Kuliah - Energi & Hukum Pertama Termodinamika Bagian I - MHS_2

8/10/2019 Kuliah - Energi & Hukum Pertama Termodinamika Bagian I - MHS_2

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"Energi &Hukum I Termodinamika"


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Step 1: Problem Statement

Tulus B.S. - Teknik Mesin USU3

In your own words,briefly state theproblem, the keyinformation given, and

the quantities to befound.

This is to make sure thatyou understand theproblem and theobjectives before youattempt to solve theproblem.


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Step 2: Schematic


Draw a realistic sketch of the physical system involved, andlist the relevant information on the figure.

The sketch does not have to be something elaborate, but itshould resemble the actual system and show the key features.

Indicate any energy and mass interactions with thesurroundings.

Listing the given information on the sketch helps one to seethe entire problem at once.

Also, check for properties that remain constant during aprocess (such as temperature during an isothermal process),and indicate them on the sketch.


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Step 3: Assumptions and ApproximationsStep 4 : Physical Laws

State any appropriate assumptions andapproximations made to simplify theproblem to make it possible to obtain asolution.

Justify the questionable assumptions.

Assume reasonable values for missing

quantities that are necessary. For example, in the absence of specific

data for atmospheric pressure, it canbe taken to be 1 atm.

However, it should be noted in theanalysis that the atmospheric pressure

decreases with increasing elevation.

Apply all the relevant basic physicallaws and principles (such as theconservation of mass), and reducethem to their simplest form byutilizing the assumptions made.

However, the region to which a

physical law is applied must be clearlyidentified first. For example, theincrease in speed of water flowingthrough a nozzle is analyzed byapplying conservation of massbetween the inlet and outlet of thenozzle.



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Step 5: Properties Determine the unknown properties

at known states necessary to solvethe problem from propertyrelations or tables.

List the properties separately, and

indicate their source, if applicable.

Step 6: Calculations Substitute the known quantities

into the simplified relations andperform the calculations todetermine the unknowns.

Pay particular attention to the units

and unit cancellations, andremember that a dimensionalquantity without a unit ismeaningless.

Also, dont give a false implicationof high precision by copying all the

digits from the screen of thecalculatorround the results to anappropriate number of significantdigits


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Step 7 : Reasoning, Verification & Discussion


Check to make sure thatthe results obtained arereasonable and intuitive,and verify the validity of

the questionableassumptions.

Repeat the calculations thatresulted in unreasonable

values. A step-by-step approach can greatlysimplify problem solving.


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Energi berasal dari bahasa Yunani :

energeia yang berarti aktivitas

energos yang berarti aktif

Energi merupakan suatu besaran fisik

skalar yang menggambarkan jumlah kerjayang dapat dilakukan oleh suatu gaya

Energi


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Energi

The term energy wascoined in 1807 by ThomasYoung, and its use inthermodynamics was

proposed in 1852 by LordKelvin.

The term internal energy

and its symbol U firstappeared in the works ofRudolph Clausius andWilliam Rankine in thesecond half of thenineteenth century, andit eventually replacedthe alternative termsinner work, internal work,and intrinsic energycommonly used at thetime.



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Energi

Satuan SI untukenergi dan kerjaadalah joule (J)

Dinamakan untukmenghormati JamesPrescott Joule danpercobaannya dalam

persamaan mekanikpanas.

1 Joule = 1 newton

meter= 1kg m2 s2



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Bentuk Energi


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Energi Dalam

Internal energy is representedby the symbol U, and thechange in internal energy in aprocess is U2 - U1.

The specific internal energy issymbolized by u orrespectively, depending onwhether it is expressed on a

unit mass or per mole basis.

The change in the totalenergy of a system is



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Energi Dalam When work is done to

compress a spring, energy

is stored within the spring. When a battery is charged,

the energy stored within itis increased.

When a gas (or liquid)initially at an equilibriumstate in a closed, insulatedvessel is stirred vigorously

and allowed to come to afinal equilibrium state, theenergy of the gas isincreased in the process.



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Energi Mekanis


The mechanical energycan be defined as the

form of energy that can beconverted to mechanical

work completely anddirectly by an idealmechanical device such asan ideal turbine.

Kinetic and potentialenergies are the familiarforms of mechanicalenergy.

Thermal energy is notmechanical energy,however, since itcannot be converted

to work directly andcompletely (thesecond law ofthermodynamics).


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Energi Kinetik & Potensial

The change in kineticenergy of the body is

The quantity mgz is thegravitational potentialenergy, PE.

The change in gravitationalpotential energy, is



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Energi Potensial

Gravitasi bumi, salah satu gaya yangmenimbulkan energi potensial. Energi potensial dari kereta roller

coaster akan maksimum saat beradapada lintasan tertinggi.



