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Chapter 3 The First Law of Thermodynamics: Closed System. 31 Introduction To The First Law of Thermodynamics. 311 Conservation of energy principle Energy can be neither created nor destroyed;it can only change forms.
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311 Conservation of energy principle
Energy can be neither created nor destroyed;it can only change forms
312 The First Law of Thermodynamics
Neither heat nor work can be destroyed;they can only change from one to another, that is:
In engineering area we would rather use a formula like this:
The net energy transferred from the system
The net energy transferred to the system

The net change in the total energy of the system
=
Work is energy in transition
321Definition:
work is the energy transfer associated with a force acting through a distance
Denoted by W kJ
work on a unitmass basis is denoted by w
wkJ/kg
work done per unit time is called power
power is denoted as
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
331Moving boundary work
Pv Chart
Is that:
Reversible Process
A process that not only system itself but also system and surrounding keeps equilibrium
System undergoes a reversible process
Heat is energy in transition
331Definition:
Heat is defined as the form of energy that is transferred between two systems due to temperature difference .
denote as Q kJ
heat transferred per unit mass of a system is denoted as qkJ/kg
We define heat absorbed by a system is positive
Radiation:
334 Thermodynamic calculation of Heat
1. Q=mCΔT
2. Consider:
Pthe source to do work
dVthe indication to show if work has been done
the source to lead to heat transfer is T, then there should be:
dxthe indication to show if heat has been transferred
Consider:
We define that x is called entropy and denoted asS
The unit of S is kJ/K
Specific entropy is denoted as s
The unit of s is: kJ/kg.K
341 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 ofthe system:
E
(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
343 Other Forms of the FirstLawRelation
1. Differential Form:
δQ = dE + δW
As to a system without macroscopic form energy
δQ = dU + δW
On a unitmass basis
δq = de + δw
δq = du + δw
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
351 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
352 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
CvdT=du
353 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
CpdT=dh
354 Specific Heats of IdealGas
A: Specific heat at constant volume
Since there are no attraction among molecules of idealgas,then:
u= f (T )
271 Internal energy and enthalpy
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