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Chapter 18Temperature, Heat, and the

First Law of Thermodynamics

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18.2 Temperature

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18.3: The Zeroth Law of Thermodynamics

If bodies A and B are each in thermal equilibrium with athird body T, then A and B are in thermal equilibrium witheach other.

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18.4 Measuring Temperature

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18.4 Measuring Temperature, The Constant Volume Gas Thermometer

A constant volume gas thermometer consists of a gas-

filled bulb connected by a tube to a mercury manometer.

By raising and lowering reservoir R, the mercury level

in the left arm of the U-tube can be brought to the zeroof the scale to keep the gas volume constant (variations

in the gas volume can affect temperature

measurements).

The temperature of any body in thermal contact with the

triple-cell bulb is :

(p3 is the pressure exerted by the gas and Cis a

constant).

If the atmospheric pressure ispo, for any pressure p,

Therefore,

Finally, for very small amounts of gas,

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18.5 The Celsius and Fahrenheit Scales

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Example, Conversion Between Temperature Scales

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18.6: Thermal Expansion

When the temperature of an object is raised, the body usually exhibit thermal expansion. With the

added thermal energy, the atoms can move a bit farther from one another than usual, against the spring-

like interatomic forces that hold every solid together.)

The atoms in the metal move farther apart than those in the glass, which makes a metal object expandmore than a glass object.

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18.6: Thermal Expansion, Linear Expansion

If the temperature of a metal rod of lengthL is raised by an amount T, its length

is found to increase by an amount

in which a is a constant called the coefficient of linear expansion.

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18.6: Thermal Expansion, Volume Expansion

If all dimensions of a solid expand with temperature,

the volume of that solid must also expand. Forliquids, volume expansion is the only meaningful

expansion parameter.

If the temperature of a solid or liquid whose volume

is V is increased by an amount DT, the increase in

volume is found to be

where b is the coefficient of volume expansion of

the solid or liquid. The coefficients of volumeexpansion and linear expansion for a solid are related

by

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18.6: Thermal Expansion, Anomalous Expansion of Water

The most common liquid, water, does not

behave like other liquids. Above about

4C, water expands as the temperature

rises, as we would expect.

Between 0 and about 4C, however, water

contracts with increasing temperature.Thus, at about 4C, the density of water

passes through a maximum.

At all other temperatures, the density ofwater is less than this maximum value.

Thus the surface of a pond freezes while

the lower water is still liquid.

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Example, Thermal Expansion of Volume:

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18.7:Temperature and Heat

Heat is the energytransferred between asystem and its environment

because of a temperaturedifference that existsbetween them.

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18 8 Th Ab ti f H t b S lid d Li id

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18.8: The Absorption of Heat by Solids and Liquids

The heat capacity C of an object is the proportionality constantbetween the heat Q that the object absorbs or loses and the

resulting temperature change Tof the object

in which Tiand Tfare the initial and final temperatures of theobject.

Heat capacity C has the unit of energy per degree or energy per

kelvin.


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18.8: The Absorption of Heat by Solids and Liquids:Specific Heat

The specific heat, c, is the heat

capacity per unit mass

It refers not to an object but to a unit

mass of the material of which the

object is made.

When quantities are expressed inmoles, specific heats must also involve

moles (rather than a mass unit); they

are then called molar specific heats.


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18.8: The Absorption of Heat by Solids and Liquids:Heat of Transformation

The amount of energy per unit mass that must be transferred as heat when a sample

completely undergoes a phase change is called the heat of transformationL. When a sample

of mass m completely undergoes a phase change, the total energy transferred is:

When the phase change is between

liquid to gas, the heat of transformation

is called the heat of vaporization LV.

When the phase change is between solid

to liquid, the heat of transformation is

called the heat of fusion LF.

