Thermodynamics

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Thermodynamics

Chapters 21
-
24


Thermodynamics
-

the study of heat and its
transformation into mechanical energy.


What is mechanical energy again?


Energy due to position or movement of an object;
potential and/or kinetic energy.


As the MOTION of atoms increases, so does the
TEMPERATURE.


What will happen to temperature if we decrease
the motion of atoms?


Absolute Zero
-

the limit at which no
more energy can be taken from a
substance and therefore the
temperature cannot be lowered.


On the
celsius

scale absolute zero is
273 degrees below zero.


On the Kelvin scale absolute zero is
0 degrees.



The Kelvin scale was named after a
british

physicist named Lord
kelvin

who
coined the term thermodynamics which
come from Greek words meaning
“movement of heat”.

1.

Which is larger a
celsius

degree or a
Kelvin?

2.
Look at figure
24.1 on
pg. 355
-

At what
temperature does water boil in
Celsius? In Kelvin?



What temperature scale do we use in
the U.S.?



Convert from Fahrenheit to Celsius
-







Celsius to Kelvin
-

ADD 273


Kelvin
to Celsius
-

SUBTRACT 273

Practice

1.
Convert 0
°
C to K.

Answer: 273K

2.
Convert 546K to
°
C.

Answer: 273
°
C

3.
Convert 100
°
F to
°
C.

Answer: 38
°
C

4.
Convert 75
°
F to K.


Answer
: 297K


We said that as the movement, or kinetic
energy of atoms decreases, so does
temperature.


What happens to the amount of space (also
known as volume) a substance will occupy as
heat is added?


Heat addition will cause expansion; heat
reduction (cold) will cause contraction.


The tendency of matter to change in volume
as a response to temperature is called
Thermal Expansion. See pages 315
-
317.


1st Law of Thermodynamics
-

when heat
is added to a system it is transformed into
an equal amount of some other form of
energy.


Heat added = increase in internal energy + external work done by
system


What is a system????? See page 356.



Heat is energy!! Also known as
thermal energy!!



Remember* Energy cannot be
created or destroyed!!!



If 25J of energy is added to a system
that does no external work, by how much
will the internal energy of that system be
raised?


Answer: 25J



If 20J of energy is added to a system
that does 7J of external work, by how
much will the internal energy of that
system be raised?


Answer: 13J

Practice



2nd Law of Thermodynamics
-

heat
only flows in ONE direction
-

hot to cold.
It will never flow from cold to hot by
itself.


What about air conditioners? They
make the air cold. See pg. 360 last
paragraph.



“Hot” and “cold” are just ways of
describing the amount of heat present.

Thermal Equilibrium


Two systems are in thermal equilibrium
when their temperatures are the same.


This is also known as the
zeroeth

law of
thermodynamics.


A heat engine is any device that changes
internal energy into mechanical work.


What are some kinds of heat engines? pg.
361


In every heat engine only some of the heat
can be transformed into work.


Every heat engine will:

1.
absorb heat from a reservoir of higher
temperature (which increases internal energy)

2.
convert some of this energy into mechanical work

3.
put the remaining energy as heat to a lower
temperature reservoir which is usually called a
sink.




French Engineer
Sadi

Carnot found that even
under ideal conditions, the amount of heat that can be
used to do useful work is dependent on the
temperature difference between the hot reservoir and
the cold sink.



The ideal efficiency, also known as Carnot
efficiency of a heat engine is expressed below
-


Ideal efficiency =
T
hot

-

T
cold



T
hot

Practice



What is the ideal efficiency of an
engine if both the hot reservoir and the
exhaust are 500K?


Answer: zero efficiency



What is the ideal efficiency of a
machine having a hot reservoir of 400K
and a cold reservoir of 300K?


Answer: 25%


Entropy
-

the measure of the amount of
disorder.


For natural processes entropy is
always increased.


This means that as entropy is
increased, the available energy of the
system for doing work decreases.


The exception to this is living
organisms
-

see pg. 366.

Types of Heat
Transfer


Conduction
-

heat transfer due
to direct contact between
objects


Convection
-

heat transfer
through a fluid (liquid or gas)


Radiation
-

transferred by
electromagnetic waves.


VIDEO
-

CLICK HERE


Specific Heat
-

the heat required to raise the
temperature of one gram of a substance one
degree
celsius
.


Calorie
-

A unit of heat and energy equal to the
amount of energy requires to raise the
temperature of 1 gram of water by 1
°

Celsius. 1
calorie = 4.184 joules



Practice Problem


The specific heat capacity of iron is 450
J/(
kg
°
C
). How much heat energy does it
take to raise the temperature of 500g from
25
°
C to 100
°
C?



C = 450 J/(
kg
°
C
)


M = .5 kg


Tf

= 100
°
C



Q = m c ΔT = .5(450)(75) = 16,875 J


Ti = 25
°
C


ΔT = 100
-
25 = 75
°
C



Latent heat is the heat
released or absorbed by a
system or a chemical
substance during a change of
state that occurs without a
change in temperature.


Example: a phase transition
such as the melting of ice or
the boiling of water.


VIDEO
-
CLICK HERE

Process

Change of State

Melting

solid to liquid

Freezing

liquid to solid

Vaporization

liquid to gas

Condensation

gas to liquid

Sublimation

solid to gas

Deposition

gas to solid