20.8:
Cyclic Processes And Isolated Systems
A system that does not exchange heat and does no work on its surroundings is called an isolated system. The change in internal energy for such systems is zero.
In non-isolated systems, the internal energy can be constant only if the process followed is cyclic.
Suppose a thermodynamic system, like air inside the lungs, at initial pressure and volume, expanded to a new final state, returns to its initial state in a thermodynamic process. Such a process is called a cyclic process.
In this process, the internal energy of the gas remains constant; therefore, the work done equals the net heat transfer.
In a pV diagram, such a cyclic process is represented by a closed path. Suppose the work done along the first path is W1, and work done along the second path is W2. The net work done in a cyclic process is represented by the area between these two closed paths.
It is positive if the process follows a clockwise cycle and is negative when it follows a counter-clockwise cycle.
20.8:
Cyclic Processes And Isolated Systems
A thermodynamic system with zero heat exchange and work is an isolated system. For these systems, the internal energy remains constant.
In the case of a non-isolated system, the change in the internal energy is zero only if the process is cyclic. A thermodynamic process is considered cyclic if the system undergoes a series of changes and returns to its initial state.
Consider a cyclic process that returns to its initial state, undergoing a four-step process. The heat transfer along each path is 40 J, −80 J, −20 J, and 100 J. What is the net work done for this process?
The known quantities are the heat transfer for each step. The unknown quantity is the net amount of work done for this cyclic process.
According to the first law of thermodynamics, the change in the internal energy of the system is the difference between the net heat transfer into the system and the net work done by the system. Since the change in internal energy is zero, the net amount of work done equals the net heat transfer. The net heat transfer is the sum of the heat transfer for each path, equal to 40 J. Hence, the net work done is 40 J.
The net positive work done implies that the work done by the system is greater than the work done on the system. The heat energy supplied to the system is used by the system in doing the external work.
Suggested Reading
- Young, H. D, and Freedman, R.A. (2012). University Physics with Modern Physics. San Francisco, CA: Pearson. pp 633.
- OpenStax. (2019). University Physics Vol. 2. [Web version]. pp 125 Retrieved from https://openstax.org/books/college-physics/pages/15-2-the-first-law-of-thermodynamics-and-some-simple-processes