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A non-spontaneous change occurs when an external effort is being done to it. Since the external effort is also a natural source , does this mean that there is no truly any non-spontaneous process in nature (universe) ?

Example to illustrate my point :

An electrochemical process (like in a galvanic cell) can be reversed by providing an external voltage in opposite direction whose magnitude is higher than the cell potential. This external voltage is given by humans. Humans are basically converting some form of energy like mechanical energy of rivers in a dam into electrical energy. Also, humans are doing so because they eat food and they get their energy to do through spontaneous metabolic processes in their body. Also, the conversion of energy in above process is spontaneous because humans are making use of a spontaneous process ( a river flowing down from a height) .

So, is there a truly non-spontaneous process ?

An_Elephant
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3 Answers3

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You define the difference between spontaneous and nonspontaneous processes when you define a system: spontaneous processes are the ones that happen without adding or subtracting anything from the system.

Definitions are arbitrary and can't be true or false.

...Although definitions can be useless if they define something that doesn't mean anything ("let $x$ represent the wavelength of the angular momentum of pi five minutes before the beginning of time"), or define a symbol to mean something completely different from what everybody else means by the symbol ("let $=$ represent velocity").

g s
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The question is not as simplistic as the down voting suggests. It alludes to the problem of how life, and in particular intelligent life that is capable of creating new things that did not exist before, should be viewed in the context of the second law. As asked, however, it is about the definition of spontaneity, which is a much easier question to answer:

A spontaneous process is one which, once initiated moves on its own without further assistance from the outside. The classical example is a box divided into two parts, each with its own pressure, temperature and composition. If we puncture the wall that separates the two parts, thus initiating the process, the rest happens entirely on its own.

Of course, humans made the box, filled it with gases and punctured the wall. But what happens after that, the equilibration of temperature, pressure and chemical potentials, requires no humans.

The waterfall (minus the hydroelectric plant) is an example of a spontaneous process that involves no humans whatsoever: water falls down the hill, not up, and in the process it converts kinetic energy into internal energy while also increasing the entropy of the universe.

Two examples of a non spontaneous process:

  • Separating the gases of the first example into the pure components

  • Pumping the water from the bottom of the waterfall back to the top

Themis
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I completely agree with @Themis and I do not understand the downvotes either. The problem of what constitutes spontaneous or non-spontaneous process is not a trivial one because the adjective describes human thinking and is not easy to define it in mathematical language. The issue is similar to the unease most of us must have felt having, the first time, learned of Lord Kelvin's formulation of the 2nd law:

It is impossible, by means of inanimate material agency, to derive mechanical effect from any portion of matter by cooling it below the temperature of the coldest of the surrounding objects

The same question could be asked how to put the "some other change" of Clausius into, say, a differential equation form:

Heat can never pass from a colder to a warmer body without some other change, connected therewith, occurring at the same time.

I have never understood what is being an "inanimate material agency" or "some other change" has to do with entropy, isothermal heating/cooling, or a differential inequality?

There has always been a stressful and strained relationship, pun intended, between real irreversible processes and classical thermo-statics whose concepts are based on reversible, i.e., non-real processes, and I believe your question is one manifestation of that. A possible way, though not explicitly emphasized by the author, is Pippard's "hole-in-the-wall" argument that formulates the 2nd law as "It is not possible to vary the constraints of an isolated system in such a way as to decrease the entropy".

If you restrict the 2nd law this way then the question what spontaneity is becomes easier to answer: spontaneous is what happens when you remove a constraint in an isolated system but do nothing else from the outside. Entropy is well-defined before and after the removal of the constraint because the isolated system starts and ends in equilibrium, thermo-statics to start and thermo-dynamics in between to proceed, and if anything happens in between that will increase the entropy. The system being isolated may include in and with all its complexity reservoirs, work bodies, fields, etc.

hyportnex
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