An ideal gas undergoes an isothermal expansion from one state to another. In this process determine whether Q, W and AU are positive, negative or zero (using the sign conventions on page 413): Q = 0, Q> 0 or Q<0 W = 0, W>0 or W<0 AU = 0, AU >0 or AU <0 O O O
An ideal gas undergoes an isothermal expansion from one state to another. In this process determine whether Q, W and AU are positive, negative or zero (using the sign conventions on page 413): Q = 0, Q> 0 or Q<0 W = 0, W>0 or W<0 AU = 0, AU >0 or AU <0 O O O
Chapter3: The First Law Of Thermodynamics
Section: Chapter Questions
Problem 17CQ: There is no change in the internal of an ideal gas undergoing an isothermal process since the...
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![In discussing thermodynamics, we often refer to particular systems. A system
is any object or set of objects that we wish to consider (see Section 14-4). Every-
thing else in the universe is referred to as its "environment" or the "surroundings."
In this Chapter, we examine the two great laws of thermodynamics. The first
law of thermodynamics relates work and heat transfers to the change in internal
energy of a system, and is a general statement of the conservation of energy. The
second law of thermodynamics expresses limits on the ability to do useful work,
and is often stated in terms of entropy, which is a measure of disorder. Besides
these two great laws, we also discuss some important related practical devices:
heat engines, refrigerators, heat pumps, and air conditioners.
15-1 The First Law of Thermodynamics
In Section 14-2, we defined the internal energy of a system as the sum total of all
the energy of the molecules within the system. Then the internal energy of a
system should increase if work is done on the system, or if heat is added to it.
Similarly the internal energy should decrease if heat flows out of the system or if
work is done by the system on something in the surroundings.
Thus it is reasonable to extend conservation of energy and propose an impor-
tant law: the change in internal energy of a closed system, AU, will be equal to
the energy added to the system by heating minus the work done by the system on
the surroundings. In equation form we write
lean bas
AU = Q - W
(15-1)
where is the net heat added to the system and W is the net work done by the system.
We must be careful and consistent in following the sign conventions for Q and W.
Because Win Eq. 15-1 is the work done by the system, then if work is done on the
system, W will be negative and U will increase. Similarly, Qis positive for heat
added to the system, so if heat leaves the system, Q is negative. [Caution: Elsewhere
you may sometimes encounter the opposite convention for W where W is defined
as the work done on the system; in that case Eq. 15-1 is written as AU = Q + W.]
Equation 15-1 is known as the first law of thermodynamics. It is one of the
great laws of physics, and its validity rests on experiments (such as Joule's) to
which no exceptions have been seen. Since Q and W represent energy transferred
into or out of the system, the internal energy changes accordingly. Thus, the first
law of thermodynamics is a general statement of the law of conservation of energy.
Note that the conservation of energy law was not able to be formulated until
the 1800s, because it depended on the interpretation of heat as a transfer of energy.
A given system does not "have" a certain amount of heat or work. Rather, work
and heat are involved in thermodynamic processes that can change the system
from one state to another; they are not characteristic of the state itself. Quantities
which describe the state of a system, such as internal energy U, pressure P, volume V,
temperature T, and mass m or number of moles n, are called state variables.
Q and W are not state variables.
FIGURE 15-3 PV diagra for
EXAMPLE 15-1 Using the first law. 2500 J of heat is added to a system,
and 1800 J of work is done on the system. What is the change in internal energy
of the system?
APPROACH We apply the first law of thermodynamics, Eq. 15-1, to our system.
SOLUTION The heat added to the system is Q = 2500 J. The work W done
by the system is -1800 J. Why the minus sign? Because 1800 J done on the
system (as given) equals - 1800 J done by the system, and it is the latter we need
for the sign conventions we used for Eq. 15-1. Hence
AU = 2500 J (-1800J) = 2500 J + 1800 J = 4300 J.
FIRST LAW OF
THERMODYNAMICS
NOTE We did this calculation in detail to emphasize the importance of keeping
careful track of signs. Both the heat and the work are inputs to the system, so we
expect AU to be increased by both.
CAUTION
Heat added is +
Heat lost is -
Work on system is-
Work by system is +
sidsvoM.
