Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

A process during which there is no heat transfer is called an adiabatic
process (Fig. 2–14). The word adiabaticcomes from the Greek word
adiabatos,which means not to be passed.There are two ways a process
can be adiabatic: Either the system is well insulated so that only a negligible
amount of heat can pass through the boundary, or both the system and
the surroundings are at the same temperature and therefore there is no
driving force (temperature difference) for heat transfer. An adiabatic process
should not be confused with an isothermal process. Even though there is
no heat transfer during an adiabatic process, the energy content and thus
the temperature of a system can still be changed by other means such
as work.
As a form of energy, heat has energy units, kJ (or Btu) being the most
common one. The amount of heat transferred during the process between
two states (states 1 and 2) is denoted by Q 12 , or just Q. Heat transfer per
unit massof a system is denoted qand is determined from

(2–14)

Sometimes it is desirable to know the rate of heat transfer(the amount of
heat transferred per unit time) instead of the total heat transferred over some
time interval (Fig. 2–15). The heat transfer rate is denoted Q

.
, where the
overdot stands for the time derivative, or “per unit time.” The heat transfer
rate Q

.

has the unit kJ/s, which is equivalent to kW. When Q

.
varies with
time, the amount of heat transfer during a process is determined by integrat-
ing Q

.

over the time interval of the process:

(2–15)

When Q

.

remains constant during a process, this relation reduces to

(2–16)

where t
t 2 t 1 is the time interval during which the process takes place.

Historical Background on Heat

Heat has always been perceived to be something that produces in us a sensa-
tion of warmth, and one would think that the nature of heat is one of the first
things understood by mankind. However, it was only in the middle of the
nineteenth century that we had a true physical understanding of the nature of
heat, thanks to the development at that time of the kinetic theory,which
treats molecules as tiny balls that are in motion and thus possess kinetic
energy. Heat is then defined as the energy associated with the random motion
of atoms and molecules. Although it was suggested in the eighteenth and
early nineteenth centuries that heat is the manifestation of motion at the
molecular level (called the live force), the prevailing view of heat until the
middle of the nineteenth century was based on the caloric theory proposed
by the French chemist Antoine Lavoisier (1744–1794) in 1789. The caloric
theory asserts that heat is a fluidlike substance called the caloricthat is a
massless, colorless, odorless, and tasteless substance that can be poured from
one body into another (Fig. 2–16). When caloric was added to a body, its

Q
Q

#¬

¢t¬¬ 1 kJ 2

Q

t 2

t 1

Q

#

dt¬¬ 1 kJ 2

q

Q

m

¬¬ 1 kJ>kg 2

Chapter 2 | 61

SURROUNDING

AIR

HEAT

BAKED POTATO

2 kJ

thermal

energy

2 kJ

thermal

energy

2 kJ

heat

System

boundary

FIGURE 2–13

Energy is recognized as heat transfer

only as it crosses the system boundary.

Q = 0

Insulation

ADIABATIC

SYSTEM

FIGURE 2–14

During an adiabatic process, a system

exchanges no heat with its surroundings.

Q = 30 k = 30 kJ

m = 2 k = 2 kg

∆t = 5 s = 5 s

Q = 6 kW = 6 kW

q = 15 k = 15 kJ/J/kg

30 k 30 kJ

heatheat

FIGURE 2–15

The relationships among q,Q, and Q

.

.