Work
Suppose a system starts from an initial state described by a pressure p
i
, a volume V
i
, and
a temperature T
i
. The final state of the system is described by a pressure p
f
, a volume V
f
, and a
temperature T
f
. The transformation from the initial state to the final state can be achieved in a
variety of ways (see for example Figure 17.2). In Figure 17.2a both pressure and volume change
simultaneously. In Figure 17.2b the pressure of the system is first lowered while keeping the
volume constant (this can for example be achieved by cooling the sample) and subsequently, the
volume is increased while keeping the pressure constant (this can be achieved by heating the gas
while increasing the volume).
If the pressure of a gas increases it can move a piston (this happens in an engine). In this
case, work is done by the system as the expanding gas lifts the piston. On the other hand, if we
increase the weight of the piston, work will be done on the system as the piston falls. The force

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