DEfinition of WORK:
work is done when the point of application of a
force moves in the direction of the force.
The product of the force and the distance moved in the direction of the
force is equal to the amount of the work done.
This
simple definition of work confines only to the area of mechanics and can not be
extended to the more complex problems in thermodynamics. Hence a new definition should be introduced
to cover mechanical as well as the other forms of work.
Positive work
is done by a system, during a given process, when sole effect external to the
system could be reduced to the lifting of a mass.
Consider a gas expanding in a piston cylinder arrangement as given
in Figure 2.1. Here no mass is actually
lifted against gravity. But if the
existing surroundings is fitted with an arrangement as given in the Figure 2.2,
there is a possibility of lifting the mass.
Hence work is said to be done by the system.
While exploring
the possibility of lifting a mass the effects that are external to the system
alone must be taken into account. For
example, a lift with a person and a suitcase is considered as a system. If the person lifts the suitcase, it should
not be taken into account, because this event occurs within the system.
UNITS:
In the
international system (SI), the unit of force is Newton (N) and that of distance
is metre (m). Hence the unit of work is
Nm which is also given a special name Joule. In most of the applications large
quantity of work is involved. Therefore kJ is commonly used.
Rate of doing
work is known as power. Hence its unit
is Nm/S or J/S which is again given a special name Watts(W).
Work done by the system on the surroundings is considered as
positive work.
· Work done on the system by
the surroundings is taken as negative work.
