Thermodynamics. Irreversible and reversible Process

1. Mar 28, 2015

Prannoy Mehta

When we were taught these in the class. There were a few terms I did not understand which my school teacher used. Firstly, they told us that the work in a reversible process occurs at the boundary of the system, an acceptable fact. All the energy is converted yo work done, and then showed us a graph of P vs V for an isothermal reversible expansion, adiabatic reversible reaction, etc. The graphs made sense. But then as we moved towards irreversible process, our sir mentioned loss in energy to the surroundings. I did not understand how, or to what type of energy. Any help is appreciated. Thanks in advance. I am sorry, if there was a conceptual mistake, if so please do correct me.

2. Mar 28, 2015

phys_samar

Let us consider some examples of irreversible processes.

A typical example of an irreversible process, accompanying many natural processes, is the above-mentioned process of friction. The work expended to overcome friction turns irreversibly into the heat, which is liberated in the course of friction.

The presence of friction always results in the absolute value of the work output of a system undergoing a direct process being smaller than the work transferred into the system from the outside during the reverse process. This, for instance, explains the fact that in reality the ball, moving from one inclined plane to the other and back, rises each time to a smaller height, until it comes to a standstill at the bottommost point. To overcome friction and the resistance offered by the surrounding medium an irreversible expenditure of energy takes place, and the process develops spontaneously in one direction, until the system comes into the state of rest.

3. Mar 28, 2015

Staff: Mentor

It sounds like you are saying that, if it were not for friction, there would be no such thing as an irreversible process. Is that what you are saying?

Chet

4. Mar 28, 2015

Staff: Mentor

Is this a high school level course that you are describing, or is it a college thermo or phys chem course? I'd like to know the math you've had so far before trying to address your question.

Chet

5. Mar 28, 2015

Prannoy Mehta

It is a chapter taught in both Physics and Chemistry in my high school. In Mathematics, I have completed higher Algebra, and Single Variable Calculus.

6. Mar 29, 2015

Staff: Mentor

In a reversible process between two thermodynamic equilibrium states of a closed system, the system is never more than slightly removed from thermodynamic equilibrium over then entire path between the two equilibrium states. So, a reversible process can be characterized as a continuous sequence of thermodynamic equilibrium states. In such a process, the temperature and pressure of the material within the system will not at any time vary with spatial position (i.e., location) throughout the system. So, if we plot a graph of pressure at the interface (with the surroundings) versus volume for a reversible process, we will also automatically be plotting the pressure throughout the system versus volume.

To carry out such a process on a closed system, we must make sure that the rate of heat transfer and the rate of doing work at the interface between the system and surroundings are both very slow. If we do not do this, and rate of heat transfer and/or the rate of doing work at the interface between the system and surroundings are high, there will be temperature and pressure variations spatially throughout the system, and the system pressure and temperature will not match those of the surroundings except at the interface. Such a process is called an irreversible process. For an irreversible process in which work is done on the surroundings, if we were to plot the pressure at the interface as a function of the volume, it would not be the same as plotting the pressure at other locations within the system as a function of volume. Of course, the work done by the system on the surroundings would still be equal to the pressure at the interface integrated over the volume change. However, the pressure at the interface could not be determined from an equation of state such as the ideal gas law, because the pressure would be varying spatially within the system.

For an reversible process of a closed system, it is possible to also operate the surroundings reversibly such that both the system and surroundings can be returned to their original thermodynamic equilibrium states. For an irreversible process, this cannot be done.

Chet