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					                                                                                   Exercises


17.15 A Constant-Volume Gas Thermometer. An experimenter                 17.25 A glass flask whose volume is 1000.00 cm3 at 0 O°C is
using a gas thermometer found the pressure at the triple point of        completely filled with mercury at this temperature. When flask
water (0.0l0C) to be 4.80 X lo4 Pa and the pressure at the normal        and mercury are warmed to 55.0°C, 8.95 cm' of mercury over-
boiling polnt (100°C) to be 6.50 X lo4 Pa. a) Assuming that the          flow. If the coefftcient of volume expansion of mercury is
pressure varies linearly wlth temperature, uqe these two data points     18.0 X lo-' K-', compute the coefficient of volume expansion of
to find the Celsius temperature at which the gas pressure would be       the glass.
zero (that is, find the Celsius temperature of absolute zero). b) Does   17.26 a) If an area measured on the surface of a solid body is A, at
the gas in this thermometer obey Eq. (17.4) precisely? If that equa-     some initial temperature and then changes by AA when the temper-
tion were precisely obeyed, and the pressure at 100°C were               ature changes by AT, show that
6.50 X 104Pa, what pressure would the experimenter have mea-
sured at 0.0l0C? (As we will learn in Section 18.1, Eq. (17.4) is
accurate only for gases at very low density.)                            where a is the coefficient of linear expansion. b) A circi~lar sheet of
                                                                         aluminum is 55.0 cm in diameter at 15.0°C. By how much does the
Section 17.4 Thermal Expansion                                           area of one side of the sheet change when the temperature increases
17.16 Air Friction and Thermal Expansion. The supersonic air-            to 27S°C?
liner Concorde is 62.1 m long when sitting on the ground on a typ-       17.27 A machinist bores a hole of diameter 1.350 cm in a steel
ical (15°C) day. It is primarily made of aluminum. In flight at twice    plate at a temperature of 25°C. What is the cross-sectional area of
the speed of sound, friction with the air warms the Concorde's skin      the hole a) at 25OC; b) when the temperature of the plate is increased
and causes the aircraft to lengthen by 25 cm. (The passenger cabin       to 175OC? Assume that the coefficient of h e a r expansion remains
is on rollers, and the airplane expands around the passengers.)                                                   Hn:
                                                                         constant over this temperature range. ( i t See Exercise 17.26.)
What is the temperature of the Concorde's skin in flight?                17.28 As a new mechanical engineer for Engines Inc., you have
17.17 The Humber Bridge in England has the world's longest s n -         been assigned to design brass pistons to slide inside steel cylinders.
gle span, 1410 m in length. Calculate the change in length of the        The engines in which these pistons will be used will operate
steel deck of the span when the temperature increases from -5.0°C        between 20°C and 150°C.Assume that the coefficients of expansion
to 18.0°C.                                                               are constant over this temperature range. a) If the piston just fits
17.18 Ensuring a Tight Fit. Aluminum rivets used in airplane             inside the chamber at 20°C, will the engines be able to run at higher
construction are made slightly larger than the rivet holes and           temperatures? Explain. b) If the cylindrical pistons are 25.000 cm in
cooled by "dry ice" (solid CO,) before being driven. If the diame-       diameter at 20°C, what should be the minimum diameter of the
ter of a hole is 4.500 mm, what should be the diameter of a rivet at     cylinders at that temperature so the pistons will operate at 150°C?
23.0°C, if its diameter is to equal that of the hole when the rivet is   17.29 The markings on an aluminum ruler and a brass ruler are
cooled to -78.0°C. the temperature of dry ice? Assume that the           perfectly aligned at O°C. How far apart will the 20.0 cm marks be
expansion coefficient remains constant at the value given in             on the two rulers at 1 OO°C, if precise alignment of the left-hand
Table 17.1.                                                              ends of the rulers is maintained'?
17.19 A U.S. one-cent coin has a diameter of 1.9000 cm at 20.0°C.        17.30 A brass rod is 185 cm long and 1.60 cm in diameter. What
The coin is made of a metal alloy (mostly zinc) for which the coef-      force must be applied to each end of the rod to prevent it from con-
ficient of linear expansion 1s 2.6 X          K-I. What would its dia-   tracting when ~tis cooled from 120°C to I O°C?
meter be a) on a hot day in Death Valley (48.0°C)? b) on a cold          17.31 a) A wire that is 1.50 m long at 20°C is found to increase in
night in the mountains of Greenland (-53.0°C)?                           length by 1.9 cm when warmed to 420°C. Compute its average
17.20 The pendulum shaft of a clock is made of brass. What is the        coefficient of linear expansion for this temperature range. b) The
fractional change in length of the shaft when it is cooled from          wire is stretched just taut (zero tension) at 420°C. Find the stress in
 19.50°C to 5.00°C?                                                      the wire if it is cooled to 20°C without being allowed to contract.
17.21 A metal rod is 40.125 cm long at 20.0°C and 40.148 cm              Young's modulus for the wire is 2.0 X 10' Pa.
long at 45.0°C. Calculate the average coefficient of linear expan-       17.32 Steel train rails are laid in 12.0-m-long segments placed
sion of the rod for this temperature range.                              end-to-end. The rails are laid on a winter day when their tempera-
17.22 Acopper cylinder is initially at 20.0°C. At what temperature       ture is -2.0°C. a) How much space must be left between adjacent
will its volume be 0.150% larger than it is at 20.0°C?                   rails if they are just to touch on a summer day when their tempera-
17.23 An underground tank with a capacity of 1700 L (1.70 m3)            ture is 33.0°C? b) If the rails are originally laid in contact, what is
is fllled with ethanol that has an initial temperature of 19.O"C.After   the stress in them on a summer day when then temperature is
the ethanol has cooled off to the temperature of the tank and            33.0°C?
ground, which is 10.O°C, how much air space will there be above
the ethanol in the tank? (Assume that the volume of the tank doesn't     Section 17.5 Quantity of Heat
change.)                                                                 17.33 Heat Loss During Breathing. In very cold weather a sig-
 17.24 A steel tank is completely filled with 2.80 m" of ethanol         nificant mechanism for heat loss by the human body is energy
when both the tank and the ethanol are at a temperature of 32.0°C.       expended in warming the air taken into the lungs with each breath.