CAUTION
P, V, T, U, m, n are state
variables. W and Q are not:
a system does not have an RUDH
amount of heat or work buite
s 101 mei 19 s-ar BAUDA
Isaiso gator obnu ang inobi
Tangih
SECT 15-2 abimanybormed Hd awon SECTION 15-1 413](/v2/_next/image?url=https%3A%2F%2Fcontent.bartleby.com%2Fqna-images%2Fquestion%2Fe591ff8b-7fbf-41eb-ab1a-65fc8e854653%2F3e42f809-9a16-4f6c-922f-967f985452fe%2Fwsp7e7n_processed.jpeg&w=3840&q=75)
Transcribed Image Text:In discussing thermodynamics, we often refer to particular systems. A system
is any object or set of objects that we wish to consider (see Section 14-4). Every-
thing else in the universe is referred to as its "environment" or the "surroundings."
In this Chapter, we examine the two great laws of thermodynamics. The first
law of thermodynamics relates work and heat transfers to the change in internal
energy of a system, and is a general statement of the conservation of energy. The
second law of thermodynamics expresses limits on the ability to do useful work,
and is often stated in terms of entropy, which is a measure of disorder. Besides
these two great laws, we also discuss some important related practical devices:
heat engines, refrigerators, heat pumps, and air conditioners.
15-1 The First Law of Thermodynamics
In Section 14-2, we defined the internal energy of a system as the sum total of all
the energy of the molecules within the system. Then the internal energy of a
system should increase if work is done on the system, or if heat is added to it.
Similarly the internal energy should decrease if heat flows out of the system or if
work is done by the system on something in the surroundings.
Thus it is reasonable to extend conservation of energy and propose an impor-
tant law: the change in internal energy of a closed system, AU, will be equal to
the energy added to the system by heating minus the work done by the system on
the surroundings. In equation form we write
lean bas
AU = Q - W
(15-1)
where is the net heat added to the system and W is the net work done by the system.
We must be careful and consistent in following the sign conventions for Q and W.
Because Win Eq. 15-1 is the work done by the system, then if work is done on the
system, W will be negative and U will increase. Similarly, Qis positive for heat
added to the system, so if heat leaves the system, Q is negative. [Caution: Elsewhere
you may sometimes encounter the opposite convention for W where W is defined
as the work done on the system; in that case Eq. 15-1 is written as AU = Q + W.]
Equation 15-1 is known as the first law of thermodynamics. It is one of the
great laws of physics, and its validity rests on experiments (such as Joule's) to
which no exceptions have been seen. Since Q and W represent energy transferred
into or out of the system, the internal energy changes accordingly. Thus, the first
law of thermodynamics is a general statement of the law of conservation of energy.
Note that the conservation of energy law was not able to be formulated until
the 1800s, because it depended on the interpretation of heat as a transfer of energy.
A given system does not "have" a certain amount of heat or work. Rather, work
and heat are involved in thermodynamic processes that can change the system
from one state to another; they are not characteristic of the state itself. Quantities
which describe the state of a system, such as internal energy U, pressure P, volume V,
temperature T, and mass m or number of moles n, are called state variables.
Q and W are not state variables.
FIGURE 15-3 PV diagra for
EXAMPLE 15-1 Using the first law. 2500 J of heat is added to a system,
and 1800 J of work is done on the system. What is the change in internal energy
of the system?
APPROACH We apply the first law of thermodynamics, Eq. 15-1, to our system.
SOLUTION The heat added to the system is Q = 2500 J. The work W done
by the system is -1800 J. Why the minus sign? Because 1800 J done on the
system (as given) equals - 1800 J done by the system, and it is the latter we need
for the sign conventions we used for Eq. 15-1. Hence
AU = 2500 J (-1800J) = 2500 J + 1800 J = 4300 J.
FIRST LAW OF
THERMODYNAMICS
NOTE We did this calculation in detail to emphasize the importance of keeping
careful track of signs. Both the heat and the work are inputs to the system, so we
expect AU to be increased by both.
CAUTION
Heat added is +
Heat lost is -
Work on system is-
Work by system is +
sidsvoM.
CAUTION
P, V, T, U, m, n are state
variables. W and Q are not:
a system does not have an RUDH
amount of heat or work buite
s 101 mei 19 s-ar BAUDA
Isaiso gator obnu ang inobi
Tangih
SECT 15-2 abimanybormed Hd awon SECTION 15-1 413
Transcribed Image Text:2. An ideal gas undergoes an isothermal expansion from one state to another. In this process
determine whether Q, W and AU are positive, negative or zero (using the sign conventions on
page 413):
Q = 0, Q> 0 or Q<0
W = 0, W> 0 or W<0
AU = 0, AU >0 or AU <0
O
O
O
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