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TRANSCRIPT
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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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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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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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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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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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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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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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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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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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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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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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G l i l E i i D t t
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Geological Engineering Department
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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?
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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
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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.
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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.
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g g g p
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Energy Transfer Mechanism
Energy transferred by the movement of a heatedsubstance is said to have been transferred by
convection.
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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
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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
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Heat & 1stLaw of Thermodynamics
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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
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Why did it happen?
Heat & 1stLaw of Thermo
L
L
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F l f E i i
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F lt f E i i
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