 When the tank and its contents have cooled to 18.0°C, what addi-        a) On a cold winter day when the temperature is -20°C, what is the
tional volume of ethanol can be put into the tank?                       amount of heat needed to warm to body temperature (37°C) the
                                                 c H A P T E R 17 I Temperature and Heat


0.50 L of air exchanged with each breath? Assume that the specific        pose that in this experiment heat transfer from the liquid to the con-
heat capacity of air is 1020 J/kg - K and that 1.0 L of air has mass      tainer or surroundings cannot be ignored. Is the result calculated in
 1.3 X lo-' kg. b) How much heat is lost per hour if the respiration      part (a) an overestimate or an underestimate of the average specific
rate is 20 breaths per minute?                                            heat capacity? Explain.
17.34 While running, a 70-kg student generates thermal energy at          17.42 You are given a sample of metal and are asked to determine
a rate of 1200 W. To maintain a constant body temperature of 37"C,        its specific heat capacity. You weigh the sample and find that its
this energy must be removed by perspiration or other mechanisms.          weight is 28.4 N. You carefully add 1.25 X lo4 J of heat energy to
If these mechanisms failed and the heat could not flow out of the         the sample and find that its temperature rises 18.0 CO.What is the
student's body, for what amount of time could a student run before        sample's specific heat capacity?
irreversible body damage occurs? (Protein structures in the body          17.43 You add 8950 J of heat to 3.00 moles of iron. a) What is the
are irreversibly damaged if body temperature rises to 44OC or             temperature increase of the iron? b) If this same amount of heat is
above. The specific heat capacity of a typical human body is              added to 3.00 kg of iron, what is the iron's temperature increase?
           -
3480 Jlkg K, slightly less than that of water. The difference is due      c) How do the results of parts (a) and (b) compare? Explain.
to the presence of protein, fat, and minerals, which have lower spe-
cific heat capacities.)                                                   Section 17.6 Calorimetry and Phase Changes
17.35 While painting the top of an antenna 225 m in height, a             17.44 As a physicist, you put heat into a 500-g solid sample at the
worker accidentally lets a 1.00-L water bottle fall from his lunch-       rate of 10.0 Idlmin, while recording its temperature as a function of
box. The bottle lands in some bushes at ground level and does not         time. You plot your data and obtain the graph shown in Fig. 17.27.
break. If a quantity of heat equal to the magnitude of the change in      a) What is the latent heat of fusion for this solid? b) What are the
mechanical energy of the water goes into the water, what is its           specific heats of the liquid and solid states of the material?
increase in temperature?
17.36 A crate of fruit with mass 35.0 kg and specific heat capacity
3650 J/kg. K slides down a ramp inclined at 36.9" below the hori-
zontal. The ramp is 8.00 m long. a) If the crate was at rest at the top
of the incline and has a speed of 2.50 rn/s at the bottom, how much
work was done on the crate by friction? b) If an amount of heat
equal to the magnitude of the work done by friction goes into the
crate of fruit and the fruit reaches a uniform final temperature, what
is its temperature change?




                                                                                                  +
17.37 An engineer is working on a new engine design. One of the
moving parts contains 1.60 kg of aluminum and 0.30 kg of iron and
is designed to operate at 210°C. How much heat is required to raise
its temperature from 20" to 210°C?                                                                                          t (min)
17.38 A nail driven into a board increases in temperature. If we
assume that 60% of the kinetic energy delivered by a 1.80-kg ham-         GgWe 17.27 Exercise 17.44.
mer with a speed of 7.80 m/s is transformed into heat that flows
into the nail and does not flow out, what is the temperature increase     17.45 A 500-g chunk of an unknown metal, which has been in
of an 8.00-g aluminum nail after it is struck ten times?                  boiling water for several minutes, is quickly dropped into an insu-
17.39 An aluminum tea kettle with mass 1.50 kg and containing             lating Styrofoam beaker containing 1.00 kg of water at room tem-
 1.80 kg of water is placed on a stove. If no heat is lost to the sur-    perature (20.0°C). After waiting and gently stirring for
roundings, how much heat must be added to raise the temperature           5.00 minutes, you observe that the water's temperature has reached
from 20.0°C to 85.0°C?                                                    a constant value of 22.0°C. a) Assuming that the Styrofoam absorbs
17.40 In an effort to stay awake for an all-night study session, a        a negligibly small amount of heat and that no heat was lost to the
student makes a cup of coffee by first placing a 200-W electric           surroundings, what is the specific heat of the metal? b) Which is
immersion heater in 0.320 kg of water. a) How much heat must be           more useful for storing heat, this metal or an equal weight of water?
added to the water to raise its temperature from 20.0°C to 80.0°C?        Explain. c) What if the heat absorbed by the Styrofoam actually is
b) How much time is required? Assume that all of the heater's             not negligible. How would the specific heat you calculated in
power goes into heating the water.                                        part (a) be in error? Would it be too large, too small, or still correct?
17.41 A technician measures the specific heat capacity of an              Explain.
unidentified liquid by immersing an electrical resistor in it. Electri-   17.46 Before going in for his annual physical, a 70.0-kg man
cal energy is converted to heat transferred to the liquid for 120 s at    whose body temperature is 37.OoC consumes an entire 0.355-L can
a constant rate of 65.0 W. The mass of the liquid is 0.780 kg, and its    of a soft drink (mostly water) at 12.0°C. a) What will be his body
temperature increases from 18.55"C to 22.54'C. a) Find the aver-          temperature after equilibrium is attained? Ignore any heating by the
age specific heat capacity of the liquid in this temperature range.       man's metabolism. The specific heat capacity of the man's body is
Assume that negligible heat is transferred to the container that          3480 J1kg.K. b) Is the change in his body temperature great
holds the liquid and that no heat is lost to the surroundings. b) Sup-    enough to be measured by a medical thermometer?
