熱力學第一定律

The First Law of Thermodynamics The First Law of Thermodynamics

• The conservation of energy

• -Energy cannot be created or destroyed; it can only change forms (the first law).

• The conservation of energy

• -Energy cannot be created or destroyed; it can only change forms (the first law).

Heat defined

021 Q Heat transfer to the system:Heat transfer to the system:

Heat transfer from the system:Heat transfer from the system: 021 Q

Work defined

021 W Work transfer to the system:Work transfer to the system:

Work transfer from the system:Work transfer from the system: 021 W

Energy Balance For Closed Systems Energy Balance For Closed Systems

Energy Balances

==Change of EnergyChange of Energyin the Systemin the SystemChange of EnergyChange of Energyin the Systemin the System

--Heat Transfer at Heat Transfer at System BoundarySystem BoundaryHeat Transfer at Heat Transfer at System BoundarySystem Boundary

Work Done byWork Done bythe Systemthe SystemWork Done byWork Done bythe Systemthe System

These terms are evaluated at the system boundary. They represent energetic interactions between the system and itssurroundings.

FIGURE 2-43The energy change of a system during a process is equal to the net work and heat transfer between the system and its surroundings.

Mechanical Work OutMechanical Work Out

Heat Loss, (W)Heat Loss, (W)

Motor

ElectricalElectricalPower InputPower Input(W)(W)

inW

Q

mout PEW

cz

g

gmF ,0,0

mm zz

Example 1Example 1

MotorinW

Q

mout PEW mm

Example Example

For steady operation, thereFor steady operation, thereis no change of the state of is no change of the state of the system, andthe system, and

0 lossoutin QWW

Cyclic processes...

Cyclic Processes

P1

P2

SS11

SS22

21,2121 byWQdU

121 121 121

byWQdU

Equivalence of heat and work

cycle

by

cycle

WQ 0

This is an important result that says This is an important result that says the cyclic the cyclic heat and work are the sameheat and work are the same. It establishes that . It establishes that heat and work are different forms of energy.heat and work are different forms of energy.

CYCLIC WORK

The Mechanical

Equivalent of Heat

Cyclic processes

III III

QW

I

II

P1

P2

Cycles & Work

Work done in the cycle is represented by Work done in the cycle is represented by the area inside the two paths.the area inside the two paths.

I

II

P1

P2

WACYCLEIII

Energy balances...

Energy Balances

Net change ofenergy in thesystem.

Heat transferinto system from surroundings.

Work done bysystem onsurroundings.(Work transferto surrounding)

Path, or process, dependentPath, or process, dependentquantities between statesquantities between states

(U+KE+PE) Q W

The energy balance as a rate equation

dt

W

dt

Q

dt

dE by

Path, or process, dependentPath, or process, dependentquantities. quantities.

Quasi-static cyclic processes

Cycle Cycle

QW

II

IIII

P1

P2

General cycle analysis...

A general cycle analysis

II. Quasistatic II. Quasistatic Compression Compression

PP1 1 == pp

PP22 = = VV

State 1State 1

IntermediateIntermediate State 2State 2

I. Quasistatic I. Quasistatic Expansion Expansion

The expansion process

IntermediateIntermediateState 2State 2

pp

pp11

VVVV11 VV22

pp22

State 1State 1Expansion ProcessExpansion Process

21,2112 byWQEE

The compression process

IntermediateIntermediateState 2State 2

pp

VVVV11 VV22

pp22

pp11

State 1State 1

12,1221 byWQEE

Compression ProcessCompression Process

Combined processes (I + II)

21,1221 byWQEE

12,2112 byWQEE

IIIbyIII WQ ,0

Cycle Cycle

byWQ 0