thermo

2-6 THE 1st LAW OF THERMODYNAMICS – CONSERVATION OF ENERGY PRINCIPLE mechanical equivalent of heat
1 Btu = 772 lbf ⋅ ft
Energy can be neither created nor destroyed during a process.
It can only change forms.

The net change in the total energy ∆E=E2 − E1 of the system Joule's experiment (1843)
is equal to the difference between the total energy entering Ein
and the total energy leaving Eout the system during the process.

James P. Joule (1818-1889) E Eout
2 ∆E

E
1

1st Law of Thermodynamics Ein (2-32)
conservation of energy (energy balance)
∆E = E2 − E1 = Ein − Eout
for any system and any kind of process

Open system (energy crossing the boundary can be in the forms of heat, Q, work, W, and mass):

( ) ( )Ein − Eout = Qin − Qout − Wout − Win + Emass,in − Emass,out (2-34)

Qnet ,in Wnet ,out Emass ,net ,in

Closed system (energy crossing the boundary is in the forms of heat, Q, and work, W, only):

W2 ∆E = Qnet ,in − Wnet ,out Cycle (∆E = 0) Q = Wnet ,in net ,out Q = Q∆t
net ,out W = W ∆t
dE 1=2
1 dt
= Qnet ,in − Wnet ,out Q = Wnet ,in net ,out
Qnet ,in

Total energy change ∆E = ∆ U + ∆ KE + ∆ PE (2-33)
E = ∆E
 V22 − V12 ( 
∆E = m u2 − u1 + 2 + g z2 − z1 ) ∆t



 + V22 − V12 g ( z2 
E = m u2 − u1 2 + − z1 )



∆E = m (u2 − u1 ) ∆E = m (u2 − u1 ) for stationary system (∆KE = ∆PE = 0)

1st Law of Thermodynamics  flow work + V22 − V12  [kJ ]
m u2 2 
V2 + v2 P2 − v1P1  [kW ]
( )Q − Wnet ,in
State 2 net ,out = − u1 + g z2 − z1 enthalpy
m h ≡ u + Pv
 + V22 − V12 
m u2 2  (5-38)
( )Q − Wnet ,in 
net ,out = − u1 + v2 P2 − v1P1 + g z2 − z1

fixed mass m V1  + V22 − V12 
steady flow m h2 2 
( )Q − Wnet ,in 
State 1 net ,out = − h1 + g z2 − z1
m
z2

 + V22 − V12 
m h2 2 
( )Q − Wnet ,in 
z1 net ,out = − h1 + g z2 − z1