                                                                                    Exercises


17.47 In the situation described in Exercise 17.46, the man's             human, 3480 Jlkg K. The heat of vaporization of water at 34°C is
metabolism will eventually return the temperature of his body (and        2.42 X lo6 Jlkg.)
of the soft drlnk that he consumed) to 37.0°C. If his body releases       17.56 In a physics lab experiment a student immersed 200 one-
energy at a rate of 7.00 X 103kJ1day (the basal metabolic rate, or        cent coins (each having a mass of 3.00 g) in boiling water. After
BMR), how long does this take? Assume that all of the released            they reached thermal equilibrium, she fished them out and dropped
energy goes into raising the temperature.                                 them into 0.240 kg of water at 20.0°C in an insulated container of
17.48 An ice cube tray of negligible mass contains 0.350 kg of            negligible mass. What was the final temperature of the coins?
water at 18.0°C. How much heat must be removed to cool the                (One-cent coins are made of a metal alloy-mostly zinc-with a
water to O.O°C and freeze it? Express your answer in joules, calo-        specific heat capacity of 390 Jlkg .K.)
ries, and Btu.                                                            17.57 A copper pot with mass 0.500 kg contains 0.170 kg of water
17.49 How much heat is required to convert 12.0 g of ice at               at a temperature of 20.0°C. A 0.250-kg block of iron at 85.0°C is
 - 10.O°C to steam at 100.O°C?Express your answer in joules, calo-        dropped into the pot. Find the final temperature, assuming no heat
ries, and Btu.                                                            loss to the surroundings.
17.50 An open container holds 0.550 kg of ice at - 15.0°C. The            17.58 A laboratory technician drops a 0.0850-kg sample of
mass of the container can be ignored. Heat is supplied to the con-        unknown material, at a temperature of 100.O°C, into a calorimeter.
tainer at the constant rate of 800 Jlmin for 500 min. a) After how        The calorimeter can, initially at 19.0°C, is madeof0.150 kg of copper
many minutes does the ice start to melt? b) After how many min-           andcontains 0.200 kg of water. The final temperature of the calorime-
utes, from the time when the heating is first started, does the tem-      ter can is 26.1°C. Compute the specific heat capacity of the sample.
perature begin to rise above O°C? c) Plot a curve showing the             17.59 An insulated beaker with negligible mass contains 0.250 kg
temperature as a function of the elapsed time.                            of water at a temperature of 75.0°C. How many kilograms of ice at
17.51 The capacity of commercial air conditioners is sometimes            a temperature of -20.0°C must be dropped in the water so that the
expressed in "tons," the number of tons of ice (1 ton = 2000 lb)          final temperature of the system will be 30.0°C?
that can be frozen from water at O°C in 24 h by the unit. Express the     17.60 A glass vial containing a 16.0-g sample of an enzyme is
capacity of a 2-ton air conditioner in Btulh and in watts.                cooled in an ice bath. The bath contains water and 0.120 kg of ice.
17.52 Steam Burns vs. Water Burns. What is the amount of heat             The sample has specific heat capacity 2250 Jlkg .K; the glass vial
input to your skin when it receives the heat released a) by 25.0 g of     has mass 6.0 g and specific heat capacity 2800 J1kg.K. How much
steam initially at 100.O°C, when it is cooled to skin temperature         ice melts in cooling the enzyme sample from room temperature
(34.0°C)? b) by 25.0 g of water initially at 100.O°C, when it is          (19.S°C) to the temperature of the ice bath'?
cooled to 34.0°C? c) What does this tell you about the relative           17.61 A 4.00-kg silver ingot is taken from a furnace, where its
severity of steam and hot water burns?                                    temperature is 750°C, and placed on a large block of ice at O°C.
17.53 What must the initial speed of a lead bullet be at a tempera-       Assuming that all the heat given up by the silver is used to melt the
ture of 25°C so that the heat developed when it is brought to rest        ice, how much ice is melted?
will be just sufficient to melt it? Assume that all the initial mechan-   17.62 A copper calorimeter can with mass 0.100 kg contains
ical energy of the bullet is converted to heat and that no heat flows     0.160 kg of water and 0.018 kg of ice in thermal equilibrium at
from the bullet to its surroundings. (Typical rifles have muzzle          atmospheric pressure. If 0.750 kg of lead at a temperature of 255°C
speeds that exceed the speed of sound in air, which is 347 mls at         is dropped into the calorimeter can, what is the final temperature?
25°C.)                                                                    Assume that no heat is lost to the surroundings.
17.54 Evaporation of sweat is an important mechanism for tem-             17.63 A vessel whose walls are thermally insulated contains
perature control in some warm-blooded animals. a) What mass of            2.40 kg of water and 0.450 kg of ice, all at a temperature of O.O°C.
water must evaporate from the skin of a 70.0-kg man to cool his           The outlet of a tube leading from a boiler in which water is boiling
body 1.00 CO?The heat of vaporization of water at body tempera-           at atmospheric pressure is inserted into the water. How many grams
ture (37°C) is 2.42 X lo6 J1kg.K. The specific heat capacity of a         of steam must condense inside the vessel (also at atmospheric pres-
typical human body is 3480 J1kg.K (see Exercise 17.34). b) What           sure) to raise the temperature of the system to 28.0°C? You can
volume of water must the man drink to replenish the evaporated            ignore the heat transferred to the container.
water? Compare to the volume of a soft-drink can (355 cm3).
17.55 "The Ship of the Desert." Camels require very little water          Section 17.7 Mechanisms of Heat Transfer
because they are able to tolerate relatively large changes in their       17.64 Use Eq. (17.21) to show that the SI units of thermal conduc-
body temperature. While humans keep their body temperatures               tivity are Wlm - K.
constant to within one or two Celsius degrees, a dehydrated camel         17.65 Suppose that the rod in Fig. 17.20 is made of copper, is
permits its body temperature to drop to 34.0°C overnight and rise to      45.0 cm long, and has a cross-sectional area of 1.25 cm'. Let
40.0°C during the day. To see how effective this mechanism is for         TH = 100.O°C and Tc = O.O°C. a) What is the final steady-state
saving water, calculate how many liters of water a 400-kg camel           temperature gradient along the rod? b) What is the heat current in
would have to drink if it attempted to keep its body temperature at       the rod in the final steady state? c) What is the final steady-state
a constant 34.0°C by evaporation of sweat during the day                  temperature at a point in the rod 12.0 cm from its left end?
(12 hours) instead of rising to 40.0°C. (The specific heat capacity       17.66 One end of an insulated metal rod is maintained at 100°C,
of a camel or other mammal is about the same as that of a typical         and the other end is maintained at O°C by an ice-water mixture. The
                                                c H A P T E R 17 1 Temperature and Heat


rod is 60.0 cm long and has a cross-sectional area of 1.25 cm2.The        of the aluminum section is maintained at 150.0°C and the free end
heat conducted by the rod melts 8.50 g of ice in 10.0 mln. Find the       of the brass piece is maintained at 20.0°C. No heat is lost through
thermal conductivity k of the metal.                                      the sides of the rod. At a steady state, what is the temperature T of
17.67 A carpenter builds an exterior house wall with a layer of           the point where the two metals are joined?
wood 3.0 cm thick on the outside and a layer of Styrofoam insula-         17.74 What is the rate of energy radiation per unit area of a black-
tion 2.2 cm thick on the inside wall surface. The wood has k =            body at a temperature of a) 273 K? b) 2730 K?
