To get the change in entropy of a system as a result of the transition from one thermodynamic equilibrium state to another, you need to dream up a reversible process path between the two states, and calculate the integral of dq/T for that path. This has nothing to do with the actual process path that caused the body to transition from the initial state to the final state (unless the actual process path just happened to be a reversible path).rajathjackson said:The equation for entropy S=delta(Q)/T is derived from reversible processes such as Carnot cycle. The delta(Q) in the equation is the reversible heat added or taken out from the system. So, why is this equation valid in the case of processes like cooling of a body which is irreversible?
Chestermiller said:What makes you think that the process of cooling a body is irreversible? Cooling of a body can be carried out reversibly. Can you dream up a reversible path that takes a body between a higher temperature and a lower temperature?
Chet
When you were talking about cooling a body, I naturally assumed you were talking about transferring heat to a colder body. But, if you want to transfer heat from a cold body to a hotter body, that can be done reversibly also. Just because you need an external source to do it (like, for example, a sequence of constant temperature baths) does not mean that it can't be done reversibly. And you can even accomplish this without doing any external work. But, of course, you will also be making a change in the constant temperature baths.rajathjackson said:But, according to 2nd law of thermodynamics heat doesn't flow from a cold to a hot body without any external work done on it. So, cooling of a body is an irreversible process because a body doesn't get hot on its own. We need to heat it up with an external source of energy.
Chestermiller said:When you were talking about cooling a body, I naturally assumed you were talking about transferring heat to a colder body. But, if you want to transfer heat from a cold body to a hotter body, that can be done reversibly also. Just because you need an external source to do it (like, for example, a sequence of constant temperature baths) does not mean that it can't be done reversibly. And you can even accomplish this without doing any external work. But, of course, you will also be making a change in the constant temperature baths.
Chet
Are you asking for the definition of a reversible process?rajathjackson said:So. if even processes in which we use an external source of energy is reversible which all processes are called irreversible and on the basis of what are they called so?
Chestermiller said:Are you asking for the definition of a reversible process?
Chet
At each location along a reversible path between the initial and final thermodynamic equilibrium states of a system, the system is only slightly removed from thermodynamic equilibrium. So the path is essentially a continuous sequence of thermodynamic equilibrium states.rajathjackson said:My understanding was that processes which can't proceed in the reverse direction without any external intervention are called as irreversible processes. But, you mentioned that even if there is an external energy source that process can be reversible. So, what is it that actually makes a process irreversible or reversible?
Entropy change in irreversible processes refers to the change in the amount of disorder or randomness in a system during a process that cannot be reversed. It is a measure of the energy that is no longer available to do work, and it always increases in irreversible processes.
The second law of thermodynamics states that the total entropy of a closed system can never decrease. This means that in irreversible processes, where entropy always increases, the total entropy of the system and its surroundings will also increase.
No, entropy change can never be negative in irreversible processes. This is because the second law of thermodynamics states that entropy always increases or remains constant in a closed system, and it cannot decrease.
The change in entropy in an irreversible process can be calculated using the equation ΔS = Qrev/T, where ΔS is the change in entropy, Qrev is the reversible heat transfer, and T is the temperature. This equation can only be used if the process is reversible. For irreversible processes, other methods such as the Clausius inequality may be used.
Some examples of irreversible processes include combustion, diffusion, and friction. These processes cannot be reversed and result in an increase in entropy. Other examples include the expansion of gases into a vacuum, mixing of different liquids, and heat transfer between two objects at different temperatures.