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What is a Thermodynamic Process?

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Question: What is a Thermodynamic Process?
What is a thermodynamic process? How do they relate to the laws of thermodynamics? Can you use them to determine the work done by a system?
Answer: A system undergoes a thermodynamic process when there is some sort of energetic change within the system, generally associated with changes in pressure, volume, internal energy, temperature, or any sort of heat transfer.

Major Types of Thermodynamic Processes

There are several specific types of thermodynamic processes that happen frequently enough (and in practical situations) that they are commonly treated in the study of thermodynamics. Each has a unique trait that identifies it, and which is useful in analyzing the energy and work changes related to the process. It is possible to have multiple processes within a single process. The most obvious example would be a case where volume and pressure change, resulting in no change in temperature or heat transfer - such a process would be both adiabatic & isothermal.

The First Law of Thermodynamics

In mathematical terms, the first law of thermodynamics can be written as:
delta-U = Q - W or Q = delta-U + W
where
  • delta-U = system's change in internal energy
  • Q = heat transferred into or out of the system.
  • W = work done by or on the system.
When analyzing one of the special thermodynamic processes described above, we frequently (though not always) find a very fortunate outcome - one of these quantities reduces to zero!

For example, in an adiabatic process there is no heat transfer, so Q = 0, resulting in a very straightforward relationship between the internal energy and work: delta-Q = -W. See the individual definitions of these processes for more specific details about their unique properties.

Reversible Processes

Most thermodynamic processes proceed naturally from one direction to another. In other words, they have a preferred direction.

Heat flows from a hotter object to a colder one. Gases expand to fill a room, but will not spontaneously contract to fill a smaller space. Mechanical energy can be converted completely to heat, but it's virtually impossible to convert heat completely into mechanical energy.

However, some systems do go through a reversible process. Generally, this happens when the system is always close to thermal equilibrium, both inside the system itself and with any surroundings. In this case, infinitesimal changes to the conditions of the system can cause the process to go the other way. As such, a reversible process is also known as an equilibrium process.

Example 1: Two metals (A & B) are in thermal contact and thermal equilibrium. Metal A is heated an infinitesimal amount, so that heat flows from it to metal B. This process can be reversed by cooling A an infinitesimal amount, at which point heat will begin to flow from B to A until they are once again in thermal equilibrium.

Example 2: A gas is expanded slowly and adiabatically in a reversible process. By increasing the pressure by an infinitesimal amount, the same gas can compress slowly and adiabatically back to the initial state.

It should be noted that these are somewhat idealized examples. For practical purposes, a system that is in thermal equilibrium ceases to be in thermal equilibrium once one of these changes is introduced ... thus the process is not actually completely reversible. It is an idealized model of how such a situation would take place, though with careful control of experimental conditions a process can be carried out which is extremely close to being fully reversible.

Irreversible Processes & The Second Law of Thermodynamics

Most processes, of course, are irreversible processes (or nonequilibrium processes). Using the friction of your brakes do work on your car is an irreversible process. Letting air from a balloon release into the room is an irreversible process. Placing a block of ice onto a hot cement walkway is an irreversible process.

Overall, these irreversible processes are a consequence of the second law of thermodynamics, which is frequently defined in terms of the entropy, or disorder, of a system.

There are several ways to phrase the second law of thermodynamics, but basically it places a limitation on how efficient any transfer of heat can be. According to the second law of thermodynamics, some heat will always be lost in the process, which is why it is not possible to have a completely reversible process in the real world.

Heat Engines, Heat Pumps, & Other Devices

We call any device which transforms heat partly into work or mechanical energy a heat engine. A heat engine does this by transferring heat from one place to another, getting some work done along the way.

Using thermodynamics, it is possible to analyze the thermal efficiency of a heat engine, and that is a topic covered in most introductory physics courses. Here are some heat engines which are frequently analyzed in physics courses:

  • Internal-Combusion Engine - A fuel-powered engine such as those used in automobiles. The "Otto cycle" defines the thermodynamic process of a regular gasoline engine. The "Diesel cycle" refers to Diesel powered engines.
  • Refrigerator - A heat engine in reverse, the refrigerator takes heat from a cold place (inside the refrigerator) and transfers it to a warm place (outside the refrigerator).
  • Heat Pump - A heat pump is a type of heat engine, similar to a refrigerator, which is used to heat buildings by cooling the outside air.

The Carnot Cycle

In 1924, French engineer Sadi Carnot created an idealized, hypothetical engine which had the maximum possible efficiency consistent with the second law of thermodynamics. He arrived at the following equation for his efficiency, eCarnot:
eCarnot = ( TH - TC) / TH
TH and TC are the temperatures of the hot and cold reservoirs, respectively. With a very large temperature difference, you get a high efficiency. A low efficiency comes if the temperature difference is low. You only get an efficiency of 1 (100% efficiency) if TC = 0 (i.e. absolute value) which is impossible.
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