0.080 W1m.K and the Styrofoam has k = 0.010 Wlm-K. The                    17.75 What is thenet rate of heat loss by radiation in Example 17.16
interlor surface temperature is 19.0°C, and the exterior surface tem-     (Section 17.7) if the temperature of the surroundings is 5.0°C?
perature is - 10.O°C. a) What is the temperature at the plane where       17.76 The emissivity of tungsten is 0.35. A tungsten sphere with
the wood meets the Styrofoam? b) What is the rate of heat flow per        radius 1.50 cm is suspended within a large evacuated enclosure
square meter through this wall?                                           whose walls are at 290 K. What power input is required to maintain
17.68 An electric kitchen range has a total wall area of 1.40 m2          the sphere at a temperature of 3000 K if heat conduction along the
and is insulated with a layer of fiberglass 4.0 cm thick. The Inside      supports is neglected?
surface of the fiberglass has a temperature of 175°C and its outside      17.77 Size of a Light-Bulb Filament. The operating temperature
surface is at 35'C. The fiberglass has a thermal conductivity of          of a tungsten filament in an incandescent light bulb is 2450 K, and its
0.040 Wlm. K. a) What is the heat current through the insulation,         emissivity is 0.35. Find the surface area of the filament of a 150-W
assuming it may be treated as a flat slab with an area of 1.40 mZ?        bulb if all the electrical energy consumed by the bulb is radiated by
b) What electric-power input to the heating element is required to        the filament as electromagnetic waves. (Only a fraction of the radia-
maintain this temperature?                                                tion appears as visible light.)
17.69 The ceiling of a room has an area of 125 ft2. The ceiling is        17.78 The Sizes of Stars. The hot glowing surfaces of stars emit
insulated to an R value of 30 (in units of ft2- Fo .h/Btuj. The surface   energy in the form of electromagnetic radiation. It is a good
in the room is maintained at 69OF, and the surface in the attic has a     approximation to assume e = 1 for these surfaces. Find the radii
temperature of 35°F. What is the heat flow through the ceiling into       of the following stars (assumed to be spherical): a) Rigel, the
the attic in 5.0 h? Express your answer in Btu and in joules.             bright blue star in the constellation Orion, that radiates energy at a
17.70 A long rod, insulated to prevent heat loss along its sides, is      rate of 2.7 X          W and has surface temperature 11,000 K;
in perfect thermal contact with boiling water (at atmospheric pres-       b) Procyon B (only visible using a telescope), that radiates energy
sure) at one end and with an ice-water mixture at the other               at a rate of 2.1 X loz3W and has surface temperature 10,000 K.
(Fig. 17.28). The rod consists of 1.00-m section of copper (one end       c) Compare your answers to the radius of the earth, the radius of
in steam) joined end-to-end to a length L2 of steel (one end in ice).     the sun, and the distance between the earth and the sun. (Rigel is
Both sections of the rod have cross-sectional areas of 4.00 cm2.The       an example of a supergiant star, and Procyon B is an example of a
temperature of the copper-steel junction is 65.0°C after a steady         white dwarf star.)
state has been set up. a) How much heat per second flows from the
steam bath to the ice-water mixture? b) What is the length L2 of the
steel sectionr?

                                                                          17.79 You propose a new temperature scale, with temperatures
                                                                          given in "M. You define O.OOMto be the normal melting point of
                                                    @@          @         mercury and 100.0' to be the normal boiling point of mercury.
                                                                          a) What is the normal boiling point of water in OM? b) A tempera-
                                                       @ @
                                                        Ice and
                                                                @         ture change of 10.0 M" corresponds to how many CO?
                                                                          17.80 Suppose that a steel hoop could be constructed to fit just
                                                                          around the earth's equator at a temperature of 20.0°C. What would
                                                                          be the thickness of space between the hoop and the earth if the tem-
       1758 Exercise 17.70.                                               perature of the hoop were increased by 0.50 CO?
                                                                          17.81 At an absolute temperature To, a cube has sides of length Lo
17.71 Apot wlth a steel bottom 8.50 mm thick rests on a hot stove.        and has density p,. The cube is made of a material with coefficient
The area of the bottom of the pot is 0.150 m2. The water inside the       of volume expansion P. a) Show that if the temperature increases to
pot is at 100.O°C, and 0.390 kg are evaporated every 3.00 min. Find       To + AT, the density of the cube becomes, approximately,
the temperature of the lower surface of the pot, which is in contact
with the stove.
17.72 You are asked to design a cylindrical steel rod 50.0 cm long,                                      +
                                                                          (Hint: Use the expression ( 1 x)" = 1 + nx, valid for 1x1 < 1.)
                                                                                                                                       <
with a circular cross-section, that will conduct 150 Jls from a fur-      Explain why this approximate result is only valid ifAT is much less
nace at 400°C to a container of boiling water under one atmo-             than 1/P, and explain why you would expect this to be the case in
sphere. What must the rod's diameter be?                                  most situations. b) A copper cube has sides of length 1.25 cm at
17.73 A rod 1.300 m long consists of a 0.800-m length of alu-             20.0°C. Find the change in its volume and density when its temper-
minum joined end-to-end to a 0.500-m length of brass. The free end        ature is increased to 70.0°C.
