PPT: First Law of Thermodynamics | Thermodynamics - Mechanical Engineering PDF Download

 Page 1


Thermodynamics
The First Law of Thermodynamics
 Heat and Work. First Law of Thermodynamics
 Heat and Work on Quasi-Static Processes for a Gas.
Page 2


Thermodynamics
The First Law of Thermodynamics
 Heat and Work. First Law of Thermodynamics
 Heat and Work on Quasi-Static Processes for a Gas.
The First Law of Thermodynamics
System
Surroundings
The system can exchange mass and energy
through the boundary with the environment.
An example of “closed system” - no mass flow- is
the gas confined in a cylinder. The boundary –in
this case real wall- is made by the cylinder and the
piston walls.
Energy exists in many forms, such as mechanical energy, heat,
light, chemical energy, and electrical energy. Energy is the
ability to bring about change or to do work.
Thermodynamics is the study of energy.
The boundary of the system
is arbitrarily chosen
Page 3


Thermodynamics
The First Law of Thermodynamics
 Heat and Work. First Law of Thermodynamics
 Heat and Work on Quasi-Static Processes for a Gas.
The First Law of Thermodynamics
System
Surroundings
The system can exchange mass and energy
through the boundary with the environment.
An example of “closed system” - no mass flow- is
the gas confined in a cylinder. The boundary –in
this case real wall- is made by the cylinder and the
piston walls.
Energy exists in many forms, such as mechanical energy, heat,
light, chemical energy, and electrical energy. Energy is the
ability to bring about change or to do work.
Thermodynamics is the study of energy.
The boundary of the system
is arbitrarily chosen
The First Law of Thermodynamics
First Law of Thermodynamics ? Conservation of Energy:
Energy can be changed from one form to another, but it cannot be created or
destroyed. The total amount of energy and matter in the Universe remains
constant, merely changing from one form to another.
The First Law of Thermodynamics (Conservation) states that energy is
always conserved, it cannot be created or destroyed. In essence,
energy can be converted from one form into another.
The energy balance of a system –as a consequence of FLT- is a
powerful tool to analyze the exchanges of energy between the system
and its environment.
We need to define the concept of internal energy of the system, E
int
as
an energy stored in the system.
Warning: It is not correct to say that a system has a large amount of
heat or a great amount of work
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookEner1.html
Page 4


Thermodynamics
The First Law of Thermodynamics
 Heat and Work. First Law of Thermodynamics
 Heat and Work on Quasi-Static Processes for a Gas.
The First Law of Thermodynamics
System
Surroundings
The system can exchange mass and energy
through the boundary with the environment.
An example of “closed system” - no mass flow- is
the gas confined in a cylinder. The boundary –in
this case real wall- is made by the cylinder and the
piston walls.
Energy exists in many forms, such as mechanical energy, heat,
light, chemical energy, and electrical energy. Energy is the
ability to bring about change or to do work.
Thermodynamics is the study of energy.
The boundary of the system
is arbitrarily chosen
The First Law of Thermodynamics
First Law of Thermodynamics ? Conservation of Energy:
Energy can be changed from one form to another, but it cannot be created or
destroyed. The total amount of energy and matter in the Universe remains
constant, merely changing from one form to another.
The First Law of Thermodynamics (Conservation) states that energy is
always conserved, it cannot be created or destroyed. In essence,
energy can be converted from one form into another.
The energy balance of a system –as a consequence of FLT- is a
powerful tool to analyze the exchanges of energy between the system
and its environment.
We need to define the concept of internal energy of the system, E
int
as
an energy stored in the system.
Warning: It is not correct to say that a system has a large amount of
heat or a great amount of work
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookEner1.html
The First Law of Thermodynamics. Heat, Work and Internal Energy
Joule’s Experiment and the First Law of Thermodynamics.
Equivalence between work and heat
Schematic diagram for Joule´s
experiment. Insulating walls are
necessary to prevent heat transfer
from the enclosed water to the
surroundings.
As the weights fall at constant speed,
they turn a paddle wheel, which does
work on water.
If friction in mechanism is negligible,
the work done by the paddle wheel on
the water equals the change of
potential energy of the weights.
1 calorie = 4.184 Joules
Work is done on water. The energy is transferred to
the water – i. e.  the system- . The energy transferred
appears as an increase in temperature.
We can replace the insulating walls by conducting
walls. We can transfer heat through the walls to the
system to produce the same increase in temperature.
The increase in temperature of the system is a
consequence of an increase in Internal Energy.
Internal energy is a state function of the system
The sum of the heat transferred into
the system and the work done on the
system equals the change in the
internal energy of the system
on in
W Q E + =
int
D
Page 5


Thermodynamics
The First Law of Thermodynamics
 Heat and Work. First Law of Thermodynamics
 Heat and Work on Quasi-Static Processes for a Gas.
The First Law of Thermodynamics
System
Surroundings
The system can exchange mass and energy
through the boundary with the environment.
An example of “closed system” - no mass flow- is
the gas confined in a cylinder. The boundary –in
this case real wall- is made by the cylinder and the
piston walls.
Energy exists in many forms, such as mechanical energy, heat,
light, chemical energy, and electrical energy. Energy is the
ability to bring about change or to do work.
Thermodynamics is the study of energy.
The boundary of the system
is arbitrarily chosen
The First Law of Thermodynamics
First Law of Thermodynamics ? Conservation of Energy:
Energy can be changed from one form to another, but it cannot be created or
destroyed. The total amount of energy and matter in the Universe remains
constant, merely changing from one form to another.
The First Law of Thermodynamics (Conservation) states that energy is
always conserved, it cannot be created or destroyed. In essence,
energy can be converted from one form into another.
The energy balance of a system –as a consequence of FLT- is a
powerful tool to analyze the exchanges of energy between the system
and its environment.
We need to define the concept of internal energy of the system, E
int
as
an energy stored in the system.
Warning: It is not correct to say that a system has a large amount of
heat or a great amount of work
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookEner1.html
The First Law of Thermodynamics. Heat, Work and Internal Energy
Joule’s Experiment and the First Law of Thermodynamics.
Equivalence between work and heat
Schematic diagram for Joule´s
experiment. Insulating walls are
necessary to prevent heat transfer
from the enclosed water to the
surroundings.
As the weights fall at constant speed,
they turn a paddle wheel, which does
work on water.
If friction in mechanism is negligible,
the work done by the paddle wheel on
the water equals the change of
potential energy of the weights.
1 calorie = 4.184 Joules
Work is done on water. The energy is transferred to
the water – i. e.  the system- . The energy transferred
appears as an increase in temperature.
We can replace the insulating walls by conducting
walls. We can transfer heat through the walls to the
system to produce the same increase in temperature.
The increase in temperature of the system is a
consequence of an increase in Internal Energy.
Internal energy is a state function of the system
The sum of the heat transferred into
the system and the work done on the
system equals the change in the
internal energy of the system
on in
W Q E + =
int
D
The First Law of Thermodynamics
Another method of
doing work.
Electrical work is
done on the
system by the
generator, which
is driven by the
falling weight.