17.1. Heat
Two
bodies brought in thermal contact will change their temperature until they
are at the same temperature. In the process of reaching thermal
equilibrium, heat is transferred from one body to the other.
Suppose
we have a system of interest at temperature TS surrounded by an environment with temperature TE. If TS > TE heat flows from
the system to the environment. If TS < TE heat flows from the environment into the system. Heat,
presented by the symbol Q and unit Joule, is chosen to be positive when
heat flows into the system, and negative if heat flows out of the system
(see Figure 17.1). Heat flow is a results of a temperature difference
between two bodies, and the flow of heat is zero if TS = TE.
Figure 17.1. Heat Flow.

Heat
is not the only way in which energy can be transferred between a system
and its environment. Energy can also be transferred between a system
and its environment by means of work (W). The unit of work is the Joule.
Another
commonly used unit is the calorie. The calorie is defined as the amount of heat
that would raise the temperature of 1 g of water from 14.5 ¡C to 15.5 ¡C. The Joule and the calorie are related as follows: 1
cal = 4.1860 J
17.2. Heat Capacity
When
heat is added to an object, its temperature increases. The change
in the temperature is proportional to the amount of heat added
The
constant C is called the heat capacity of the object. The heat capacity of an
object depends on its mass and the type of material of which it is made. The
heat capacity of an object is proportional to its mass, and the heat
capacity per unit mass, c, is commonly used. In that case
where
m is the mass of the object. The molar heat capacity is the heat capacity per mole of material. For
most materials the molar heat capacity is 25 J/mol K.
In
order to determine the heat capacity of a substance we not only need to
know how much heat is added, but also the conditions under which the heat
transfer took place. For gases, adding heat under constant pressure
and under constant temperature will lead to very different values of the
specific heat capacity.
17.3. Heat of Transformation
When
heat is added to a solid or a liquid, the temperature of the sample does
not necessarily rise. During a phase change (melting, boiling) heat
is added to the sample without an increase in temperature. The amount
of heat transferred per mass unit during a phase change is called the heat
of transformation (symbol L) for the process. The amount
of heat needed/released is given by
where
m is the mass of the sample.
Problem
28P
What
mass of steam of 100¡C must be mixed with 150 g of ice at 0¡C, in a thermally
insulated container, to produce liquid water at 50¡C ?
We
start with calculating the heat required to transform 150 g of ice at 0¡C to 150 g
of liquid at 0¡C. The heat of transformation of water is 333
kJ/kg (see Table 202, page 555). The transformation of ice into
water therefore requires a total heat given by
The
heat required to change the temperature of 150 g of water from 0¡C to 50¡C is given
by
The
total heat that needs to be added to the system is therefore equal to 81.5
kJ. This heat must be supplied by the steam. Heat will be released
when the steam is transformed into liquid, The heat of transformation
for this process is 2260 kJ/kg. Suppose the mass of the steam is
m. The total heat released in the conversion of steam into water
is given by
The
heat released when the steam cools down from 100¡C to 50¡C is given by
The
total heat released by the cooling of the steam is therefore equal to 2470
m kJ. The total heat required is 81.5 kJ, and we therefore conclude
that the mass of the steam must be equal to 33 g.
17.4. Work
Suppose
a system starts from an initial state described by a pressure pi, a volume Vi, and a temperature
Ti. The final state of the system
is described by a pressure pf, a volume Vf, and a temperature Tf. The transformation from the initial state to the final state can
be achieved in a variety of ways (see for example Figure 17.2). In
Figure 17.2a both pressure and volume change simultaneously. In Figure
17.2b the pressure of the system is first lowered while keeping the volume
constant (this can for example be achieved by cooling the sample) and subsequently,
the volume is increased while keeping the pressure constant (this can be
achieved by heating the gas while increasing the volume).
If
the pressure of a gas increases it can move a piston (this happens in an
engine). In this case, work is done by the system as the expanding gas lifts
the piston. On the other hand, if we increase the weight of the piston,
work will be done on the system as the piston falls. The force exerted
by the gas on the piston is equal to p A, where A is the area of the
piston and p is the gas pressure. If the piston is displaced by a
distance ds, the amount of work done can be calculated as follows:
Figure 17.2. Two possible ways to get from
the initial state to the final state.

