PPT: Introduction to First Law and SFEE | Thermodynamics - Mechanical Engineering PDF Download

 Page 1


The First Law of Thermodynamics
ü The quantity (Q – W) is the same for all processes
ü It depends only on the initial and final states of the system 
ü Does not depend at all on how the system gets from one to the other 
ü This is simply conservation of energy
(Q is the heat absorbed and W is the work done by the system)
The internal energy E of a system tends to increase, if energy is added as heat Q and tends to 
decrease if energy is lost as work W done by system 
dE = dQ – dW ( first law)
Page 2


The First Law of Thermodynamics
ü The quantity (Q – W) is the same for all processes
ü It depends only on the initial and final states of the system 
ü Does not depend at all on how the system gets from one to the other 
ü This is simply conservation of energy
(Q is the heat absorbed and W is the work done by the system)
The internal energy E of a system tends to increase, if energy is added as heat Q and tends to 
decrease if energy is lost as work W done by system 
dE = dQ – dW ( first law)
Internal Energy 
• Internal energy (u): that portion of total energy E which is not kinetic or potential 
energy. It includes thermal, chemical, electric, magnetic, and other forms of energy 
• Change in the specific internal energy  
du = CdT
• In case of gases internal energy is given by u = C
v
dT
• Specific heat changes with temperature is given by 
C = C
o
(a+bT), a and b are constants and C
o
is specific heat at 0
o
C
• The total energy in the mass m is the sum of internal energy as well as PE and KE in the mass
E = U + PE + KE = m .u + mg. Z + 
!
"
m V
2
For unit mass E = em = u + 
$
%
+ 
!
"
V
2    
(p =	??gZ)
Page 3


The First Law of Thermodynamics
ü The quantity (Q – W) is the same for all processes
ü It depends only on the initial and final states of the system 
ü Does not depend at all on how the system gets from one to the other 
ü This is simply conservation of energy
(Q is the heat absorbed and W is the work done by the system)
The internal energy E of a system tends to increase, if energy is added as heat Q and tends to 
decrease if energy is lost as work W done by system 
dE = dQ – dW ( first law)
Internal Energy 
• Internal energy (u): that portion of total energy E which is not kinetic or potential 
energy. It includes thermal, chemical, electric, magnetic, and other forms of energy 
• Change in the specific internal energy  
du = CdT
• In case of gases internal energy is given by u = C
v
dT
• Specific heat changes with temperature is given by 
C = C
o
(a+bT), a and b are constants and C
o
is specific heat at 0
o
C
• The total energy in the mass m is the sum of internal energy as well as PE and KE in the mass
E = U + PE + KE = m .u + mg. Z + 
!
"
m V
2
For unit mass E = em = u + 
$
%
+ 
!
"
V
2    
(p =	??gZ)
Enthalpy
1
Q
2
=U
2 
- U
1 
+ P
2
V
2 
- P
1
V
1
= (U
2
+ P
2
V
2
) - (U
1
+ P
1
V
1
) 
u = h - pv
The enthalpy is especially valuable for analyzing isobaric processes 
Enthalpy h =  u + pv
s
• The heat transfer in a constant-pressure quasi-equilibrium process is equal to the change in
enthalpy, which includes both the change in internal energy and the work for this particular
process
• This is by no means a general result
• It is valid for this special case only because the work done during the process is equal to the
difference in the PV product for the final and initial states
• This would not be true if the pressure had not remained constant during the process
Page 4


