Problems
601
The rectangular plate shown in Figure P19.55 has an area
A
i
equal to !w. If the temperature increases by "T, each di-
mension increases according to the equation "L # &L
i
"
T,
where & is the average coefficient of linear expansion.
Show that the increase in area is "A # 2&A
i
"
T. What
approximation does this expression assume?
55.
r
2
r
1
θ
(a)
(b)
Figure P19.54
Charles D. Winters
w
w
+ ∆w
!
+ ∆!
!
T
i
T
+
∆T
T
i
Figure P19.55
(a)
T
250 m
T + 20
°C
(b)
y
Figure P19.61 Problems 61 and 62.
below the surface, the volume of the balloon at the sur-
face, the pressure at the surface, and the density of the wa-
ter. (Assume water temperature does not change with
depth.) (b) Does the buoyant force increase or decrease as
the balloon is submerged? (c) At what depth is the buoy-
ant force half the surface value?
59.
A copper wire and a lead wire are joined together, end to
end. The compound wire has an effective coefficient of lin-
ear expansion of 20.0 ) 10
!
6
(°C)
!
1
. What fraction of the
length of the compound wire is copper?
60.
Review problem. Following a collision in outer space, a
copper disk at 850°C is rotating about its axis with an an-
gular speed of 25.0 rad/s. As the disk radiates infrared
light, its temperature falls to 20.0°C. No external torque
acts on the disk. (a) Does the angular speed change as the
disk cools off? Explain why. (b) What is its angular speed at
the lower temperature?
61.
Two concrete spans of a 250-m-long bridge are placed end
to end so that no room is allowed for expansion (Fig.
P19.61a). If a temperature increase of 20.0°C occurs, what
is the height y to which the spans rise when they buckle
(Fig. P19.61b)?
56.
Review problem. A clock with a brass pendulum has a period
of 1.000 s at 20.0°C. If the temperature increases to 30.0°C,
(a) by how much does the period change, and (b) how
much time does the clock gain or lose in one week?
57.
Review problem. Consider an object with any one of the
shapes displayed in Table 10.2. What is the percentage in-
crease in the moment of inertia of the object when it is
heated from 0°C to 100°C if it is composed of (a) copper
or (b) aluminum? Assume that the average linear expan-
sion coefficients shown in Table 19.1 do not vary between
0°C and 100°C.
58.
(a) Derive an expression for the buoyant force on a spheri-
cal balloon, submerged in water, as a function of the depth
62.
Two concrete spans of a bridge of length L are placed end
to end so that no room is allowed for expansion (Fig.
P19.61a). If a temperature increase of "T occurs, what is
the height y to which the spans rise when they buckle (Fig.
P19.61b)?
63.
(a) Show that the density of an ideal gas occupying a vol-
ume V is given by , # PM/RT, where M is the molar mass.
(b) Determine the density of oxygen gas at atmospheric
pressure and 20.0°C.
64.
(a) Use the equation of state for an ideal gas and the defi-
nition of the coefficient of volume expansion, in the form
' #
(1/V ) dV/dT, to show that the coefficient of volume
expansion for an ideal gas at constant pressure is given by
' #
1/T, where T is the absolute temperature. (b) What
value does this expression predict for ' at 0°C? Compare
this result with the experimental values for helium and air
in Table 19.1. Note that these are much larger than the
coefficients of volume expansion for most liquids and
solids.
Starting with Equation 19.10, show that the total pressure
P in a container filled with a mixture of several ideal gases
is P # P
1
$
P
2
$
P
3
$ + + +
, where P
1
, P
2
, . . . , are the pres-
sures that each gas would exert if it alone filled the con-
tainer (these individual pressures are called the partial pres-
65.