Thermodynamics

How "cool" it must be to tell your friends that you are studying "thermodynamics" in physics, even though you probably have little to no idea what it entails. The word "thermos" in the Greek language translates to "hot," As most of our physics terminologies are taken from the Greek language, it follows that "dynamics" owes its existence to the Greeks as well; it means "force" or "power." Now you can boast to your friends that thermodynamics is the study of... "hot forces??" The answer is a little more complicated than that and this is where "heat" comes in. Thermodynamics is the study of heat, temperature, and work and how they relate to other physical properties of matter, such as internal energy, entropy, pressure, etc. The change in any of these quantities usually results in changes in the others, putting the "dynamics" in "thermodynamics." Since thermodynamics deals with cooling as much as it does heating, it seems as if it is pretty "cool" after all. In this article, we will learn about the basics of thermodynamics and how it applies to everyday life.

Thermodynamics Definition

It would be quite difficult to provide an exact definition for thermodynamics as it is the study of many interrelated quantities but a simple definition would be one that is similar to what we've mentioned above.

We usually consider matter that is in the form of a gas, more specifically, an ideal gas, but thermodynamics applies to all phases of matter. If we measure each of the variables, e.g. temperature, pressure, energy, etc. that describes an object/substance at a specific moment in time, then we can say that the object is in a particular state. If the value for any of those quantities changes, the state of the substance changes as well, which is called a thermodynamic process. The substance in this case can also be called the system, and these quantities that describe the state are known as state functions or state variables.

The First Law of Thermodynamics

Our focus in this topic will be on only one of the four laws; the first law of thermodynamics. In simple terms, the first law of thermodynamics is a statement of the law of conservation of energy. Energy can neither be created nor destroyed, it can only be converted from one form to another.

The concepts of internal energy \(U\) and thermal energy \(Q\) will be dealt with in detail in subsequent articles. Mathematically, the first law can be represented as \[\boxed{\Delta U=Q+W,}\] where \(W\) is the work done on the system. There is sure to be some confusion when energy is leaving the system, rather than entering, so we can also adopt the following sign convention to make things simpler :

\(Q>0:\) thermal energy is added to the system.

\(Q<0:\) thermal energy is removed from the system.

\(W>0:\) work is done on the system.

\(W<0:\) work is done by the system.

The sign convention places a minus sign on values of quantities that indicate energy is leaving the system. The values are positive if energy is entering the system. A positive value for the work done means that work is done by an external force on the system and its internal energy increases.

As an example, imagine a gas that is trapped by a moveable piston in a cylinder. If the gas is heated and expands, it will apply pressure on the piston, causing it to move. It gains thermal energy, \(Q>0\), from being heated but loses energy by doing work against the piston, \(W<0\), and the change in internal energy of the gas \(\Delta U\) can be calculated using the first law \[\Delta U=Q+W.\] A simple illustration of this scenario is shown in Fig. 1 below.

All of the thermal energy introduced to the gas will not necessarily be used to do mechanical work on the piston and so the internal energy is likely to change.

Entropy in Thermodynamics

At some point, you may have the word "entropy" and also heard it being described as the measure of disorder or randomness of a thermodynamic system. This is quite unscientific, so we need a more solid definition of entropy.

To make this more relatable, imagine that you are holding an egg above the ground. It slips from your grasp, and despite desperate attempts to catch it, it falls to the floor and shatters. Pieces of yolk and shell lie scattered everywhere. If you consider the egg to be in a closed system, the kinetic energy it gained from converting its initial gravitational potential energy was converted into heat, noise, etc. Those forms of energy could not be used to do useful work and the entropy increased, even though energy itself was conserved. Entropy \(S\) is measured in SI units of \(\text{joules per kelvin, }\) \(\mathrm{J\,K^{-1}}.\)

Thermodynamic Cycles

A process will usually take a thermodynamic system between states, that is, work done or thermal energy transfer will change the volume, pressure, entropy, etc. of that system. A cycle in thermodynamics is a set of processes, all of which are different, that link together to eventually bring the system back to its original state. There are different types of processes that can be used together to create a thermodynamic process. Without going into the detail of each process, some of these processes are:

• Isothermal processes: occur at constant temperature.
• Isobaric processes: occur at constant pressure.
• Isovolumetric processes: occur at constant volume. No work is done for isovolumetric processes, \(W=0\)
• Adiabatic processes: no thermal energy enters or leaves the system, \(Q=0.\)

Let's look at an example below that shows the relationship between the pressure and volume of a thermodynamic system.

PVT Diagrams in Thermodynamics

We have seen a two-dimensional plot of pressure against volume give us a PV diagram, but we can also plot pressure, volume, and temperature on a three-dimensional coordinate system to interpret the relationship between all three thermodynamic variables; we call this a PVT diagram. This is beyond the scope of this course but it is important to note that the PVT diagram provides more information than a PV diagram. Fig. 3 below shows a PVT diagram for a substance that expands when freezing, such as water.

Thermodynamics Examples

Now that we have seen a brief overview of what is to be expected in the topic of thermodynamics, we can test our understanding of what we have covered thus far.

Let's now look at an example that includes a PV diagram.

Applications of Thermodynamics

One of the most common applications of thermodynamics exists in power plants. Nuclear, coal, and even geothermal power plants use different methods of heating water. When heated, water in liquid form expands and turns into steam. Its volume increases and is hence able to do work on turbines that generate electricity.

Another application of thermodynamics is in refrigeration and air conditioning in which the temperature of a closed environment can be controlled via the control of the internal energy of that environment. It is clear then, that thermodynamics happens all around us and the applications are too many to mention.