Hk i Termodinamika

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HK-I TERMODINAMIKA

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Internal Energy (U)

1.Definition:Internal energy is all kinds of micro-energy in system.

2. Internal energy is propertyIt include:a) Kinetic energy of molecule (translational kinetic,

vibration, rotational energy)b) Potential energy

c) Chemical energyd) Nuclear energy

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E = Ek+Ep+UEk = mv2/2Ep = mgzΔ E = Δ Ek+Δ Ep+Δ U

The equation( inlet energy of system) – (outlet

energy of system) = (the change of thetotal energy of the system)

Ein-Eout =Δ Esystem

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FIGURE 5.7 Internal energy, a state function, depends only on the present state of the system andnot on the path by which it arrived at that state. The internal energy of 50 g of water at 25°C isthe same whether the water is cooled from a higher temperature to 25°C or is obtained bymelting 50 g of ice and then warming to 25°C.

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FIGURE 5.8 When a battery is discharged in lighting a flashlight, all the energy of the batteryappears as radiant energy and heat; no work is done. When the battery is used in the toy car,work is done in moving the car from place to place. Thus, the work done by the system (thebattery) is not a state function because its magnitude depends on the particular path by which

the system gets from its initial state to its final state.

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THE FIRST LAW IN CLOSED SYSTEM

1. The equationEin-Eout =Δ Esystem

WQ

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THE FIRST LAW IN CLOSED SYSTEM

Q - W = Δ Esystem = Δ Usystem

Q=Δ

U+W

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THE FIRST LAW IN OPEN SYSTEM

Steady flowFor steady flow, the following conditions are fulfilled:

① The matter of system is flowing steadily, so that theflow rate across any section of the flow has the samevalue;

② The state of the matter at any point remains constant;③ Q, W flow remains constant;

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The first law in open system

Flow work

w flow=pv p

V

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The first law in open system

5. Enthalpyfor flow fluid energy:

+mc f 2/2+mgzU+pV

H =U+pV unit: J, kJ

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§ 2-5. The first law in open system

6. Energy equation for steady flow open system

U1+p1V1H1 , mc f12/2, mgz 1

U2+p2V2H2 , mc f22

/2, mgz 2

Q

W

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The Natu re of Energ y

Systems and Surroundings

System: part of the universewe are interested in.Surroundings: the rest ofthe universe.

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Internal Energy

• Internal Energy: total energy of a system.• Cannot measure absolute internal energy.• Change in internal energy, Δ U( Δ E) = U(E) final - U(E) initial

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Changing the Internal Energy

U(=E) is a “state” function --- depends uniquely on the state ofthe system in terms of p , V, T etc.

(e.g. For a classical ideal gas, E= U = k )

WORK done by the systemon the environment

Thermal reservoir

HEAT is the transfer of thermal energy into the system from the surroundings

There are two ways to change the internal energy of a system:

Work and Heat are process energies , not s ta te funct ion s .

W b y = -W o n

Q

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1st Law of Thermodynamics

• statement of energy conservation for a thermodynamic system• internal energy E (=U) is a state variable• W , Q process dependent

systemdonework: positive

systemaddedheat: positive)(

by

to

W

Qteknik W QU

Energy In = Energy Out

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-- Heat and work are forms of energy transfer and energy isconserved.

The First Law of Thermodynamics (FLT)

U = Q + Won (sains)

work doneon the system

change intotal internal energy

heat addedto system

or

U = Q - W by

(teknik)

State Func t ion Process Func t ions

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Recap: – 1st Law of Thermodynamics

– energy conservation

Q = U + W (sains)

Heat flowinto system

Increase in internalenergy of system

Work done by system

V

P

U depends only on T (U = 3nRT/2 = 3pV/2)

point on p-V plot completely specifiesstate of system (pV = nRT)

work done is area under curve for complete cycle

U=0 Q=W

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Energy cannot be created or destroyed.Energy of (system + surroundings) is constant.

Any energy transferred from a system must betransferred to the surroundings (and vice versa ).

From the first law of thermodynamics:when a system undergoes a physical or chemical change, the change ininternal energy is given by the heat added to or absorbed by the system

plus the work done on or by the system:∆ U = q + w (Sains)

Fi r s t Law of Therm od ynamics

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Relating U ( E) to Heat and Work ( Sains )

Fi r s t Law of Therm od ynamics

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