chapter 3 the first law of thermodynamics: closed system
Post on 22-Dec-2015
243 views
TRANSCRIPT
Chapter 3 The First Law of Thermodynamics:
Closed System
3-1-1 Conservation of energy principle
Energy can be neither created nor destroyed;it can only change forms
3-1 Introduction To The First Law of Thermodynamics
3-1-2 The First Law of Thermodynamics
Neither heat nor work can be destroyed;they can only change from one to another, that is:
WQ
3-1-2 The shortcomings of Q=W
• Can’t be employed in engineering calculation
•Can’t show the quality difference between heat and work
In engineering area we would rather use a formula like this:
The net change in the total energy of the system
The net energy transferred to the system
- The net energy transferred from the system
=
3-2 Work3-2-1 Definition:
work is the energy transfer associated with a force acting through a distance
W
Work is energy in transition
Denoted by W ----------kJ
work on a unit-mass basis is denoted by w
w----------kJ/kg
work done per unit time is called power
power is denoted as
3-2-2 Positive and negative
Since work is the energy transferred between system and its boundary, then we define that:
work done by a system is positive; and work done on a system is negative
3-3-1 Moving boundary work
FdsW 21
PAds 21
A
dVPA 2
1 PdV 21
3-3 Mechanical forms of Work
Pdvw 21
work done per unit:
P-v Chart
Pdvw or
Reversible Process
A process that not only system itself but also system and surrounding keeps equilibrium
Pdvw 21The condition of the formula Is that:
System undergoes a reversible process
FdsW 21
mgdz 21
dzmg 21
)( 12 zzmg
3-3-2 Gravitational work
maF )1(dt
dm
V
dt
dsV )2(dtds V
3-3-3 Accelerational work
FdsW 21 dt
dt
dm V
V
2
1)(
2
1 21
22 VV m
3-3 Heat Transfer3-3-1 Definition:
Heat is defined as the form of energy that is transferred between two systems due to temperature difference .
Heat is energy in transition
m
denote as Q ----------kJ
heat transferred per unit mass of a system is denoted as q----------kJ/kg
We define heat absorbed by a system is positive
3-3-3 Modes of Heat Transfer:
Conduction:
3-3-2 Historical Background
x
TkAQcond
Convection:
)( fsconv TThAQ
Radiation:
4ATQrad
3-3-4 Thermodynamic calculation of Heat
1. Q=mCΔT
2. Consider:
PdVW
P------the source to do work
dV-----the indication to show if work has been done
the source to lead to heat transfer is T, then there should be:
TdxQ
What is dx here?
dx-----the indication to show if heat has been transferred
Consider:
T
Qdx
We define that x is called entropy and denoted as SThe unit of S is kJ/K Specific entropy is denoted as s The unit of s is: kJ/kg.K
Tdsq Since
T-s chart
Tdsq 21then
Also needs the condition of reversible process!
3-4 The First Law of Thermodynamics
3-4-1 Modeling
1. The net energy transfer to the system:
Win , Qin
2. The net energy transfer from the system:
Wout , Qout
3. The total Energy of the system:
E
3-4-2 The First-Law Relation
(Qin + Win) - (Qout + Wout) = ΔE
(Qin - Qout) + (Win - Wout) = ΔE
Consider the algebraic value of Q and W
(Qin - ∑Qout) - (∑Wout - ∑Win) =ΔE
Q - W = ΔE
Q = ΔE + W
3-4-3 Other Forms of the First-Law Relation
1. Differential Form:
δQ = dE + δW
As to a system without macroscopic form energy
δQ = dU + δW
On a unit-mass basis
δq = de + δw
δq = du + δw
2. Reversible Process
δQ = dE + PdV
or δQ = dU + PdV
On a unit-mass basis
δq = de + pdv
δq = du + pdv
3. Cycle
δq = du + δw
∮ δq = ∮du + ∮δw
since ∮du = 0
then ∮δ q = ∮δw
if ∮δ q = 0
∮δw =0
This can illustrate that the first kind of perpetual motion machine can’t be produced
4.Reversible Process under a Constant Pressure
δQ = dU + PdV
Since p=const
δQ = dU + d(pV)
5.Isolated System
dE = 0
3-5 Specific Heats3-5-1 Definition of specific heat
The energy required to raise the temperature of the unit mass of a substance by one degree
Then q=CT or δq=CdT
3-5-2 Specific heat at constant volume The specific heat at constant volume Cv can be viewed as the
energy required to raise the temperature of unit mass of substance by 1 degree as the volume is maintained constant.
At constant volume , δq = du
Cv dT=duv
v T
uC
3-5-3 Specific heat at constant pressure
The specific heat at constant volume Cp can be viewed as the energy required to raise the temperature of unit mass of substance by 1 degree as the volume is maintained constant pressure.
Similarly, at constant pressure
δq = du+ δ w=du+Pdv=du+d(Pv)=dh
Cp dT=dhp
p T
hC
A: Specific heat at constant volume
Since there are no attraction among molecules of ideal-gas,then:
u= f (T )
vv dT
duC
3-5-4 Specific Heats of Ideal-Gas
B: Specific heat at constant pressure
Since: u= f (T )
h=u+pv
= f (T ) +RT
=f ’( T )
pp dT
dhC
3-6 The internal energy, enthalpy of Ideal-Gas
2-7-1 Internal energy and enthalpy
vv dT
duC
dTCdu v
TCu v
pp dT
duC
dTCdh p
TCh p
We define that u =0 while T =0, then: TCu v
pvuh
RTuh Obviously, h =0 while T =0, then:
TCh p
Meanwhile:
RTuh
RTTCTC vp
RCC vp
We define k =Cp / Cv
vp
vp
CC
RCC
k
RC p 1
k
k
1
k
RvC
This Chapter is over
Thank you!