Chapter 18

Temperature, Heat, and the First Law of Thermodynamics

In this chapter we will explore the following topics:

Temperature and the zeroth law of thermodynamics Thermometers and temperature scales

Thermal expansion Temperature and heat Specific heat Heat of transformation Heat, work, and the first law of thermodynamics Heat transfer mechanisms

If the temperature of a metal rod of length is increased by an amount , its lengthalso increases by an amount .The term is known as the

and

LTL L T

Linear Expansion

coefficient of linear expansion it depends on the material of the rod. L L T

The constant does not change very much with temperature. Thus for most practicalpurposes it can be taken as a constant.

All three dimensions of a solid expand with temperature, and t

Volume Expansionhus the volume also

expands. For liquids this is the only meaningful expansion parameter. For a liquid of volume , an increase in temperature results in an increase in volume

. The term V T

V V T

is known as the coefficient of volume expansion.For a solid, 3 .

V V T

Consider the two objects shown in the figure. One is calledthe "system" and the other the "environment." In the examplein the text the system is a can of cola and the environment

Temperature and Heat

is the kitchen table. If the system temperature is bigger thanthe environment temperature (see fig. ) then energy is transferred from the system to the environment. This energyis known as "

S

E

TT a

" (symbol ). In this case the heat is taken as negative. If (see fig. ) we have no heat transfer. If (see fig. ) heat is transferred from the environment to the system. In this

S E

S E

QT T b

T T c

heat

case 0.Thus heat is the energy transferred between a system and itsenvironment because a temperatu

SI unit for : tre difference

he joule. Anoexists betwee

ther non-SI un

the two nit is th. ecal

o

r

Q

Q

ie (cal) defined as 1 cal = 4.1868 J.

If an object whose initial temperature is absorbs an amount of heat, its temperature increases to a value according to the equation

. The proportionality constant

i

f

f i

T QT

Q C T C T T

Heat Capacity

is known as the

object's " "

The heat capacity of an object is proportional to its mass .

Thus the equation above can be written as . The constant

is known af i

C

C m

Q mc T T c

Specific Heatheat capacity.

s the " " and it depends on the material out of which the object is made.

If we measure the heat capacity that corresponds to one mole of a substance, this is known as "m

Molar Specific Heat

specific heat

23

olar specific heat." It is a convenient

parameter because 1 mole of any substance contains the same number (6.02 10 )of atoms or molecules.

An object can exist in one of three "states" or "phases." These are the solid, the liquid,and the gas states. In the solid phase the atoms are locked in a rigid structure. In

Heats of Transformation

the liquid phase the object does not have a rigid structure but takes the shape of thecontainer. In the gas state the atoms are free of one another and completely fill thevolume of the container. When the state of an object of mass changes, the objectcan absorb or release an amount of heat without any change in temperature .The heat is given by the equation . The constant is kn

mQ T

Q Q Lm L own as the " " If the object changes from the liquid to the gas phase the heat of transformation is known as the " " If the object changes from solid to

VLheat of transformation.

heat of vaporization.liquid the heat of transformation is known as the

" "An object heat to go from the solid to the liquid or from the liquid to thegas phase. Conversely, an object heat

FL

absorbs

releases

heat of fusion.

to go from the liquid to thesolid or from the gas to the liquid phase.

Prob. 40, Chap. 18, Page 502:Calculate the specific heat of a metal from the following data. A container made of the metal has a mass of 3.6 kg and contains 14 kg of water. A 1.8 kg piece of the metal initially at a temperature of 180°C is dropped into the water. The container and water initially have a temperature of 16.0°C, and the final temperature of the entire (insulated) system is 18.0°C.