Page 131 - 9780077418427.pdf

/Users/user-f465/Desktop
tiL12214_ch04_085-114.indd Page 108 9/1/10 9:38 PM user-f465
tiL12214_ch04_085-114.indd Page 108 9/1/10 9:38 PM user-f465 /Users/user-f465/Desktop

CONCEPTS Applied
EXAMPLE 4.8
A heat engine operates with 65.0 kcal of heat supplied and exhausts Thermodynamics in Action
40.0 kcal of heat. How much work did the engine do?
The laws of thermodynamics are concerned with changes
in energy and heat. This application explores some of
SOLUTION these relationships.
Obtain an electric blender and a thermometer. Fill the
List the known and unknown quantities:
blender halfway with water, then let it remain undisturbed
heat input Q H = 65.0 kcal
until the temperature is constant as shown by two con-
heat rejected Q L = 40.0 kcal
secutive temperature readings.
mechanical equivalent of heat 1 kcal = 4,184 J
Remove the thermometer from the blender, then run
The relationship between these quantities is found in equation 4.8, the blender at the highest setting for a short time. Stop

W = J(Q H – Q L ). This equation states a relationship between the heat and record the temperature of the water. Repeat this
supplied to the engine from a high-temperature source (Q H ), which procedure several times.
is partly converted to work (W), with the rest rejected in a lower- Explain your observations in terms of thermodynamics.
temperature exhaust (Q L ). Th e work accomplished is therefore the dif- See if you can think of other experiments that show
ference between the heat input and the heat output (Q H – Q L ), so the relationships between changes in energy and heat.
work accomplished represents the heat used, where J is the mechanical
equivalence of heat (1 kcal = 4,184 J). Th erefore,
W = J(Q H – Q L )
_
J
= 4,184 (65.0 kcal − 40.0 kcal)

kcal
J
_
= 4,184 (25.0 kcal)

kcal
J . kcal
_
= 4,184 × 25.0
kcal
= 104,600 J
= 105 kJ
SUMMARY

The kinetic theory of matter assumes that all matter is made up of molecule to molecule. Substances vary in their ability to conduct heat,

molecules. Molecules can have vibrational, rotational, or translational and those that are poor conductors are called insulators. Gases, such as
kinetic energy. Th e temperature of an object is related to the average air, are good insulators. The best insulator is a vacuum. Convection is

kinetic energy of the molecules making up the object. A measure of the transfer of heat by the displacement of large groups of molecules

temperature tells how hot or cold an object is on a temperature scale. with higher kinetic energy. Convection takes place in fluids, and the

Zero on the Kelvin scale is the temperature at which all random molec- fluid movement that takes place because of density differences is called

ular motion ceases to exist. a convection current. Radiation is radiant energy that moves through
Th e external energy of an object is the observable mechanical space. All objects with an absolute temperature above zero give off
energy of that object as a whole. Th e internal energy of the object is radiant energy, but all objects absorb it as well.

the mechanical energy of the molecules that make up the object. Heat The transition from one phase of matter to another that happens
refers to the total internal energy and is a transfer of energy that takes at a constant temperature is called a phase change. A phase change
place because of (1) a temperature diff erence between two objects or always absorbs or releases a quantity of latent heat not associated with
(2) an energy-form conversion. An energy-form conversion is actu- a temperature change. Latent heat is energy that goes into or comes
ally an energy conversion involving work at the molecular level, so all out of internal potential energy. Th e latent heat of fusion is absorbed or
energy transfers involve heating and working. released at a solid-liquid phase change.
A quantity of heat can be measured in joules (a unit of work or Molecules of liquids sometimes have a high enough velocity to
energy) or calories (a unit of heat). One kilocalorie is 1,000 calories, escape the surface through the process called evaporation. Evaporation
another unit of heat. A Btu, or British thermal unit, is the English sys- is a cooling process. Vapor molecules return to the liquid state through
tem unit of heat. Th e mechanical equivalent of heat is 4,184 J, or 1 kcal. the process called condensation. Condensation is the opposite of evap-
Th e specifi c heat of a substance is the amount of energy (or heat) oration and is a warming process. When the condensation rate equals
needed to increase the temperature of 1 gram of a substance 1 degree the evaporation rate, the air is said to be saturated.

Celsius. Th e specific heat of various substances is not the same because Th ermodynamics is the study of heat and its relationship to
the molecular structure of each substance is diff erent. mechanical energy, and the laws of thermodynamics describe these
Heat transfer takes place through conduction, convection, or relationships. Th e first law of thermodynamics states that the energy

radiation. Conduction is the transfer of increased kinetic energy from supplied to a thermodynamic system in the form of heat, minus the

108 CHAPTER 4 Heat and Temperature 4-24