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Energy Transfer by Heat

Energi dapatberupa panas &kerja

Energy can cross the boundaries of a closedsystem in the form of heat and work.



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Temperaturedifference is thedriving force for heattransfer.

The larger thetemperaturedifference, the higheris the rate of heat

transfer.



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Proses Adiabatik


A process during whichthere is no heat transferis called an adiabaticprocess.

The word adiabatic

comes from the Greekword adiabatos, whichmeans not to be passed.

There are two ways aprocess can beadiabatic: Either thesystem is well insulatedso that only a negligible

amount of heat can passthrough the boundary,or both the system andthe surroundings are atthe same temperatureand therefore there is

no driving force(temperaturedifference) for heattransfer.


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During an adiabaticprocess, a systemexchanges no heatwith its surroundings.


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Proses Adiabatik

NO HEAT EXCHANGE : Q = 0

Q = U + W ; W = -U or U = -W

Work done at EXPENSE of internal energy

INPUT Work INCREASES internal energy

Work Out Work In

U +U

Q = 0

W = -U U = -W



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Energy Transfer by Work


If the energy crossingthe boundary of aclosed system is notheat, it must be work.

The work done perunit time is calledpower and is denotedW


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Heat and work aredirectional quantities,and thus the completedescription of a heat or

work interactionrequires thespecification of boththe magnitude and

direction.Specifying the directions of heat and work


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Example : Heating of a Potato in an Oven


A potato initially atroom temperature(25C) is being bakedin an oven that is

maintained at 200C. Is there any heat

transfer during thisbaking process?


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Solution


Analysis : This is not a well-definedproblem since the system is notspecified.

Let us assume that we are observingthe potato, which will be our

system. Then the skin of the potato can be

viewed as the system boundary. Part of the energy in the oven will

pass through the skin to the potato. Since the driving force for this

energy transfer is a temperaturedifference, this is a heat transferprocess.

A potato is being baked inan oven.

It is to be determined

whether there is any heattransfer during thisprocess.


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Example : Burning of a Candle in an Insulated Room


A candle is burning in a

well-insulated room.

Taking the room (the airplus the candle) as thesystem, determine

(a) if there is any heat transfer

during this burning process

and

(b) if there is any change in

the internal energy of the

system.

Solution : A candleburning in a well-insulatedroom is considered.

It is to be determined

whether there is any heattransfer and any change ininternal energy.


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Analisa


Analysis (a) The interior surfaces of the

room form the systemboundary, as indicated by thedashed lines in Figure.

As pointed out earlier, heat isrecognized as it crosses the

boundaries.

Since the room is well insulated,we have an adiabatic system and

no heat will pass through theboundaries.

Therefore, Q= 0 for this process.

(b) The internal energy involvesenergies that exist in variousforms (sensible, latent,chemical, nuclear).

During the process just

described, part of the chemicalenergy is converted to sensibleenergy.

Since there is no increase ordecrease in the total internal

energy of the system, U = 0for this process.


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Shaft Work


Shaft work is proportionalto the torque applied andthe number of revolutionsof the shaft.

Energy transmissionthrough rotating shafts iscommonly encountered inpractice


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EXAMPLE Power Transmission by the Shaft of a Car


Determine the power

transmitted through theshaft of a car when thetorque applied is 200 Nmand the shaft rotates at a

rate of 4000 revolutionsper minute (rpm).

Solution The torque andthe rpm for a car engineare given.

The power transmitted isto be determined.

Analysis A sketch of the car is given

in Figure. The shaft power is determined

directly from

Discussion Note that powertransmitted by a shaft isproportional to torque and therotational speed.


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Work Done to Raise or to Accelerate a Body


When a body is raisedin a gravitationalfield, its potentialenergy increases.

Likewise, when abody is accelerated,its kinetic energyincreases.

The energy transferred to a body whilebeing raised is equal to the change inits potential energy.


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EXAMPLE Power Needs of a Car to Climb a Hill


Consider a 1200-kg carcruising steadily on a levelroad at 90 km/h.

Now the car starts climbing a

hill that is sloped 30 fromthe horizontal .

If the velocity of the car is toremain constant during

climbing, determine theadditional power that must bedelivered by the engine.

Solution The power required toaccelerate a car to a specifiedvelocity is to be determined.


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Analisa & Diskusi


Analysis

The work needed toaccelerate a body is simplythe change in the kinetic

energy of the body,

Discussion

This is in addition to thepower required toovercome friction, rollingresistance, and other

imperfections.


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Electrical Energy



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Bagaimana perbedaan energi dengan exergi ?

Tulus B S Teknik Mesin USU35

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Kuliah - Energi & Hukum Pertama Termodinamika Bagian I - MHS_2