The
total work done during a finite displacement of the piston is now easy
to calculate
If
W is positive, work was done by the system (for example, the expanding
gas lifts the piston). A negative value of W tells you that work
was done on the system (the piston is pressed down in order to compress
the gas).
The
amount of work done is equal to the area under the curve in the pV diagrams
shown in Figure 17.2. Clearly, the amount of work done depends on
the path chosen. The work W for the path shown in Figure 17.2a is
significantly more than the work W for the path shown in Figure 17.2b. Any
change in the system in which the volume does not change will not produce/cost
any work. The work done for the paths shown in Figure 17.2 can be
calculated easily
No
work is done for the path shown in Figure 17.2b between (pi, Vi) and (pf, Vi) since there is no change in volume. The work
done to move from (pf, Vi) to (pf, Vf) is calculated easily
Clearly,
W2b is always less then W2a, and we can make the amount of work done as small or as large as we want. For
example no work would be done if the transition follows the following path:
(pi, Vi) ¨ (0, Vi) ¨ (0,
Vf) ¨ (pf, Vi)
A
system can be taken from a given initial state to a given final state by
an infinite number of processes. In general, the work W and also
the heat Q will have different values for each of these processes. We
say that heat and work are pathdependent quantities.
From
the previous discussion neither Q nor W represents a change in some intrinsic
properties of the system. Experimentally, however, it is observed
that the quantity Q  W is the same for all processes. It depends
only on the initial and final states and it does not matter at what path
is followed to get from one to the other. The quantity Q  W is called
the change in the internal energy U of the system:
ÆU = Uf  Ui = Q  W
This
equation is called the first law of thermodynamics. For small changes the first law of thermodynamics
can be rewritten as
dU = dQ
 dW
17.4.1. Adiabatic Processes
If
a system is well insulated, no transfer of heat will occur between it and
its environment. This means that Q = 0, and the first law of thermodynamics
shows that
ÆU = 
W
If
work is done by the system (positive W) its internal energy decreases. Conversely,
if work is done on the system (negative W) its internal energy will increase. For
gases, the internal energy is related to the temperature: a higher internal
energy means a higher temperature. Adiabatic expansion of a gas will lower its temperature; adiabatic
compression of a gas will increase its temperature.
17.4.2. Constant Volume Processes
If
the volume of a system is held constant, the system can do no work (W =
0 J). The first law of thermodynamics then shows that
ÆU = Q
If
heat is added to the system its internal energy will increase; if heat
is removed from the system its internal energy will decrease.
17.4.3. Cyclical Processes
Processes
which, after certain interchanges of heat and work, are restored to their
initial state are called cyclical processes. In this case, no intrinsic
properties of the system are changed, and therefore ÆU = 0. The first
law of thermodynamics now immediately yields
Q = W
17.4.4. Free Expansion
Free
expansion is an adiabatic process in which no work is done on or by the
system. This means that Q = W = 0 J, and
the first law of thermodynamics now requires that
ÆU = 0
J
17.5. Transfer of Heat
The
transfer of heat between a system and its environment can occur
in a variety of ways. Three different mechanisms of heat transfer
will now be discussed: conduction, convection, and radiation.
17.5.1. Conduction
Consider the slab of material shown in Figure 17.3. The
left end of the beam is maintained at a temperature TH; the right end of the beam is
maintained at a temperature TC. As a result of the temperature difference heat
will flow through the slab, from its hot end to its cold end. Experimentally
it is shown that the rate of heat transfer (Q/t) is proportional to the
crosssectional area of the slab, proportional to the temperature difference,
and inversely proportional to the length of the slab
Here,
k is the thermal conductivity, which is a constant that depends on the type of material. Large
values of k define good heat conductors. The thermal resistance R is related to the thermal conductivity k in the following
manner
Figure 17.3. Conduction.

Thus,
the lower the thermal conductivity of the material, the higher the thermal
resistance R. From the definition of R it immediately follows that
Consider
a composite slab is made up out of two different materials, with length
L1 and L2, and with thermal conductivity
k1 and k2, is placed between two heat baths (see Figure 17.4). Suppose
that the temperature of the interface between the two slabs is equal to
Tx. The amount of heat flowing from TH to Tx is given by
The
amount of heat flowing from Tx to
TC is given by
Of
course, the heat flowing through slab 1 must equal the heat flowing through
slab 2. Thus
Figure 17.4. Heat transport through a composite slab.

This
equation can be used to obtain the temperature at the interface between
slab 1 and slab 2:
The
heat flowing through the slab can now be calculated easily
A
composite slab therefore has a thermal resistance equal to the sum of the
thermal resistance of each of the individual slabs.
17.5.2. Convection
Heat
transfer by convection occurs when a fluid, such as air or water, is
in contact with an object whose temperature is higher than the temperature
of its surroundings. The temperature of the fluid increases and (in
most cases) the fluid expands. Being less dense than the surrounding
cooler fluid, it rises because of buoyant forces. The surrounding
cooler fluid falls to take the place of the rising warmer fluid and a convective
circulation is set up.
17.5.3. Radiation
Every
object emits electromagnetic radiation. The energy spectrum of the radiation emitted
depends on the temperature of the object; the average energy increases
with increasing temperature.
Problem
57P
A
container of water has been outdoors in cold weather until a 5.0 cmthick
slab of ice has formed on its surface. The air above the ice is at
10¡C. Calculate the rate of formation of ice (in
centimeters per hour) on the bottom surface of the ice slab. Take
the thermal conductivity of ice to be 0.0040 cal/s . cm . ¡C
and the density to be 0.92 g/cm3.
The
water at the boundary between the water and the ice will be at a temperature
of 0¡C. The
heat transported through 5 cm of ice is equal to
This
heat is released when water is transformed into ice. The heat of
transformation of this process is 79.5 cal/g. Suppose a mass m of
water is transformed each second into ice. This produces a total
heat equal to
H = 79.5
m cal/s
This
must be equal to the heat flow through the ice:
79.5
m = 0.0080 A
A
mass m of ice (covering an area A) will have a thickness d, where d is
given by
Combining
the last two expression we obtain for the rate of ice formation:
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