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# The second law of thermodynamics says that in a cyclic process:(A) Work cannot be converted into heat(B) Heat cannot be converted into work(C) Work cannot be completely converted into heat(D) Heat cannot be completely into work

Last updated date: 14th Sep 2024
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We should know that thermodynamics is the branch of physics that deals with the relationships between heat and other forms of energy. In particular, it describes how thermal energy is converted to and from other forms of energy and how it affects matter. Properties can be combined to express internal energy and thermodynamic potentials, which are useful for determining conditions for equilibrium and spontaneous processes. With these tools, thermodynamics can be used to describe how systems respond to changes in their environment. Thermodynamics, science of the relationship between heat, work, temperature, and energy. In broad terms, thermodynamics deals with the transfer of energy from one place to another and from one form to another. The key concept is that heat is a form of energy corresponding to a definite amount of mechanical work. Based on this concept we have to solve this question.

We can see how entropy is defined by recalling our discussion of the Carnot engine. We noted that for a Carnot cycle, and
hence for any reversible processes,
$\dfrac{Q_{\mathrm{c}}}{Q_{\mathrm{h}}}=\dfrac{T_{\mathrm{c}}}{T_{\mathrm{h}}}$
Rearranging terms yields
$\dfrac{Q_{\mathrm{c}}}{T_{\mathrm{c}}}=\dfrac{Q_{\mathrm{h}}}{T_{\mathrm{h}}}$
for any reversible process. $Q_{\mathrm{C}}$ and $Q_{\mathrm{h}}$ are absolute values of the heat transfer at temperatures $T_{\mathrm{C}}$ and $T_{\mathrm{h}}$ , respectively. This ratio of $\dfrac{Q}{T}$
is defined to be the change in entropy $\Delta S$ for a reversible process, $\Delta S=\left(\dfrac{Q}{T}\right)_{\mathrm{rev}}$
Where $Q$ is the heat transfer, which is positive for heat transfer into and negative for heat transfer out of, and $T$ is the absolute temperature at which the reversible process takes place. The SI unit for entropy is joules per kelvin (J/K). If temperature changes during the process, then it is usually a good approximation (for small changes in temperature) to take $T$ to be the average temperature, avoiding the need to use integral calculus to find $\Delta$ S.
Let us start by defining the various laws of thermodynamics.
So, the First Law of Thermodynamics states that energy can be changed from one form to another, but it cannot be created or destroyed. The total amount of energy and matter in the Universe remains constant, merely changing from one form to another.
Now, the Second law of thermodynamics states that the total entropy of an isolated system can never decrease over time, and is constant if and only if all processes are reversible. Isolated systems spontaneously evolve towards thermodynamic equilibrium, the state with maximum entropy.
And lastly, the Third Law of Thermodynamics states that the entropy of a perfect crystal of a pure substance approaches zero as the temperature approaches zero. The alignment of a perfect crystal leaves no ambiguity as to the location and orientation of each part of the crystal.
Hence, it can be said that the Second law of thermodynamics states that in a cyclic process heat cannot be completely into work.

Hence, the correct answer is option (D).

Note
It should be known that a cyclic process consists of a series of changes which return the system back to its initial state. In noncyclic processes the series of changes involved do not return the system back to its initial state. The net work involved in a cyclic process is the area enclosed in a P-V diagram. If the cycle goes clockwise, the system does work. If the cycle goes anticlockwise, then the work is done on the system every cycle. An example of such a system is a refrigerator or air conditioner. The change in energy in a cyclic process is zero, since the initial and final states are the same. The work done and the quantity of heat gained in such a process are therefore the same with opposite signs.