熱力學第一定律. The First Law of Thermodynamics The conservation of energy -Energy cannot be created or destroyed; it can only change forms (the first law).

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Presentation transcript:

熱力學第一定律

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 Heat transfer to the system: Heat transfer to the system: Heat transfer from the system: Heat transfer from the system:

Work defined Work transfer to the system: Work transfer to the system: Work transfer from the system: Work transfer from the system:

Energy Balance For Closed Systems

Energy Balances= Change of Energy in the System Change of Energy in the System - Heat Transfer at System Boundary Heat Transfer at System Boundary Work Done by the System Work Done by the System These terms are evaluated at the system boundary. They represent energetic interactions between the system and its surroundings.

FIGURE 2-43 The 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 Out Heat Loss, (W) Motor Electrical Power Input (W) m z Example 1

Motor m Example For steady operation, there is no change of the state of the system, and

Cyclic processes...

Cyclic Processes P1P1 P2P2 S1S1S1S1 S2S2S2S2

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

CYCLIC WORK The Mechanical Equivalent of Heat

Cyclic processes I II P1P1 P2P2

Cycles & Work Work done in the cycle is represented by the area inside the two paths. I II P1P1 P2P2

Energy balances...

Energy Balances Net change of energy in the system. Heat transfer into system from surroundings. Work done by system on surroundings. ( Work transfer to surrounding) Path, or process, dependent quantities between states  (U+KE+PE) QQ WW

The energy balance as a rate equation Path, or process, dependent quantities.

Quasi-static cyclic processesIII P1P1 P2P2

General cycle analysis...

A general cycle analysis II. Quasistatic Compression P 1 = p P 2 = V State 1 Intermediate State 2 State 2 I. Quasistatic Expansion

The expansion process Intermediate State 2 p p1p1p1p1 V V1V1V1V1 V2V2V2V2 p2p2p2p2 State 1 Expansion Process

The compression process Intermediate State 2 pV V1V1V1V1 V2V2V2V2 p2p2p2p2 p1p1p1p1 State 1 Compression Process

Combined processes (I + II)