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    Geological Engineering Department

    Faculty of Engineering

    Physics for Scientists and Engineers

    Heat and The First Law of

    Thermodynamics

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    Geological Engineering Department

    Faculty of Engineering

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    Geological Engineering Department

    Faculty of Engineering

    INTRODUCTION

    Until about 1850, the fields of thermodynamics and mechanics wereconsidered two distinct branches of science, and the law of

    conservation of energy seemed to describe only certain kinds of

    mechanical systems.

    However, mid19th century experiments performed by theEnglishman 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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    Geological Engineering Department

    Faculty of Engineering

    HEAT & INTERNAL ENERGY

    Internal energy is all the energy of a system

    that is associated with its microscopic

    components atoms and molecules

    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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    Geological Engineering Department

    Faculty of Engineering

    Units of Heat

    calor ie (cal), which is defined as the amount of energytransfer necessary to raise the temperature of 1 g of

    water from 14.5C to 15.5C (1 degree Celcius).

    Bri t ish th ermal unit (BTU), which is defined as theamount of energy transfer required to raise the

    temperature of 1 lb of water from 63F to 64F.

    Scientists are increasingly using the SI unit of energy,thejou le, when describing thermal processes. In this

    course, heat and internal energy are usually measured

    in joules.

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    Geological Engineering Department

    Faculty of Engineering

    Mechanical equivalent of heat.

    1 calorie (cal) is the amount of energy requiredto raise the temperature of 0.001 kg of waterfrom 14.5 C to 15.5 C (T = 1.00 C).

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

    James Prescott Joule (1818-1889) showed

    1 calorie of heat = 4.186 Joule

    1 J = 1 Nm

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    Geological Engineering Department

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    HEAT CAPACITY & SPECIFIC HEAT

    The heat capacity C of a particular sample of asubstance is defined as the amount of energy

    needed to raise the temperature of that sample

    by 1C.

    From this definition, we see that if heat Q

    produces a change T in the temperature of a

    substance.

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    HEAT CAPACITY & SPECIFIC HEAT

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

    Thus, if energy Q transferred by heat to mass m

    of a substance changes the temperature of thesample by T.

    For example, the energy required to raise the temperature of 0.500

    kg of water by 3.00C is (0.500 kg) x (4186 J/kg C) x (3.00C) =

    6.28 x1000 J.

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    Geological Engineering Department

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    HEAT CAPACITY & SPECIFIC HEAT

    The quantity of energy required to raise thetemperature of a given mass of a substance by

    some amount varies from one substance to

    another. For example,

    the quant i ty o f energy required to raise the

    temperatu re o f 1 kg o f water by 1C is 4186 J.

    the quant i ty o f energy required to raise thetemperatu re o f 1 kg o f copper by 1C is on ly

    387 J.

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    Geological Engineering Department

    Faculty of Engineering

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    Geological Engineering Department

    Faculty of Engineering

    CONSERVATION OF ENERGY: CALORYMETER

    One technique for measuring specific heat involves heating asample 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 theprocess, 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 thisenergy transfer occurs are called calorimeters.

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    Faculty of Engineering

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    Geological Engineering Department

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    LATENT HEAT

    A substance often undergoes a change in temperaturewhen energy is transferred between it and its

    surroundings.

    There are situations, however, in which the transfer ofenergy 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 change.

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    Geological Engineering Department

    Faculty of Engineering

    As you might expect, different substances responddifferently 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 to

    change the phase of a mass m of a substance, 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 whenthe phase change is from solid to liquid (to fuse

    means tocombine 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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    Geological Engineering Department

    Faculty of Engineering

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    Geological Engineering Department

    Faculty of Engineering

    G l i l E i i D

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    G l i l E i i D t t

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    Geological Engineering Department

    Faculty of Engineering

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    G l i l E i i D t t

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    Geological Engineering Department

    Faculty of Engineering

    Home Assignment

    Berapa massa uap yang awalnya memiliki suhu130 C diperlukan untuk menghangatkan 200 gr

    air dalam wadah gelas 100 gr dari 20,0 C

    sampai 50,0 C?

    Geological Engineering Department

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    Geological Engineering Department

    Faculty of Engineering

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

    Touching a hot stove - Conduction

    Feeling the air rising from it - Convection

    Feeling the glow - Radiation

    Geological Engineering Department

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    Geological Engineering Department

    Faculty of Engineering

    No Talking!Think! Confer!

    How does heat get to the earth from the sun? What factors are

    important in the average temperature of the planet?(1) conduction; (2) convection; (3) radiation; (4) conduction and

    convection; (5) convection and radiation.

    Geological Engineering Department

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    Geological Engineering Department

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    Energy Transfer Mechanism

    The energy transferprocess that is most

    clearly associated with

    a temperature

    difference is thermalconduction.

    Conduction occurs onlyif there is a difference

    in temperature between

    two parts of the

    conducting medium.

    Geological Engineering Department

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    Geological Engineering Department

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    Geological Engineering Department

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    g g g p

    Faculty of Engineering

    Geological Engineering Department

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    Energy Transfer Mechanism

    Energy transferred by the movement of a heatedsubstance is said to have been transferred by

    convection.

    Geological Engineering Department

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    g g g p

    Faculty of Engineering

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

    form of electromagnetic waves produced by

    thermal vibrations of the molecules.

    = 5.6696 x 10-8W/m2K4

    Geological Engineering Department

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    g g g p

    Faculty of Engineering

    an object would eventually radiate all its energy, and its temperaturewould 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 resultof radiation is

    Geological Engineering Department

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    Faculty of Engineering

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    Faculty of Engineering

    Heat & 1stLaw of Thermodynamics

    Geological Engineering Department

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    Faculty of Engineering

    Applications:

    Adiabatic (Q =0)E = -W

    Isovolumetric (V =0) E = Q

    Isothermal (T =0)

    W =V1V2 PdV = V iV f (nRT/V) dV

    W = n R T ln(Vf/Vi)

    Heat & 1stLaw of Thermo

    Geological Engineering Department

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    Faculty of Engineering

    Geological Engineering Department

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    Faculty of Engineering

    Why did it happen?

    Heat & 1stLaw of Thermo

    L

    L

    Geological Engineering Department

    F l f E i i

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    Faculty of Engineering

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    F lt f E i i

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    Faculty of Engineering