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Nikitin
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So my professor keeps bringing up that a reversible process must always be done infinitesimally slowly up. But why is that? I can't recall the explanation.
The process has to be done slowly enough that the system is arbitrarily close to thermodynamic equilibrium during the process. If that is the case, an arbitrarily small change in conditions will cause the process to reverse direction.Nikitin said:So my professor keeps bringing up that a reversible process must always be done infinitesimally slowly up. But why is that? I can't recall the explanation.
It must be arbitrarily close to thermodynamic equilibrium so that an infinitesimal change in conditions will reverse the direction of the process. This is what we mean by a reversible process.Nikitin said:So the point is the system must never leave thermodynamic equilibrium during a reversible process? Why is that the case?
"Reversible" MEANS that the process can be reversed without materially changing any property of the system or surroundings.Nikitin said:Could you please explain it less formally, and more intuitively?Sent from my iPhone using Physics Forums
Andrew Mason said:"Reversible" MEANS that the process can be reversed without materially changing any property of the system or surroundings.
I don't know how your intuition works. Perhaps you could explain what it is about this definition that you are having difficulty grasping.
Reversibility is a thermodynamic concept that can be approached but never actually achieved in practice.
AM
Andrew Mason said:If the process involves heat transfer between the system and surroundings it has to be done with an infinitessimal temperature difference between the system and surroundings (so that an infinitessimal change in temperature willl cause the heat flow direction to reverse). This effectively means that the heat transfer will take an arbitrarily long time.
Andrew Mason said:"Reversible" MEANS that the process can be reversed without materially changing any property of the system or surroundings.
AM
Andrew Mason said:If the process involves heat transfer between the system and surroundings it has to be done with an infinitessimal temperature difference between the system and surroundings (so that an infinitessimal change in temperature willl cause the heat flow direction to reverse). This effectively means that the heat transfer will take an arbitrarily long time.
If it is an adiabatic process, an arbitrarily small change in the pressure of the surroundings will result in a change in the direction of the process. This means that the net pressure on or by the surroundings (Psurr - Psys) has to be arbitrarily close to 0 so it will proceed at an infinitessimally slow pace.
AM
An infinitesimal change in conditions is arbitrarily close to no change in conditions. If there is no change in conditions but the process reverses direction, then it is a reversible process.Soumalya said:You are changing the properties of the system or surroundings to reverse the direction of process which contradicts your previous statement
The question is: how? A Carnot engine will operate on its own and perform useful work but the reverse process requires work being done on the system. Where does that mechanical energy come from? It comes from the Carnot engine doing work which is stored as potential energy. If you can get back to the original initial state by operating the engine in reverse using ONLY the energy produced from the forward process, then the process is reversible.Soumalya said:I think I understood what Nikitin wanted to ask!
For a reversible process the only fundamental requirement is that both the system and the surroundings be restored to their initial states after a reversal.
Because more work is needed to restore the system and surroundings to the initial state than was produced in the forward process.What is the difference if we make a heat transfer between the system and surroundings with a considerably large temperature difference?Why won't the system and surroundings be restored to their initial states?
Because if it is not in equilibrium more than an infinitesimal change is needed to reverse the direction of the process. If there is a finite positive temperature difference between the hot reservoir and the system (i.e Th - Tsystem > δ > 0) the heat flow from hot reservoir to system will not reverse direction with an arbitrarily small change in temperature.While it's clear you must ensure an infinitesimal difference in conditions to establish a quasi equilibrium process could you explain as to why a reversible process must be a quasi equilibrium process?
Andrew Mason said:Because more work is needed to restore the system and surroundings to the initial state than was produced in the forward process.
AM
Andrew Mason said:Because if it is not in equilibrium more than an infinitesimal change is needed to reverse the direction of the process. If there is a finite positive temperature difference between the hot reservoir and the system (i.e Th - Tsystem > δ > 0) the heat flow from hot reservoir to system will not reverse direction with an arbitrarily small change in temperature.
In thermodynamics, a reversible process refers to a process that can be reversed without causing any change to the system or its surroundings. This is important because it allows us to accurately measure and calculate the thermodynamic properties of a system. It also serves as an idealized model for understanding the behavior of real systems.
An example of a reversible process is an ideal gas expanding or compressing slowly and without friction in a perfectly insulated container. This process can be reversed by compressing or expanding the gas back to its original state, without any loss of energy or change in the system's properties.
Yes, irreversible processes can occur in thermodynamics. In fact, most real-life processes are irreversible. For example, the combustion of fuel in an engine, or the transfer of heat from a hot object to a colder one, are both irreversible processes.
The reversibility of a process is directly related to the efficiency of a system. A reversible process is considered to be the most efficient since it does not incur any energy losses. However, in real systems, there are always some irreversible processes that result in energy losses, making the system less efficient.
Yes, reversible processes have several practical applications in thermodynamics. One example is the use of reversible heat engines in power plants, where the heat energy is converted into mechanical work with maximum efficiency. Another application is the reversible expansion and compression of gases in refrigeration and air conditioning systems, where the heat is either removed or added to the refrigerant to maintain a desired temperature.