QUICK QUIZ 20.1
(end of section 20.2)
Consider the experiment described in Example 20.2. You
realize that your calorimeter is not perfect, that is, although you
would like the system to be isolated, heat can transfer between
the calorimeter and the room. To obtain the best results for your
experiment, you should use water with an initial temperature
that is a) slightly less than the temperature of the room, b)
exactly the same as the temperature of the room, c) slightly
greater than the temperature of the room, or d) any starting
temperature will yield the same results.
QUICK QUIZ 20.1 ANSWER
(a). Recall from the experiment that the final equilibrium
temperature of the ingot-water mixture is higher than the initial
temperature of the water. Since some heat can enter or leave the
system from the room, you would like the heat entering the
system at the start of the experiment, when the water is cold, to
be compensated by the heat leaving the system at the end of the
experiment, when the water is warm. To do this, you should
start with water that is at a temperature slightly below room
temperature and finish with water (and ingot) at a temperature
that is slightly above room temperature.
QUICK QUIZ 20.2
(end of section 20.3)
After reading the section of this book on latent heat, you come up with
an interesting idea for a sleeping bag. You would like to use the latent
heat of freezing water to warm you as you sleep. Your idea is to patent a
sleeping bag with an outer water compartment that surrounds the entire
sleeping bag. If you use this sleeping bag, it will a) work as you predict,
warming you as the water starts to freeze, b) work as you predict, but
only warming you once all the water is completely frozen, c) work only
as the water cools down to 0°C, but not as the water starts to freeze, or
d) not work since heat will be transferred from you to the water rather
than the other way around.
QUICK QUIZ 20.2 ANSWER
(c). Heat is transferred from a body at a higher temperature to
a body at a lower temperature. As the water freezes, heat is
transferred from the water (at 0°C) to the surrounding air, at a
temperature less than 0°C. In addition, since your body
temperature is higher than 0°C, heat will transfer from you
into the water. Also, water has a higher thermal conductivity
than air so the heat will be transferred from you to the water
much more quickly. A better solution would be to set a
number of barrels of water around the campsite so that heat is
transferred to the air and the air is in contact with you.
QUICK QUIZ 20.3
(end of section 20.3)
Physical properties for three metals are combined in the table
below. From these data, there appears to be a correlation
between the specific heat and a) the bulk modulus, b) the
density, c) the thermal conductivity, or d) none of the above.
Material
Bulk Modulus Density
(N/m2)
(g/cm3)
Thermal
Conductivity
W/(m°C)
Specific heat
J/(kg°C)
Copper
14 x 1010
8.92
397
387
Aluminum
7 x 1010
2.70
238
900
Iron (or Steel)
6 x 1010
7.86
79.5
448
QUICK QUIZ 20.3 ANSWER
(b). The specific heat is greatest for aluminum, then iron and then copper. The
only other physical quantity that varies such that aluminum and copper
represent the two extremes is density. The density is greatest for copper, then
iron, and then aluminum. This is not an isolated fact for these three metals. In
general, the specific heat for elemental metals increases as the density
decreases. In essence, there are more atoms per kilogram, in a light metal like
aluminum, to absorb the energy. If one measures the specific heat in units that
relate to the heat transferred per atom, for example J/(mole°C), the values
obtained for the different metals are very similar. For aluminum, iron and
copper, one obtains,
Aluminum:
Iron:
Copper:
900J .0270kg
24.3J
x

,
kg o C
mole
mole o C
448J .0558kg
25.0J
x

,
kg o C
mole
mole o C
387J .0635kg
24.6J
x

.
o
o
kg C
mole
mole C
QUICK QUIZ 20.4
(end of section 20.5)
The energy transferred by work between a system and its
surroundings depends on the path of the process and, in
addition, the energy transferred by heat between a system
and its surroundings depends on the path of the process.
The first law of thermodynamics combines these two
quantities in the equation, DEint = Q + W. One can say that
a) the change in internal energy of a system always depends
on the path of the process, b) the change in internal energy
of a system sometimes depends on the path of the process,
or c) the change in internal energy of a system never
depends on the path of the process.
QUICK QUIZ 20.4 ANSWER
(c). The first law would seem to imply that the internal
energy must be dependent on the path of the process since
it is the sum of two quantities that are each dependent on
the path of the process. However, the internal energy is a
path independent quantity and therefore only depends on
the state of the system. It is a very interesting consequence
of the first law that such a path independent quantity can
exist that is the sum of two path dependent quantities.
QUICK QUIZ 20.5
(end of section 20.7)
A fire poker is used to push charcoal or logs around in a fireplace. From the
issue of safety, you would like the fire poker to be made from a material a)
that has a high specific heat and a high thermal conductivity, b) that has a
low specific heat and a low thermal conductivity, c) that has a low specific
heat and a high thermal conductivity, or d) that has a high specific heat and a
low thermal conductivity.
QUICK QUIZ 20.5 ANSWER
(d). You would like the specific heat of the poker to be
high so that it can absorb a lot of heat with only a small
increase in temperature. In addition, you would like the
thermal conductivity to be low so that very little heat is
conducted from the fire to your hand.