Kuliah Fisdas 2 Bab 1 Panas & Hukum I Termodinamika 2013

Geological Engineering DepartmentFaculty of Engineering

Physics for Scientists and Engineers

Heat and The First Law of Heat and The First Law of Thermodynamics Thermodynamics

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INTRODUCTIONINTRODUCTION

• Until about 1850, the fields of thermodynamics and mechanics were considered two distinct branches of science, and the law of conservation of energy seemed to describe only certain kinds of mechanical systems.

• However, mid–19th century experiments performed by the Englishman James Joule and others showed that energy may be added to (or removed from) a system either by heat or by doing work on the system (or having the system do work).

• Today we know that internal energy, which we formally define in this chapter, can be transformed to mechanical energy.

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HEAT & INTERNAL ENERGY

• Internal energy Internal energy is all the energy of a system that is associated with its microscopic components —atoms and molecules

• Heat Heat is defined as the transfer of energy across the boundary of a system due to a temperature difference between the system and its surroundings.

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Units of HeatUnits of Heat

• calorie (cal), which is defined as the amount of energy transfer necessary to raise the temperature of 1 g of water from 14.5°C to 15.5°C (1 degree Celcius).

• British thermal unit (Btu), which is defined as the amount of energy transfer required to raise the temperature of 1 lb of water from 63°F to 64°F.

• Scientists are increasingly using the SI unit of energy, the the joulejoule, when describing thermal processes. In this course, heat and internal energy are usually measured in joules.

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Mechanical equivalent of heat.Mechanical equivalent of heat.

• 1 calorie (cal) is the amount of energy 1 calorie (cal) is the amount of energy required to raise the temperature of required to raise the temperature of 0.001 0.001 kg kg of water from 14.5 C to 15.5 C of water from 14.5 C to 15.5 C ((∆T = ∆T = 1.00 C)1.00 C)..

• 1 BTU is the heat to raise 1 Lb by 1 ⁰F.1 BTU is the heat to raise 1 Lb by 1 ⁰F.

• James Prescott Joule (1818-1889) showed James Prescott Joule (1818-1889) showed

1 calorie of heat = 4.186 Joule1 calorie of heat = 4.186 Joule

1 J = 1 N1 J = 1 N‧‧mm

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HEAT CAPACITY & SPECIFIC HEATHEAT CAPACITY & SPECIFIC HEAT

• The heat capacity C of a particular sample of a substance is defined as the amount of energy needed to raise the temperature of that sample by 1°C.

• From this definition, we see that if heat Q produces a change ΔT in the temperature of a substance.

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• The specific heat c of a substance is the heat capacity per unit mass.

• Thus, if energy Q transferred by heat to mass m of a substance changes the temperature of the sample by ΔT.

For example, the energy required to raise the temperature of 0.500 kg of water by 3.00°C is (0.500 kg) x (4186 J/kg °C) x (3.00°C) = 6.28 x1000 J.

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• The quantity of energy required to raise the temperature of a given mass of a substance by some amount varies from one substance to another. For example,

• the quantity of energy required to raise the temperature of 1 kg of water by 1°C is 4186 J.

• the quantity of energy required to raise the temperature of 1 kg of copper by 1°C is only 387 J.

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CONSERVATION OF ENERGY: CALORYMETERCONSERVATION OF ENERGY: CALORYMETER

• One technique for measuring specific heat involves heating a sample to some known temperature Tx, placing it in a vessel containing water of known mass and temperature and measuring the temperature of the water after equilibrium has been reached.

• Because a negligible amount of mechanical work is done in the process, the law of the conservation of energy requires that the amount of energy that leaves the sample (of unknown specific heat) equal the amount of energy that enters the water.

• This technique is called calorimetry, and devices in which this energy transfer occurs are called calorimeters.

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LATENT HEATLATENT HEAT

• A substance often undergoes a change in temperature when energy is transferred between it and its surroundings.

• There are situations, however, in which the transfer of energy does not result in a change in temperature. This is the case whenever the physical characteristics of the substance change from one form to another; such a change is commonly referred to as a phase changephase change.

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• As you might expect, different substances respond differently to the addition or removal of energy as they change phase because their internal molecular arrangements vary.

• If a quantity Q of energy transfer is required Q of energy transfer is required to change the phase of a mass to change the phase of a mass m of a m of a substancesubstance, the ratio L = Q/m characterizes an important thermal property of that substance.

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• Latent heat of fusion Lf is the term used when the phase change is from solid to liquid (to fuse means “to combine by melting”)

• Latent heat of vaporization Lv is the term used when the phase change is from liquid to gas (the liquid “vaporizes”)

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Home Assignment

• Berapa massa uap yang awalnya memiliki suhu 130 °C diperlukan untuk menghangatkan 200 grair dalam wadah gelas 100 gr dari 20,0 °C sampai 50,0 °C?


How does heat get from one place to How does heat get from one place to another?another?Examples:

• Touching a hot stove - Conduction

• Feeling the air rising from it - Convection

• Feeling the glow - Radiation


No Talking!No Talking!Think!Think! Confer!Confer!

How does heat get to the earth from the sun? What factors How does heat get to the earth from the sun? What factors are important in the average temperature of the planet? are important in the average temperature of the planet?

(1) conduction; (2) convection; (3) radiation; (4) conduction (1) conduction; (2) convection; (3) radiation; (4) conduction and convection; (5) convection and radiation.and convection; (5) convection and radiation.

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Energy Transfer Mechanism

• The energy transfer process that is most clearly associated with a temperature difference is thermal conduction.

• Conduction occurs only if there is a difference in temperature between two parts of the conducting medium.

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Energy Transfer Mechanism

• Energy transferred by the movement of a heated substance is said to have been transferred by convection.

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• The third means of energy transfer is radiationradiation. • All objects radiate energy continuously in the

form of electromagnetic waves produced by thermal vibrations of the molecules.

σ = 5.6696 x 10-8 W/m2‧K4

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• an object would eventually radiate all its energy, and its temperature would reach absolute zero.

• The energy an object absorbs comes from its surroundings, which consist of other objects that radiate energy. If an object is at a temperature T and its surroundings are at a temperature To , then the net energy gained or lost each second by the object, as a result of radiation is

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(0 (0 thth) Law of Thermodynamics) Law of Thermodynamics

If objects A and B are separately in If objects A and B are separately in thermal thermal equilibrium equilibrium with a third object C, then objects A with a third object C, then objects A

and B are in thermal equilibrium with each and B are in thermal equilibrium with each other.other.

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Heat & 1Heat & 1stst Law of Thermodynamics Law of Thermodynamics


Applications:Applications:

Adiabatic Adiabatic (∆Q =0)(∆Q =0)∆∆E = -WE = -W

Isovolumetric (∆V =0)Isovolumetric (∆V =0)∆∆E = ∆QE = ∆Q

Isothermal Isothermal (∆T =0)(∆T =0)

W =W =∫∫V1V1V2 V2 PdV = ∫PdV = ∫V iV i

V f (V f (nRT/V) dVnRT/V) dV

W = n R T ln(VW = n R T ln(Vf f /V/Vi i ) )

Heat & 1Heat & 1stst Law of Thermo Law of Thermo

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Why did it happen?Why did it happen?

Heat & 1Heat & 1stst Law of Thermo Law of Thermo

ΔL = α LΔT

ΔL

ΔL

Kuliah Fisdas 2 Bab 1 Panas & Hukum I Termodinamika 2013

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