Entropy and The Second Law of Thermodynamics
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One Way Processes: |
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The processes which occur in one direction only are called irreversible or one way processes. The examples are as follows:
| (i) | When two objects at different temperatures are placed in thermal contact with each
other, energy always flows by heat from the warmer to the cooler object. The flow
of
energy in the
reverse way is not possible.
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| (ii) | A rubber ball dropped to the ground bounces several times and eventually
comes to rest,
but a ball lying on the ground never begins bouncing on its own.
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| (iii) | An oscillating pendulum
eventually comes to rest because of the collisions with air
molecules and friction at the point of suspension. The mechanical energy of the
system
is converted to internal energy in the air, the pendulum, and the suspension; the
reverse
conversion of energy never occurs.
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These processes only run in forward direction. They are never observed to be running in backward direction. Thus, the one-way processes run in one direction or in forward direction only.
The Zeroth law thermodynamics and First law of thermodynamics
involve the concepts of
temperature and internal energy respectively. Temperature and internal energy, both are state
functions. Similar to these, another state function called entropy is related to second law of
thermodynamics. The entropy depends upon the state of the system. And the change in entropy,
when a system moves between any two equilibrium states depends only on initial and final
states. Thus, the change in entropy (dS) is independent of the actual path followed. It is defined as
the amount of energy transferred by heat (dQ) per unit temperature
(T):
The entropy is a measure of the system’s thermal energy per unit temperature that is
unavailable for doing useful work. Since the work is obtained from ordered molecular motion, the
amount of entropy is also a measure of randomness of a system. The isolated systems tend
toward disorder. The cause of this is easily explained by distinguishing between the microstates
and macro states of a system. A microstate is a particular description of the properties of the
individual molecules of the system. And a macro state is a description of the conditions of the
system from a macroscopic point of view and makes use of macroscopic variables such as
pressure, density, and temperature. In all real processes, the entropy of the universe increases.
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