                                                                                      Problems


17.82 A 250-kg weight is hanging from the ceiling by a thin cop-           length is permitted to change by an amount AL when its tempera-
per wire. In its fundamental mode, this wire vibrates at the fre-          ture changes by AT, the stress is equal to
quency of concert A (440 Hz). You then increase the temperature of
the wire by 40 CO.a) By how much will the fundamental frequency
change? Will it increase or decrease? b) By what percent will the
speed of a wave on the wire change? c) By what percent will the            where F is the tension on the rod, L , is the original length of the
wavelength of the fundamental standing wave change? Will it                rod, A its cross-sectional area, a its coefficient of linear expansion,
increase or decrease?                                                      and Y its Young's modulus. b) A heavy brass bar has projections at
17.83 You are making pesto for your pasta and have a cylindri-             its ends, as in Fig. 17.29. Two
cal measuring cup 10.0 c m high made of ordinary glass                                                                   Steel wires Brass a
                                                                           fine steel wires, fastened
 ( P = 2.7 X 10-"CO)-') that is filled with olive oil                      between the projections, are just
(/I 6.8 X I O - ~ ( C ' ) ' )to a height of 1.OO mm below the top of
     =                                                                     taut (zero tension) when the
the cup. Initially, the cup and oil are at room temperature (22.0°C).      whole system is at 20°C. What is
You get a phone call and forget about the olive oil, which you inad-       the tensile stress in the steel 1
vertently leave on the hot stove. The cup and oil heat up slowly, and      wires when the temperature of
have a common temperature. At what temperature will the olive oil          the system is raised to 140°C? Figure 1729 Problem 17.89.
start to spill out of the cup?                                             Make any simplifying assump-
17.84 Use Fig. 17.1 1 to find the approximate coefficient of vol-          tions you think are justified, but state what they are.
ume expansion of water at 2.0°C and at 8°C.                                17.90 Asteelrod witha length of 0.350 m and an aluminum rod with
17.85 As in Problem 17.90, a steel rod and an aluminum rod are             a length of 0.250 m, both with the same diameter, are placed end-to-
placed end-to-end between rigid supports. But now 0.20 cm has              end between rigid supports with no initial stress in the rods. The tem-
been cut from each rod, so the steel rod is 0.348 m long, the alu-         peratureof the rods isnow raisedby 60.0 CO.    What is thestress in each
minum rod is 0.248 m long and there is a 0.40 cm gap between                      Hn:
                                                                           rod? ( i tThe length of the combined rod remains the same, but the
them. Initially, each rod has a temperature of 20.0°C. If both rods        lengths of the individual rods do not. See Problem 17.89.)
are either heated or cooled to the same final temperature, at what         17.91 A steel ring with a 2.5000-in. inside diameter at 20.0°C is to
temperature will the gap just become closed?                               be warmed and slipped over a brass shaft with a 2.5020-in. outside
17.86 At a temperature of 20.0°C the volume of a certain glass             diameter at 20.0°C. a) To what temperature should the rlng be
flask, up to a reference mark on the long stem of the flask, is            warmed? b) If the ring and the shaft together are cooled by some
exactly 100 cm3.The flask is filled to this point with a liquid whose      means such as liquid air, at what temperature will the ring just slip
coefficient of volume expansion is 8.00 X               K-', with both     off the shaft?
flask and liquid at 20.0°C. The coefficient of volume expansion of         17.92 Bulk Stress Due to a Temperature Increase. a) Prove that,
the glass is 2.00 X lo-' K-I. The cross-sectional area of the stem         if an object under pressure is raised in temperature but not allowed
is 50.0 mm2 and can be considered constant. a) Explain why it is a         to expand, the increase in pressure is
good approximation to ignore the change in the cross-sectional area
of the stem. b) How far will the liquid rise or fall in the stem when
the temperature is raised to 50.0°C?                                       where the bulk modulus B and the average coefficient of volume
17.87 A metal rod that is 30.0 cm long expands by 0.0650 cm                expansion p are both assumed positive and constant. b) What pres-
when its temperature is raised from 0°C to 100°C. A rod of a dif-          sure is necessary to prevent a steel block from expanding when ~ t s
ferent metal and of the same length expands by 0.0350 cm for the           temperature is increased from 20.0°C to 35.0°C?
same rise in temperature. A third rod, also 30.0 cm long, is made up       17.93 A liquid is enclosed in a metal cylinder that is provided with
of pieces of each of the above metals placed end-to-end and                a piston of the same metal. The system is originally at a pressure of
expands 0.0580 cm between O°C and 100°C. Find the length of                1.00 atm ( 1.013 X lo5 Pa) and at a temperature of 30.0°C. The
each portion of the composite bar.                                         piston is forced down until the pressure on the liquid is increased
17.88 On acool(4"C) Saturday morning, apilotfills the fuel tanks of        by 50.0 atm, and then clamped in this position. Find the new tem-
her Pitts S-2C (a two-seat aerobatic airplane) to their full capacity of   perature at which the pressure of the liquid is again 1 .OO atm.
 106.0 L. Before flying on Sunday morning, when the temperature is         Assume that the cylinder is sufficiently strong so that its volume is
again4"C, shechecks the fuel level and finds only 103.4 Lof gasoline       not altered by changes in pressure, but only by changes in temper-
in thetanks. Sherealizes thatitwashot onSaturday afternoon, andthat        ature. Use the result derived in Problem 17.92. (Hint. Sec-  See
thermal expansion of the gasoline caused the missing fuel to empty         tion 11.4.)
out of the tank's vent. a) What was themaximum temperature (in "C)
                                                                             Compressibility of liquid: k   =   8.50 X IO-"'Pa-l
reached by the fuel and the tank on Saturday afternoon? The coeffi-
                                                       ~
cient of volumeexpansion of gasolineis 9.5 X 1o - K-I , and the tank         Coefficient of volumeexpansionof liquid: P = 4.80 X              K-I
                            n
is made of aluminum. b) L order to have the maximum amount offuel
                                                                             Coefficient of volume expansion of metal: P = 3.90 X             K '
available for flight, when should the pilot have filled the fuel tanks?
17.89 a) Equation (17.12) gives the stress required to keep the            17.94 A thirsty mechanic cools a 2.00-L bottle of a soft drink
length of a rod constant as its temperature changes. Show that if the      (mostly water) by pouring it into a large aluminum mug with mass
                                                 c H A P T E R 17 I Temperature and Heat

0.257 kg and adding 0.120 kg of ice initially at - 15.0°C. If the soft    you will see in Chapter 20, this event is allowed by energy conser-
drink and mug are initially at 20.0°C, what is the final temperature      vation but is prohibited by the second law of thermodynamics.)
