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

4–3
SPECIFIC HEATS

We know from experience that it takes different amounts of energy to raise

the temperature of identical masses of different substances by one degree.

For example, we need about 4.5 kJ of energy to raise the temperature of 1 kg

of iron from 20 to 30°C, whereas it takes about 9 times this energy (41.8 kJ

to be exact) to raise the temperature of 1 kg of liquid water by the same

amount (Fig. 4 –17). Therefore, it is desirable to have a property that will

enable us to compare the energy storage capabilities of various substances.

This property is the specific heat.

The specific heatis defined as the energy required to raise the temperature

of a unit mass of a substance by one degree(Fig. 4 –18). In general, this

energy depends on how the process is executed. In thermodynamics, we are

interested in two kinds of specific heats:specific heat at constant volumecv

andspecific heat at constant pressurecp.

Physically, the specific heat at constant volume cvcan be viewed as the

energy required to raise the temperature of the unit mass of a substance

by one degree as the volume is maintained constant. The energy required to

178 | Thermodynamics

(c) Under stated assumptions and observations, the energy balance on the

system can be expressed as

Net energy transfer Change in internal, kinetic,

by heat, work, and mass potential, etc., energies

Notice that even though the water is expanding during this process, the sys-

tem chosen involves fixed boundaries only (the dashed lines) and therefore

the moving boundary work is zero (Fig. 4–16). Then W
0 since the system

does not involve any other forms of work. (Can you reach the same conclu-

sion by choosing the water as our system?) Initially,

The quality at the final state is determined from the specific volume

information:

Then

Substituting yields

Discussion The positive sign indicates that the assumed direction is correct,

and heat is transferred to the water.

Qin
 1 5 kg 231 104.88104.83 2 kJkg 4 
0.25 kJ

104.88 kJ>kg

104.83 kJ>kg 1 2.3 10 ^521 2304.3 kJ>kg 2

u 2 
ufx 2 ufg

x 2 

v 2 vf

vfg

0.0020.001

43.340.001

2.3 10 ^5

u 1 uf @ 25°C
104.83 kJ>kg

Qin
¢U
m 1 u 2 u 12

EinEout¬


¬

¢Esystem

Vacuum

P = 0

W = 0

H 2 O Heat

FIGURE 4 –16

Expansion against a vacuum involves
no work and thus no energy transfer.

20 30°C

IRON

1 kg

←

4.5 kJ

20 30°C

WATER

1 kg

←

41.8 kJ

FIGURE 4 –17

It takes different amounts of energy to
raise the temperature of different
substances by the same amount.

Specific heat = 5 kJ/kg ·°C

∆T = 1°C

m = 1 kg

5 kJ

FIGURE 4 –18

Specific heat is the energy required to
raise the temperature of a unit mass of
a substance by one degree in a
specified way.

⎭⎪⎬⎪⎫ ⎭⎪⎬⎪⎫

SEE TUTORIAL CH. 4, SEC. 3 ON THE DVD.

INTERACTIVE

TUTORIAL