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may be interpreted graphically as the area under the curve between the
limits V and V .
1
2
According to the convention stated above, the work is positive when a
system expands. Thus, if a system expands from 1 to 2 in Fig.2.3.2, the

area is regarded as positive. A compression from 2 to 1 would give rise to a
negative area; when a system is compressed, its volume decreases and it
does negative work on its surroundings.

If the pressure remains constant while the volume changes, then the
work is
A  p V  V  (2.3.4)
2 1

2.4 Heat Capacity

The heat capacity C of the system is determined as the amount of heat
(expressed in joules) needed to raise the system temperature by one
degree (expressed in Kelvin).

 Q
C  (2.4.1)
 T

It is expressed in units of thermal energy per degree of temperature. The
SI unit of heat capacity is joule per kelvin,

The amount of heat Q needed to increase the temperature of one
mole   1 mole  of a substance by one degree  T  1 K  is the molar

heat capacity.

 Q
C M  (2.4.2)
   T

 J 
It is expressed in joule per mole per degree Kelvin  C M  


 mole K 
The amount of heat needed to increase the temperature of one kilogram of
a substance by one degree is the specific heat capacity.

 Q
c  (2.4.3)
m  T

From (2.4.2) and (2.4.3) we obtain relation between specific an molar heat
capacity.

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