Second Law of thermodynamics

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27 Οκτ 2013 (πριν από 4 χρόνια και 8 μήνες)

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THERMODYNAMICS

CH 15

THERMODYNAMICS

Thermodynamics

is the study of
processes in which energy is transferred
as heat and as work

System

is any object or set of objects that
we wish to consider

Environment

is everything else in the
universe

OPEN AND CLOSED SYSTEMS

Closed system

is one for which no mass
enters or leaves (but energy may be
exchanged with the environment)

Ex: idealized systems studied in physics

Open system

is one for which mass may
enter or leave (as well as energy)

Ex: plants and animals

Isolated System

Isolated system

is a closed system where
no energy in any form passes across its
boundaries

1
ST

Law of Thermodynamics

The change in internal energy of a closed
system,
∆U
, will be equal to the sum of the
energy transferred across the system
boundary by heat (
Q
) and the energy
transferred by work (
W
)

∆U = Q + W

Sign Convention

Energy of any kind that goes into the
system is +

Energy of any kind that comes out of the
system is

First Law of Thermodynamics is
conservation of energy

It is one of the great laws of physics

Its validity rests on experiments (such as
Joule’s) in which no exceptions have been seen

Internal energy is the sum total of all the energy
the molecules of the system. It is a property of a
system like pressure, volume and temperature

Work and heat are not properties of a system

First Law of Thermodynamics
applied to some simple systems

Isothermal Process is an idealized process
that is carried out at constant temperature
(

T = 0)

An ideal gas in a cylinder fitted with a
movable piston

PV diagram for an ideal gas
undergoing isothermal processes

If the temperature is to remain
constant, the gas must expand
and do an amount of work W
on the environment (it exerts a
force on the piston and moves
it through a distance)

U =
3

N
k

T = 0 (since the

2 temperature is

kept constant)

U = Q + W = 0

W =
-
Q (
the work done by the
gas in an isothermal process
equals the heat lost to the
environment
)

Adiabatic process is one in which no heat
is allowed to flow into or out of the system:
Q = 0

It can occur if the system is extremely well
insulated, or the process happens so
quickly that the heat
-
which flows slowly
-

has no time to flow in or out.

Since Q = 0,

U = W
(The internal energy
decreases if the gas
expands)

The temperature
decreases as well since

U =
3

N
k

T

2

compression work is
done on the system so U
and T increases

Isobaric Process

Isobaric process

is one in which the
pressure is kept constant, so the process
is represented by a straight line on the PV
diagram

Isochoric Process

Isochoric

or
isovolumetric

process is
one which the volume does not change

PV diagram for Different
processes

Work done in volume changes

W = F d = Pad

W =
-

P

V

Work done by a gas is equal to the area
under the PV curve

The Second Law of
Thermodynamics

The First Law of Thermodynamics states
that energy is conserved

Some processes in nature do not occur in
reverse even though they wouldn’t violate
the First Law

Ex: broken glass back to be together
spontaneously

Essential Question

Which processes occur in nature and
which do not?

On the second half of xix century scientists
came to formulate a new principle known
as the second law of thermodynamics

It is a statement about which processes
occur in nature and which do not

The second law of
thermodynamics

It is stated in a variety of ways

A general statement is based on the study
of heat engines

Heat Engines

A
heat engine

is any device that changes
thermal energy into mechanical work

Ex: steam engines ( most electricity today
is generated with steam turbines)

Car engines (internal combustion engine)

Heat Engine Diagram

A heat input Q
H

at a
high Temperature T
H

is partially
transformed into work
W and partially
exhausted as heat Q
L

at a lower
temperature T
L

Q
H

= W + Q
L
(Conservation of energy)

Diagram of Reciprocating type of
steam engine

Diagram of Turbine steam Engine

Internal Combustion Engine

In an internal
combustion engine,
the high temperature
is achieved by
burning the gasoline
-
air mixture in the
cylinder itself (ignited
by the spark plug)

Why a

T is needed to run a heat
engine?

Same temperature would mean that the
pressure of the gas being exhausted
would be the same as that on intake.

Work would be done by the gas on the
piston when it expanded but the same
amount of work would be done by the
piston to force the gas to exhaust.
No net

work
!

Efficiency of a Heat Engine

A net amount of work is obtained only if
there is a difference in temperature.

Q
H

= W + Q
L
(
Conservation of energy
)

The efficiency,
e
, of any heat engine can
be defined as the ratio of the work it does,
W
, to the heat input,
Q
H

e

=
W

=
QH

QL

= 1
-

QL

Q
H
Q
H

Q
H

Carnot Engines

It is an ideal engine, investigated by the
-
1832)
to see how to increase the efficiency of a
real engine

No Carnot engine actually exists, but as a
theoretical engine it played an important
role in the development of
thermodynamics

PV Diagram of Carnot cycle

1
-
2: Isothermal
expansion

2
-
expansion

3
-
4: Isothermal
compression

4
-
compression

Ideal x Real Process

Ideal
-

the process is
reversible
, that is, is done
so slow that can be considered a series of
equilibrium states, so the whole process can be
done in reverse with no change in the magnitude
of work done or heat exchanged.

Real
-

the process is done more quickly so there
is heat lost because of friction and turbulence,
so the process cannot be done precisely in
reverse, the process is then called
irreversible
.

Carnot efficiency

For ideal engines

Q
˜T (T in kelvin)

e
ideal
=
T
H

T
L
= 1
-
T
L

T
H

T
H

Real engines that are well designed
reach 60 to 80 percent of the
Carnot efficiency

Entropy

It was not until the latter half of the nineteenth
century the second law of thermodynamics was
finely stated in a general way in terms of a
quantity called
entropy
.

Entropy is a measure of order or disorder of a
system

Entropy is a function of state of a system

Like potential energy, it is the change in entropy
during a process that is important not the
absolute amount

Entropy

The change in entropy

S of a system
when an amount of heat Q is added to it
by a reversible process at constant
temperature:

S =
Q

(T in Kelvin)

T

Entropy

The entropy of an isolated system never
decreases. It can only stay the same
(ideal, reversible processes) or increase
(real processes)

S > 0

If the system is not isolated:

S =

S
S

+

S
env

≥ 0

Second Law of thermodynamics
(General Statement)

The total entropy of any system plus that
of its environment increases as a result of
any natural process

Or

Natural processes tend to move toward a
state of greater disorder

Order to Disorder

The second law introduces a new quantity,
S
, that is not conserved in natural
processes, it always increase in time.

Entropy concept is very abstract. To get a
better feel of it, we can relate it to order
and disorder

Entropy of a system is a measure of the
disorder of the system

Which process occur in nature and
which not?

The normal course of events is an
increase of disorder (entropy)

Ex: a solid coffee cup is a more “orderly”
object than the pieces of a broken cup

Cups break when they fall, but they do not
spontaneously mend themselves

Thermodynamocs Laws

0
th
: A equilibrium B
» A equilibrium C

B equilibrium C

1
st
: Conservation of energy: ∆U = Q

W

2
nd
: Which processes occur in nature and
which do not (Entropy). Natural processes
tend to move toward a state of greater
disorder (∆S ≥ 0)

3
rd
: From careful experimentation absolute
zero is unattainable.

Diagram of energy transfer for a
heat pump

The operating
principle of
refrigerators, air
conditioners, and heat
pumps is just the
reverse of a heat
engine.

Each operates to
transfer heat out of a
cool environment into
a warm environment

Heat engine Diagram

Heat Pump

Refrigerator

Heat pump or air
conditioner