“A prescribed, identifiable and fixed collection of matter which is completely enclosed with in a definite region, and whose behaviour is being investigated”
The system may be a quantity
of steam, a mixture of vapour and gas, or an internal combustion engine and its
components. The term surroundings represent the environments (a
combination of matter and space external to the system) which are affected by
changes occurring within the system. The boundary separating a system
from its surrounding may be real or imaginary, rigid and confined in the space
or flexible, i.e., it may change in shape, volume, position and / or
orientation.
Fig. Shows the
conventional representation of a system, its boundary and surroundings. The
surroundings can also be regarded as a system and hence we are largely
concerned with the interactions between systems.
When the system executes a process (undergoes
a change from one state to another), there are mass and energy interactions
between the system and the surroundings. For a closed system the
same mass matter remains within the system and only work and heat energies
cross the systemboundaries. The physical nature and chemical composition of the
matter can, however, change. Thus a liquid may evaporate, a gas may condense or
a chemical reaction may take place between two or more constituents of a
system. Examples of closed system are :
(1) gas expanding in a cylinder by displacing
a piston
(2) mixture of water and steam in a closed
vessel
(3) Bomb calorimeter
When the mass flow of substance also takes
place simultaneously with the transfer of energy, due system is known as an open
system. Examples of an open system are :
(1) steam flowing through a turbine
(2) water entering a boiler and leaving as
steam
(3) water wheel suspended in a
water stream
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