Chapter 15 Lecture

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Oct 27, 2013 (3 years and 1 month ago)

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Chapter 15

Lecture

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Chapter 15: Thermodynamics


The First Law of Thermodynamics


Thermodynamic Processes (isobaric, isochoric, isothermal, adiabatic)


Reversible and Irreversible Processes


Heat Engines


Refrigerators and Heat Pumps


The Carnot Cycle


Entropy (The Second Law of Thermodynamics)


The Third Law of Thermodynamics

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§
15.1 The First Law of
Thermodynamics

The first law of thermodynamics

says the change in
internal energy of a system is equal to the heat flow into the
system plus the work done on the system (conservation of
energy).

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§
15.2 Thermodynamic Processes

A
state variable

describes the state of a system at time t,
but it does not reveal how the system was put into that state.
Examples of state variables: pressure, temperature, volume,
number of moles, and internal energy.

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A PV diagram can be used to represent the state changes
of a system, provided the system is always near
equilibrium.

The area under a PV curve
gives the magnitude of the
work done on a system.
W>0 for compression and
W<0 for expansion.

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The work done on a system depends on the path taken in
the PV diagram. The work done on a system during a
closed cycle can be nonzero.

To go from the state (V
i
, P
i
) by the path (a) to the state (V
f
,
P
f
) requires a different amount of work then by path (b). To
return to the initial point (1) requires the work to be nonzero.

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An isothermal process
implies that both P and
V of the gas change
(PV

T).

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§
15.3 Thermodynamic Processes
for an Ideal Gas

No work is done on a system
when its volume remains
constant (isochoric process).
For an ideal gas (provided the
number of moles remains
constant), the change in internal
energy is

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For a constant pressure (isobaric) process, the change in
internal energy is

C
P

is the
molar specific heat at constant
pressure
. For an ideal gas C
P

= C
V
+ R.

where

and

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For a constant temperature (isothermal) process,

U = 0
and the work done on an ideal gas is

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§
15.4 Reversible and Irreversible
Processes

A process is
reversible

if it does not violate any law of
physics when it is run backwards in time. For example an
ice cube placed on a countertop in a warm room will melt.
The reverse process cannot occur: an ice cube will not
form out of the puddle of water on the countertop in a warm
room.

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A collision between two billiard balls is reversible.
Momentum is conserved if time is run forward; momentum
is still conserved if time runs backwards.

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Any process that involves dissipation of energy is not
reversible.

Any process that involves heat transfer from a hotter object
to a colder object is not reversible.

The second law of thermodynamics

(Clausius Statement):
Heat never flows spontaneously from a colder body to a
hotter body.

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§
15.5 Heat Engines

A heat engine is a device designed to convert disordered
energy into ordered energy. The net work done by an
engine during one cycle is equal to the net heat flow into
the engine during the cycle (

U = 0).

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§
15.6 Refrigerators and Heat
Pumps

In a heat engine, heat flows from hot to cold, with work as
the output. In a heat pump, heat flows from cold to hot, with
work as the input.

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§
15.7 Reversible Engines and Heat
Pumps

A reversible engine can be used as an engine (heat input
from a hot reservoir and exhausted to a cold reservoir) or
as a heat pump (heat is taken from cold reservoir and
exhausted to a hot reservoir).

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From the second law of thermodynamics, no engine can
have an efficiency greater than that of an ideal reversible
engine that uses the same two reservoirs. The efficiency of
this ideal reversible engine is

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The ideal engine of the previous section is known as a
Carnot engine. The Carnot cycle has four steps:

1.
Isothermal expansion: takes in heat from hot reservoir;
keeping the gas temperature at T
H
.

2.
Adiabatic expansion: the gas does work without heat
flow into the gas; gas temperature decreases to T
C
.

3.
Isothermal compression: Heat Q
C

is exhausted; gas
temperature remains at T
C
.

4.
Adiabatic compression: raises the temperature back to
T
H
.

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The Carnot cycle
illustrated

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§
15.8 Entropy

Heat flows from objects of high temperature to objects at low
temperature because this process increases the disorder of
the system.
Entropy

is a measure of a system’s disorder.
Entropy is a state variable and is not a conserved quantity.

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If an amount of heat Q flows into a system at constant
temperature, then the change in entropy is

Every irreversible process increases the total entropy of the
universe. Reversible processes do not increase the total
entropy of the universe.

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The second law of thermodynamics

(Entropy Statement):
The entropy of the universe never decreases.

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§
15.9 The Third Law of
Thermodynamics

It is impossible to cool a system to absolute zero.

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Summary


The Three Laws of Thermodynamics


Thermodynamic Processes


Reversible and Irreversible Processes


Heat Engines and Heat Pumps


Efficiency of an Engine


Entropy

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Homework


pg. 550


Conceptual


3,5,6,8,11,13


Multiple Choice


2,3,5,9,11