of the system, assuming no heat losses?                                   17.99 Hot Air in a Physics Lecture. a) A typical student listening
17.95 Spacecraft Reentry. A spacecraft made of aluminum cir-              attentively to a physics lecture has a heat output of 100 W. How
cles the earth at a speed of 7700 mls. a) Find the ratio of its kinetic   much heat energy does a class of 90 physics students release into a
energy to the energy required to raise its temperature from 0°C to        lecture hall over the course of a 50-min lecture? b) Assume that all
600°C. (The melting point of aluminum is 660°C. Assume a con-             the heat energy in part (a) is transferred to the 3200 m3 of air in the
stant specific heat capacity of 910 J/kg. K. ) b) Discuss the bearing     room. The air has specific heat capacity 1020 J1kg.K and density
of your answer on the problem of the reentry of a manned space            1.20 kglm! If none of the heat escapes and the air conditioning sys-
vehicle into the earth's atmosphere.                                      tem is off, how much will the temperature of the air in the room rise
17.96 A capstan is a rotating drum or cylinder over which a rope          during the 50-min lecture? c) If the class is taking an exam, the heat
or cord slides in order to provide a great amplification of the           output per student rises to 280 W. What is the temperature rise dur-
rope's tension while keeping both ends free (Fig. 17.30). Since the       ing 50 min in this case?
added tension in the rope is due to friction, the capstan generates       17.100 The molar heat capacity of a certain substance varies with
thermal energy. a) If the difference in tension between the two           temperature according to the empirical equation
ends of the rope is 520 N and the capstan has a diameter of
10.0 cm and turns once in 0.900 s, find the rate at which thermal
energy is generated. Why does the number of turns not matter?             How much heat is necessary to change the temperature of 3.00 mol
b) If the capstan is made of iron and has mass 6.00 kg, at what           of this substance from 27°C to 227"C? (Hint: Use Eq. (17.18) in the
rate does its temperature rise? Assume that the temperature in the        form dQ = nC dT and integrate.)
capstan is uniform and that all the thermal energy generated flows        17.101 a) An ice cube with mass 0.075 kg is taken from a
into it.                                                                  freezer, where the cube's temperature was - IO.OoC, and dropped
                                                                          into a glass of water at O.O°C. If no heat is gained or lost from out-
                                                                          side, how much water will freeze onto the cube? b) Is it possible
                                                                          to add so much ice to the glass that all of the water freezes?
                                                                          Explain.
                                                                          17.102 Hot Water versus Steam Heating. In a household hot-
                                                                          water heating system, water is delivered to the radiators at 70.0°C
                                                                          (158.0°F) and leaves at 28.0°C (82.4"F). The system is to be
                                                                          replaced by a steam system in which steam at atmospheric pressure
                                                                          condenses in the radiators and the condensed steam leaves the radi-
                                                                          ators at 35.0°C (95.0QF).How many kilograms of steam will sup-
                                                                          ply the same heat as was supplied by 1.OO kg of hot water in the
                                                                          first system?
                                                                          17.103 A copper calorimeter can with mass 0.446 kg contains
r~guw
    17.31) Problem 17.96.                                                 0.0950 kg of ice. The system is initially at 0.O0C. a) If 0.0350 kg of
                                                                          steam at 100.O°C and 1.00 atm pressure is added to the can, what is
17.97 Debye's T~ Law. At very low temperatures the molar heat             the final temperature of the calorimeter can and its contents? b) At
capacity of rock salt varies with temperature according to Debye's        the final temperature, how many kilograms are there of ice, how
T' law:                                                                   many of liquid water, and how many of steam?
                                     T1                                   17.104 In a container of negligible mass, 0.140 kg of ice initially
                              C=k-                                        at - 15.0°C is added to 0.190 kg of water that has a temperature of
                                     8"
                                                                          35.0°C. a) If no heat is lost to the surroundings, what is the final
where k = 1940 J1mol-K and 8 = 281 K. a) How much heat is                 temperature of the system? b) At the final temperature, how many
required to raise the temperature of 1.50 mol of rock salt from           kilograms are there of ice and how many of liquid water?
10.0 K to 40.0 K? (Hint: Use Eq. (17.18) in the form dQ = nC dT           17.105 In a container of negligible mass, 0.0400 kg of steam at
and integrate.) b) What is the average molar heat capacity in this        100°C and atmospheric pressure is added to 0.200 kg of water at
range? c) What is the true molar heat capacity at 40.0 K?                 50.OoC. a) If no heat is lost to the surroundings, what is the final
17.98 A person of mass 70.0 kg is sitting in the bathtub. The bath-       temperature of the system? b) At the final temperature, how many
tub is 190 cm by 80 cm; before the person got in, the water was           kilograms are there of steam and how many of liquid water?
10 cm deep. The water is at a temperature of 37.0°C. Suppose that         17.1 06 A tube leads from a 0.150-kg calorimeter to a flask in
the water were to cool down spontaneously to form ice at O.O°C,           which water is boiling under atmospheric pressure. The calorimeter
and that all the energy released was used to launch the hapless           has specific heat capacity 420 J/kg.K, and it originally contains
bather vertically into the air. How high would the bather go? (As         0.340 kg of water at 15.0°C. Steam is allowed to condense in the
                                                                                     Problems


calorimeter at atmospheric pressure until the temperature of the          to form an ice sheet. Why doesn't freezing occur throughout the
calorimeter and contents reaches 7 l.O°C, at which point the total        entire volume of the lake? b) Show that the thickness of the ice
mass of the calorinieter and its contents is found to be 0.525 kg.        sheet formed on the surface of a lake is proportional to the square
Conlpute the heat of vaporization of water from these data.               root of the time if the heat of fusion of the water freezing on the
17.107 In a container of negligible mass, 0.150 kg of ice at O0C          underside of the ice sheet is conducted through the sheet.
and 0.0950 kg of steam at IOO°C are added to 0.200 kg of water            c) Assuming that the upper surface of the ice sheet is at - 10°C and
that has a temperature of 50.0°C. a) If no heat is lost to the sur-       that the bottom surface is at O°C, calculate the time it will take to
roundings and the pressure in the container is a constant 1.OO atm,       form an ice sheet 25 cm thick. d) If the lake in part (c) is uniformly
what is the final temperature of the system'? b) At the final temper-     40 m deep, how long would it take to freeze all the water in the
ature, how many kilograms are there of ice, how many of liquid            lake? Is this likely to occur?
