, and then of a 1 mm layer of still air. Express both answers in SI units.

(b) In the United States, *R *values of building materials are normally given in English units, °F.ft^{2 }.hr/Btu. A Btu, or British thermal unit, is the energy needed to raise the temperature of a pound of water by 1°F. Work out the conversion factor between the SI and English units for *R *values. Convert your answers from part (a) to English units.

(c) Prove that for a compound layer of two different materials sandwiched together (such as air and glass, or brick and wood), the effective total *R *value is the sum of the individual *R *values.

(d) Calculate the effective *R *value of a single piece of plate glass with a 1.0mm layer of still air on each side. (The effective thickness of the air layer will depend on how much wind is blowing; 1 mm is of the right order of magnitude under most conditions.) Using this effective *R *value, make a revised estimate of the heat loss through a I_m^{2 }single-pane window when the temperature in the room is 20°C higher than the outdoor temperature.

Q58:

According to a standard reference table, the *R *value of a 3.5inch-thick vertical air space (within a wall) is 1.0 (in English units), while the *R *value of a 3.5-inch thickness of fiberglass batting is 10.9. Calculate the *R *value of a 3.5-inch thickness of *still *air, then discuss whether these two numbers are reasonable. (Hint: These reference values include the effects of convection.)

Q59:

Make a rough estimate of the total rate of conductive heat loss through the windows, walls, floor, and roof of a typical house in a cold climate. Then estimate the cost of replacing this lost energy over the course of a month. If possible, compare your estimate to a real utility bill. (Utility companies measure electricity by the kilowatt-hour, a unit equal to 3.6 MJ. In the United States, natural gas is billed in therms, where 1 therm = 10^{5 }Btu. Utility rates vary by region; I currently pay about 7 cents per kilowatt-hour for electricity and 50 cents per therm for natural gas.)

Q60:

A frying pan is quickly heated on the stovetop to 200°C. It has an iron handle that is 20 cm long. Estimate how much time should pass before the end of the handle is too hot to grab with your bare hand. (Hint: The crosssectional area of the handle doesn't matter. The density of iron is about 7.9 *g/cm** ^{3 }*and its specific heat is 0.45 J/g·°C).

Q61:

Geologists measure conductive heat flow out of the earth by drilling holes (a few hundred meters deep) and measuring the temperature as a function of depth. Suppose that in a certain location the temperature increases by 20°C per kilometer of depth and the thermal conductivity of the rock is 2.5 W /*m·K. *What is the rate of heat conduction per square meter in this location? Assuming that this value is typical of other locations over all of earth's surface, at approximately what rate is the earth losing heat via conduction? (The radius of the earth is 6400 km.)

Q62:

Consider a uniform rod of material whose temperature varies only along its length, in the *x *direction. By considering the heat flowing from both directions into a small segment of length ∆*x, *derive the heat equation,

where *K *= *kt**I**cp, *c is the specific heat of the material, and *p *is its density. (Assume that the only motion of energy is heat conduction within the rod; no energy enters or leaves along the sides.) Assuming that *K *is independent of temperature, show that a solution of the heat equation is

where *T _{o} *is a constant background temperature and

*A*is any constant. Sketch (or use a computer to plot) this solution as a function of

*x,*for several values of

*t.*Interpret this solution physically, and discuss in some detail how energy spreads through the rod as time passes.

Q63:

At about what pressure would the mean free path of an air molecule at room temperature equal 10 em, the size of a typical laboratory apparatus?

Q64:

Make a rough estimate of the thermal conductivity of helium at room temperature. Discuss your result, explaining why it differs from the value for air.

Q65:

Pretend that you live in the 19th century and don't know the value of Avogadro's number* (or of Boltzmann's constant or of the mass or size of any molecule). Show how you could make a rough estimate of Avogadro's number from a measurement of the thermal conductivity of a gas, together with other measurements that are relatively easy.

Q67:

Make a rough estimate of how far food coloring (or sugar) will diffuse through water in one minute.

Q68:

Suppose you open a bottle of perfume at one end of a room. Very roughly, how much time would pass before a person at the other end of the room could smell the perfume, *if *diffusion were the only transport mechanism? Do you think diffusion *is *the dominant transport mechanism in this situation?

Q69:

Imagine a narrow pipe, filled with fluid, in which the concentration of a certain type of molecule varies only along the length of the pipe (in the *x *direction). By considering the flux of these from both directions into a short segment ∆*x, *derive Fick's second law

Noting the similarity to the heat equation derived in Q62, discuss the implications of this equation in some detail.

Q70:

In analogy with the thermal conductivity, derive an approximate formula for the diffusion coefficient of an ideal gas in terms of the mean free path and the average thermal speed. Evaluate your formula numerically for air at room temperature and atmospheric pressure, and compare to the experimental value quoted in the text. How does *D *depend on *T, *at fixed pressure?

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