The First Law of Thermodynamics
ü The quantity (Q – W) is the same for all processes
ü It depends only on the initial and final states of the system 
ü Does not depend at all on how the system gets from one to the other 
ü This is simply conservation of energy
(Q is the heat absorbed and W is the work done by the system)
The internal energy E of a system tends to increase, if energy is added as heat Q and tends to 
decrease if energy is lost as work W done by system 
dE = dQ – dW ( first law)
Internal Energy 
• Internal energy (u): that portion of total energy E which is not kinetic or potential 
energy. It includes thermal, chemical, electric, magnetic, and other forms of energy 
• Change in the specific internal energy  
du = CdT
• In case of gases internal energy is given by u = C
v
dT
• Specific heat changes with temperature is given by 
C = C
o
(a+bT), a and b are constants and C
o
is specific heat at 0
o
C
• The total energy in the mass m is the sum of internal energy as well as PE and KE in the mass
E = U + PE + KE = m .u + mg. Z + 
!
"
m V
2
For unit mass E = em = u + 
$
%
+ 
!
"
V
2    
(p =	??gZ)
Enthalpy
1
Q
2
=U
2 
- U
1 
+ P
2
V
2 
- P
1
V
1
= (U
2
+ P
2
V
2
) - (U
1
+ P
1
V
1
) 
u = h - pv
The enthalpy is especially valuable for analyzing isobaric processes 
Enthalpy h =  u + pv
s
• The heat transfer in a constant-pressure quasi-equilibrium process is equal to the change in
enthalpy, which includes both the change in internal energy and the work for this particular
process
• This is by no means a general result
• It is valid for this special case only because the work done during the process is equal to the
difference in the PV product for the final and initial states
• This would not be true if the pressure had not remained constant during the process
Source: http://www4.ncsu.edu/~kimler/hi322/Rumford-expt.gif
Sir Benjamin Thompson
Count Rumford
• Thompson’s theory of heat was demonstrated by a test tube full 
of water within wooden paddles
• Water boiled due to friction
• The heat of friction is unlimited
Page 5


The First Law of Thermodynamics
ü The quantity (Q – W) is the same for all processes
ü It depends only on the initial and final states of the system 
ü Does not depend at all on how the system gets from one to the other 
ü This is simply conservation of energy
(Q is the heat absorbed and W is the work done by the system)
The internal energy E of a system tends to increase, if energy is added as heat Q and tends to 
decrease if energy is lost as work W done by system 
dE = dQ – dW ( first law)
Internal Energy 
• Internal energy (u): that portion of total energy E which is not kinetic or potential 
energy. It includes thermal, chemical, electric, magnetic, and other forms of energy 
• Change in the specific internal energy  
du = CdT
• In case of gases internal energy is given by u = C
v
dT
• Specific heat changes with temperature is given by 
C = C
o
(a+bT), a and b are constants and C
o
is specific heat at 0
o
C
• The total energy in the mass m is the sum of internal energy as well as PE and KE in the mass
E = U + PE + KE = m .u + mg. Z + 
!
"
m V
2
For unit mass E = em = u + 
$
%
+ 
!
"
V
2    
(p =	??gZ)
Enthalpy
1
Q
2
=U
2 
- U
1 
+ P
2
V
2 
- P
1
V
1
= (U
2
+ P
2
V
2
) - (U
1
+ P
1
V
1
) 
u = h - pv
The enthalpy is especially valuable for analyzing isobaric processes 
Enthalpy h =  u + pv
s
• The heat transfer in a constant-pressure quasi-equilibrium process is equal to the change in
enthalpy, which includes both the change in internal energy and the work for this particular
process
• This is by no means a general result
• It is valid for this special case only because the work done during the process is equal to the
difference in the PV product for the final and initial states
• This would not be true if the pressure had not remained constant during the process
Source: http://www4.ncsu.edu/~kimler/hi322/Rumford-expt.gif
Sir Benjamin Thompson
Count Rumford
• Thompson’s theory of heat was demonstrated by a test tube full 
of water within wooden paddles
• Water boiled due to friction
• The heat of friction is unlimited
The Joules Experiment
Rise in 
temperature
One calorie corresponds to the amount 
of heat that is needed to get one gram 
of water from 14.5 C to 15.5 C 
1 Cal = 4.1840 J
??	????? =	?????
A paddle wheel turns 
in liquid water 
proportionality constant - mechanical equivalent of heat 
W
W
H
Insulating walls 
prevent heat transfer from the 
enclosed water to the surroundings
As the weight falls 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