water, and how many of steam? c) Repeat parts (a) and (b) if              17.114 A rod is initially at a uniform temperature of 0°C through-
0.350 kg of ice at O°C and 0.012 kg of steam at 100°C are added to        out. One end is kept at O°C, and the other is brought into contact
0.200 kg of water that has a temperature of 40.OoC.                       with a steam bath at 100°C. The surface of the rod is insulated so
17.108 One experimental method of measuring an insulating                 that heat can flow only lengthwise along the rod. The cross-sec-
material's thermal conductivity is to construct a box of the material     tional area of the rod is 2.50 cm2, its length is 120 cm, its thermal
and measure the power input to an electric heater inside the box that     conductivity is 380 W/m .K, its density is 1.OO X lo4 kg/m7, and
maintains the interior at a measured temperature above the outside        its specific heat capacity is 520 Jlkg. K. Consider a short cylindri-
surface. Suppose that in such an apparatus a power input of 180 W         cal element of the rod 1.OO cm in length. a) If the temperature gra-
is required to keep the interior surface of the box 65.0 CO(about         dient at the cooler end of this element is 140 CO/m,how many
 120 FO)  above the temperature of the outer surface. The total area      joules of heat energy flow across this end per second? b) If the
of the box is 2.18 m2, and the wall thickness is 3.9 cm. Find the         average temperature of the element is increasing at the rate of
thermal conductivity of the material in SI units.                         0.250 COls,what is the temperature gradient at the other end of the
17.109 Effect of a Window in a Door. A carpenter builds a solid           element?
wood door with dimensions 2.00 m X 0.95 m X 5.0 cm. Its ther-             17.115 If the solar radiation energy incident per second on the
mal conductivity is k = 0.120 W/m. K. The air films on the inner          frozen surface of a lake is 600 w/m2 and 70% of this energy is
and outer surfaces of the door have the same combined thermal             absorbed by the ice, how much time will it take for a 2.50-cm-thick
resistance as an additional 1.8 cm thickness of solid wood. The           layer of ice to melt? The ice and the water beneath it are at a tem-
inside air temperature is 20.0°C and the outside air temperature is       perature of 0°C.
 -8.0°C. a) What is the rate of heat flow through the door? b) By         17.116 The rate at which radiant energy from the sun reaches the
what factor is the heat flow increased if a window 0.50 m on a side       earth's upper atmosphere is about 1.50 kw/m2. The distance from
is inserted in the door? The glass is 0.45 cm thick and the glass has     the earth to the sun is 1.50 X 10" m, and the radius of the sun is
a thermal conductivity of 0.80 W1rn.K. The air films on the two           6.96 X lo8 m. a) What is the rate of radiation of energy per unit
sides of the glass have a total thermal resistance that is the same as    area from the sun's surface? b) If the sun radiates as an ideal black-
an additional 12.0 cm of glass.                                           body, what is the temperature of its surface?
17.110 A wood ceiling with thermal resistance R, is covered with          17.117 A Thermos for Liquid Helium. A physicist uses a cylin-
a layer of insulation with thermal resistance R,. Prove that the          drical metal can 0.250 m high and 0.090 m in diameter to store liq-
effective thermal resistance of the combination is R = Rl + R,.           uid helium at 4.22 K; at that temperature the heat of vaporization of
17.111 Compute the ratio of the rate of heat loss through a single-       helium is 2.09 X 104J/kg. Completely surrounding the metal can
pane window with area 0.15 m2 to that for a double-pane window            are walls maintained at the temperature of liquid nitrogen, 77.3 K,
with the same area. The glass of a single pane is 4.2 mm thick, and       with vacuum between the can and the surrounding walls. How
the air space between the two panes of the double-pane window is          much helium is lost per hour? The emissivity of the metal can is
7.0 mm thick. The glass has thermal conductivity 0.80 W1m.K.              0.200. The only heat transfer between the metal can and the sur-
The air films on the room and outdoor surfaces of either window           rounding walls is by radiation.
have a corllbined thermal resistance of 0.15 m'. K/W.                     17.118 Thermal Expansion of an Ideal Gas. a) The pressure p,
17.112 Rods of copper, brass, and steel are welded together to            volume number of moles n, and Kdvin temperature T of an ideal
form a Y-shaped figure. The cross-sectional area of each rod is           gas are related by the equation pV = nRT, where R is a constant.
2.00 cm'. The free end of the copper rod is maintained at 100.O°C         Prove that the coefficient of volume expansion for an ideal gas is
and the free ends of the brass and steel rods at O.O°C. Assume that       equal to the reciprocal of the Kelvin temperature if the expansion
there is no heat loss from the surfaces of the rods. The lengths of the   occurs at constant pressure. b) Compare the coefficients of volume
rods are as follows: copper, 13.0 cm; brass, 18.0 cm; steel, 24.0 cm.     expansion of copper and air at a temperature of 20°C. Assume that
a) What is the temperature of the junction point? b) What is the          air may be treated as an ideal gas and that the pressure remains con-
heat current in each of the three rods?                                   stant.
17.113 Time Needed for a Lake to Freeze Over. a) When the air             17.119 An engineer is developing an electric water heater to pro-
temperature is below O°C, the water at the surface of a lake freezes      vide a continuous supply of hot water. One trial deslgn is shown in
                                                  c H A P T E R 17 1 Temperature and Heat




                                                                                                                                        pT
Fig. 17.3 1. Water is flowing at the rate of 0.500 kglmin, the inlet       17.1 2 2 A steam pipe with a
                                                                                                                         -----------
                                                                                                                               ------
thermometer registers 18.0°C, the voltmeter reads 120 V, and the
ammeter reads 15.0 A (corresponding to a power input of ( 120 V )
 X ( 15.0 A ) = 1800 W). a) When a steady state is finally
                                                                           radius of 2.00 cm, carrying
                                                                           steam at 140°C, is surrounded
                                                                           by a cylindrical jacket with inner
                                                                                                                  (@f                2 00cm cm
                                                                                                                                    Steam pipe
reached, what is the reading of the outlet thermometer? b) Why is it       and outer radii 2.00 cm and
unnecessary to take into account the heat capacity mc of the appa-         4.00 cm and made of a type of
                                                                                                                            Stvrnfqam
ratus itself?                                                              cork with thermal conductivity
                                                                           4.00 X I 0-2 ~ / K. This in
                                                                                                 m                       -      - -    ChaUenge
                                                                           turn is surrounded by a cylindri-                   I7.l22.
                                                                           cal jacket made of a brand of
                                                                           Styrofoam with thermal conductivity 1.00 X 10-2W/m-K and
                                                                           having inner and outer radii 4.00 cm and 6.00 cm (Fig. 17.32). The
                                                                           outer surface of the Styrofoam is in contact with air at 15°C.
                                                                           Assume that this outer surface has a temperature of 15°C. a) What
                              Voltmeter                                    is the temperature at a radius of 4.00 cm, where the two insulating
                                                                           layers meet? b) What is the total rate of transfer of heat out of a
rlgure i1 . ~ 1
              Problem 17.119.