E l H Sl i W

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Example, Hot Slug in Water:

E l H Ch T

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Example, Heat to Change Temperature:

E l H t t Ch T t t

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Example, Heat to Change Temperature, cont.:

18 9: A Closer Look at Heat and Work

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18.9: A Closer Look at Heat and Work

18 10: The First Law of Thermodynamics

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18.10: The First Law of Thermodynamics

The quantity (Q

W) is the same for all processes. It depends only on theinitial and final states of the system and does not depend at all on how the

system gets from one to the other.

All other combinations ofQ and W, including Q alone, Walone, Q +W,

and Q -2W, are path dependent; only the quantity (Q

W) is not.

(Q is the heat and W is the work done by the system).

18 11: Some Special Cases of the First Law of Thermodynamics

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18.11: Some Special Cases of the First Law of Thermodynamics

1. Adiabatic processes. An adiabatic process is one that occurs so rapidly oroccurs in a system that is so well insulated that no transfer of thermal

energy occursbetween the system and its environment. Putting Q=0 in

the first law,

2. Constant-volume processes. If the volume of a system (such as a gas) isheld constant,so thatsystem can do no work. Putting W=0 in the firstlaw,

3. Cyclical processes. There are processes in which, after certaininterchanges of heat and work, the system is restored to its initial state.

No intrinsic property of the systemincluding its internal energycan

possibly change. PuttingEint= 0 in the first law

4. Free expansions. These are adiabatic processes in which no heat transferoccurs between the system and its environment and no work is done on or

by the system. Thus, Q =W =0, and the first law requires that

18 11: Some Special Cases of the First Law of Thermodynamics

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18.11: Some Special Cases of the First Law of Thermodynamics

Example First Law of Thermodynamics:

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Example, First Law of Thermodynamics:

Example

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Example,

First Law of Thermodynamics,

cont.:

18 12: Heat Transfer Mechanisms: Conduction

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18.12: Heat Transfer Mechanisms: Conduction

A slab of face area A and thicknessL, have faces

maintained at temperatures THand TCby a hot

reservoir and a cold reservoir. IfQ be the energy

that is transferred as heat through the slab, fromits hot face to its cold face, in time t, then the

conduction ratePcond(the amount of energy

transferred per unit time) is

Here k, called the thermal conductivity, is a

constant that depends on the material of which

the slab is made.

The thermal resistanceR, or the R-value of a

slab of thicknessL is defined as:


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Letting TXbe the temperature of

the interface between the two

materials, we have:

For n materials making up the slab,

Fig. 18-19 Heat is transferred at a steady rate

through a composite slab made up of two

different materials with different thicknesses and

different thermal conductivities.

The steady-state temperature at the interface ofthe two materials is TX.

18 12: Heat Transfer Mechanisms: Convection

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18.12: Heat Transfer Mechanisms: Convection

In convection, energy transfer occurs when a fluid, such as air or

water, comes in contact with an object whose temperature is higher

than that of the fluid.

The temperature of the part of the fluid that is in contact with the hot

object increases, and (in most cases) that fluid expands and thus

becomes less dense.

The expanded fluid is now lighter than the surrounding cooler fluid,

and the buoyant forces cause it to rise.

Some of the surrounding cooler fluid then flows so as to take the place

of the rising warmer fluid, and the process can then continue.

18 12: Heat Transfer Mechanisms: Radiation

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18.12: Heat Transfer Mechanisms: Radiation

In radiation, an object and its environment can exchange energy as heat via electromagnetic

waves. Energy transferred in this way is called thermal radiation.

The ratePradat which an object emits energy via electromagnetic radiation depends on the

objects surface area A and the temperature T of that area in K, and is given by:

Here s =5.6704 x10-8 W/m2 K4 is called the StefanBoltzmann constant, andeis the emissivity.

If the rate at which an object absorbs energy via thermal radiation from its environment isPabs,

then the objects net ratePnetof energy exchange due to thermal radiation is:

Example Thermal Conduction Through a Layered Wall:

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Example, Thermal Conduction Through a Layered Wall:

ch18 lecture 9th edi 60957

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