                                                                           2.00-m length of pipe? (Hint: Use the expression derived in part (c)
17.120 Food Intake of a Hamster. The energy output of an ani-              of Challenge Problem 17.121.)
mal engaged in an activity is called the basal metabolic rate (BMR)        17.123 Suppose that both ends of the rod in Fig. 17.20 are kept at
and is a measure of the conversion of food energy into other forms         a temperature of O°C, and that the initial temperature distribution
of energy. A simple calorimeter to measure the BMR consists of an                                                                      ,
                                                                           along the rod is given by T = (lOO°C) sin ~ x l L where x is mea-
insulated box with a thermometer to measure the temperature of the         sured from the left end of the rod. Let the rod be copper, with
air. The air has density 1.20 kg/m3 and specific heat capacity             length L = 0.100 m and cross-sectional area 1.OO cm2. a) Show the
1020 Jlkg .K. A 50.0-g hamster is placed in a calorimeter that con-        initial temperature distribution in a diagram. b) What is the final
tains 0 0500 m' of air at room temperature. a) When the hamster is         temperature distribution after a very long time has elapsed?
running in a wheel, the temperature of the air in the calorimeter          c) Sketch curves that you think would represent the temperature
rises 1.60 COper hour. How much heat does the running hamster              distribution at intermediate times. d) What is the initial temperature
generate in an hour? Assume that all this heat goes into the air in the    gradient at the ends of the rod? e) What is the initial heat current
calorimeter. You can ignore the heat that goes into the walls of the       from the ends of the rod into the bodies malung contact with its
box and Into the thermometer, and assume that no heat is lost to the       ends? f) What is the initial heat current at the center of the rod?
surroundings. b) Assuming that the hamster converts seed into heat         Explain. What is the heat current at this point at any later time'?
with an efficiency of 10% and that hamster seed has a food energy          g) What is the value of the thermal d ~ f i s i v i t y
                                                                                                                                 klpc for copper, and
value of 24 Jlg, how many grams of seed must the hamster eat per           rn what unit is it expressed? (Here k is the thermal conductivity,
hour to supply this energy?                                                p = 8.9 X 10' kg/m3 is the density, and c is the specific heat
                                                                           capacity.) h) What is the initial time rate of change of temperature
                                                                           at the center of the rod? i) How much time would be required for
Challenge Problems                                                         the center of the rod to reach its final temperature if the tempera-
                                                                           ture continued to decrease at thts rate? (This time is called the
17.121 a) A spherical shell has inner and outer radii a and b,             relaxation time of the rod.) j) From the graphs in part (c), would
respectively, and the temperatures at the inner and outer surfaces         you expect the magnitude of the rate of temperature change at the
are T, and T,. The thermal conductivity of the material of which the       midpoint to remain constant, increase, or decrease as a function of
shell is made is k. Derive an equation for the total heat current          time? k) What is the initial rate of change of temperature at a point
through the shell. b) Derive an equation for the temperature varia-        m the rod 2.5 cm from its left end?
tion within the $hell in part (a); that is, calculate T as a function of   17.1 24 Temperature Change in a Clock. A pendulum clock is
i the distance from the center of the shell. c) A hollow cylindef has
 ;                                                                         designed to tick off one second on each side-to-side swing of the
length L. inner radius a, and outer radius b, and the temperatu~s     at   pendulum (two ticks per complete period). a) Will a pendulum
the inner and outer surfaces are T2 and T , . (The cylinder could rep-     clock gain time in hot weather and lose it in cold, or the reverse?
resent an insulated hot-water pipe, for example.) The thermalcon-          Explain your reasoning. b) A particular pendulum clock keeps cor-
ductivity of the material of which the cylinder is made is k. Derive       rect time at 20.0°C. The pendulum shaft is of steel, and its mass can
an equation for the total heat current through the walls of the cylin-     be ignored compared with that of the bob. What is the fractional
der. d) For the cylinder of part (c), derive an equation for the tem-      change in length of the shaft when it is cooled to 10.O°C? c) How
perature variation inside the cylinder walls. e) For the spherical         many seconds per day will the clock galn or lose at 10.OOC? How     d)
shell of part (a) and the hollow cylinder of part (c), show that the       closely must the temperature be controlled if the clock is not to gain
equation for the total heat current in each case reduces to Eq.            or lose more than 1.OO s a day? Does the answer depend on the
(17.21) for linear heat flow when the shell or cylinder is very thin.      period of the pendulum?
                                                                         Challenge Problems


17.125 A solid cylindrical copper rod 0.200 m long has one end           54 J/h - CO.m2, the exposed skin area A,,,, is 1.5 m2, the air temper-
maintained at a temperature of 20.00 K. The other end is blackened       ature T,,, is 47"C, and the skin temperature T,,,, is 36°C; iii) the
and exposed to thermal radiation from surrounding walls at 500 K.        skin absorbs radiant energy from the sun at a rate of 1400 W/m2;
The sides of the rod are insulated, so no energy is lost or gained       iv) the skin absorbs radiant energy from the environment, which
except at the ends of the rod. When equilibrium is reached, what is      has temperature 47°C. a) Calculate the net rate (in watts) at that the
the temperature of the blackened end? (Hint: Since copper is a very      person's skin is heated by all four of these mechanisms. Assume
good conductor of heat at low temperature, with k = 1670 W/m K           that the emissivity of the skin is e = 1 and that the skin temperature
at 20 K, the temperature of the blackened end is only slightly           is initially 36°C. Which mechanism is the most important? h) At
greater than 20.00 K.)                                                   what rate (in L/h) must perspiration evaporate from this person's
17.126 A Walk in the Sun. Consider a poor lost soul walking at           skin to maintain a constant skin temperature? (The heat of vapor-
5 krnlh on a hot day in the desert, wearing only a bathing suit. This    ization of water at 36'C is 2.42 X 10" Jlkg.) c) Suppose instead the
person's skin temperature tends to rise due to four mechanisms:          person is protected by light-colored clothing ( e = 0 ) so that the
i) energy is generated by metabolic reactions in the body at a rate of   exposed skin area is only 0.45 m'. What rate of perspiration 1s
280 W, and almost all of this energy is converted to heat that flows     required now? Discuss the usefulness of the traditional clothlng
to the skin; ii) heat is delivered to the skin by convection from the    worn by desert peoples.
outside air at a rate equal to k ' , ~ (T,,, - T,,,,), where k' is

				
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