Laws of Thermodynamics

Introduction

We can say that “Everything present in the world is thermodynamics.” It is not just one concept that reserves for the realm of physics and also not a set of laws that you’ll only find in refrigerators, electronics, planes, cars, etc. It is one of the scientific concepts that weaves itself into the very fabric of life. Thermodynamics is nothing but just the way energy works, so we can say that it is easy to miss. For example, when someone cleans their office week after week, which seems to get messier by the day, their Second Law of Thermodynamics is applicable at work, where everything leads to an increased state of disorganization. Or we can take one more example, when we cook that delicious steak on the grill this weekend, their First Law of Thermodynamics is applicable at work while the energy is transfering to our food in the form of heat. We can say that thermodynamics is not just a single study of heat and work; it’s the study of how energy and the stuff that around us is made of, works. So thermodynamics is nothing but the study of life.

Systems and Surroundings

We all live in a system where energy and matter are being exchanged continuously in an endless flow. Let us look at the example given below: in the process of eating where we take the chemical energy present in food and convert that energy into a form that can be used to do work by our body. Our body has gained energy from food, now it can go about doing some kind of work out in the world.

Above mentioned process of exchange of energy where the energy transforms from one state to another state that all happens in a set of systems and surroundings. When we turn on our electric tea kettle to drink tea in the morning, the kettle has water enclosed within a metal container, here the kettle is a system. The rest part of the kitchen, even the rest part of the house, are said to be  surroundings.

When the tea kettle is starting to boil, it can transform some amount of the water into the steam which can release from the spout present at the top. This can convert energy that crosses a boundary from the system. This happens within the metal container and the surroundings outside the metal container. This is nothing but thermodynamics at work, the transference of matter and energy between  the systems and surroundings.

Image 1:Every thermodynamic system is surrounded by a boundary and surroundings. (Image source)

Systems in thermodynamics are defined by the observer, so it is varying person by person, for one person the tea kettle  used might be the system in thermodynamics. For another person the entire house might be the system in thermodynamics and all the neighborhood and area are the surroundings, it all depends on our perspective. The main point being noticed is that every system present in thermodynamics is contained in the defined boundary, and on the other side of the boundary are the surroundings. 

Basically there are three types of systems present in thermodynamics:

• An open system: where energy and matter can be exchanged between a system and its surroundings.

• A closed system: where only energy can be exchanged between a system and its surroundings, not matter.

• An isolated system: where neither energy nor matter is exchanged between a system and its surroundings. A truly isolated system is rare.

The First Law of Thermodynamics

The First Law of Thermodynamics is also known as the Law of the Conservation of Energy, so basically this law says that energy cannot be created or destroyed, it can only change its form. There are lots of types of energy present. Figure given below explains different types of energy :

Image 2: Energy comes in a variety of different forms. (Image source)

We learned that Energy cannot be created or destroyed; it simply changes its form from one form to another. Take an example of turning on a light, here the switch does not create any type of energy, it just converts electrical energy into radiant energy (light) and thermal energy (heat).

Image 3: Practical examples of transformation of energy in action. (Image source)

Basically in the First Law of thermodynamics there are three related concepts: work, heat, and internal energy. Heat is nothing but the transfer of thermal energy between two systems. Work is the force that can transfer energy between a system and its surroundings. By producing work we can create heat inside or outside the system. Internal energy, which is all the energy which is present in a system. When heat, work, and internal energy  all interact together, the energy is transformed. 

We can express this relationship mathematically as:

Here, 

ΔU is the total change or internal energy present in a system, 

Q is the exchanged heat between the system and its surroundings, 

W is the work done by or to the system.

When a system doing some kind of work or releases heat the internal energy of the system decreases. Likewise, if we add heat to a system, or we do work on a system, the internal energy of the system will increase. Any kind of energy released by a system is absorbed by its surroundings, and any kind of energy lost by a surrounding is absorbed into a system. In all of these examples, we can not create or destroy energy; it is not but just moving from one place to another. Expressed mathematically, this looks like:

Here, ΔUsystem is the total internal energy present in a system and is always equal to  the ΔUsurroundings the total energy in the surroundings.

One important thing is to be noticed that the First Law is that the transformation of energy is not 100% efficient. In our light bulb example you can transform electrical energy into a usable form of light energy, but in the process, you create unusable energy in the form of heat.

When related to electronics, First Law of Thermodynamics has a similar resemblance to Kirchhoff’s Current Law. This Is a  well-known law which states that the amount of current that enters inside a node is equal to the amount of current leaving outside a node. It doesn’t matter how many nodes you have, what goes in, must come out.

In the image below we have two currents entering a node, and three currents leaving the node. According to Kirchhoff’s Current Law, the relationship between the current entering and exiting the nodes can be represented as:

Image 4: Kirchhoff’s Current Law. (Image source)

The Second Law of Thermodynamics

The Second Law of Thermodynamics which is also known as the Law of Increased Entropy, It can be said that over time the state of  the entropy or disorganization in a system will always increase. What do we mean by this? Take one example – In the office why does the workers desk always get messier as the week progresses? Or more importantly, why doesn’t the worker's office go from messy to clean without you having to do work on it? This is the arrow of time in thermodynamics. As time increases, so too does disorganization.

This phenomenon happens in any system. Over The period of time, the usable energy will eventually give its way to unusable energy. While energy cannot be created or destroyed according to the First Law, it can change from a useful state to a less-useful state, like thermal energy (heat).

Image 5: Over time, every system moves from a state of low to high entropy. 

(Image source)

As we discuss above in our light bulb example, the more time we leave our light bulb on, electrical energy gets converted into radiant energy, it results in the more usable energy we continue to convert into unusable energy in the form of heat. As usable energy within a system decreases and unusable energy increases, then we say that the entropy of a system has increased. Stated mathematically:

Here, the total entropy ΔSuniverse within the universe equals the total entropy within a system ΔSsys plus the total energy within all surroundings ΔSsurr, all of which cannot be less than 0. Why? It is because at all times, at all hours in the day, all energy is being transformed from one form into the another form, and one of those forms of energy is unusable energy. Driving in your car uses mechanical energy to produce the kinetic energy of motion, but in the process, you also transform a ton of energy into heat. It’s an inevitable byproduct.

There is one another way to think about entropy which is with probabilities. For an example take a box which is filled with puzzle pieces. What’s the probability that you dump all of the puzzle pieces out of the box, and one of the pieces randomly lands where it connects perfectly with another piece? It’s a low probability. In that same box, what’s the probability of a piece landing randomly where it doesn’t fit with another piece? It’s a high probability.

Image 6: Total chaos! Entropy gets the upper hand with probability. (Image source)

In this puzzle example, the randomly placed puzzle piece represents a higher form of disorder or entropy. This is why tires release air when punctured, or why ice cubes left out at room temperature eventually melt, or why the electrons in a circuit flow from negative to positive. Sure, it could be possible for all of these actions to occur in reverse, but the probability of them occurring is so low, and the cards of increasing probability are stacked so high, that they simply never occur.

In electronics, we see the Second Law of Thermodynamics at work with the Seebeck Effect. This phenomenon occurs when heat is applied to one of two conductors, which causes heated electrons to flow toward the cooler conductor. If you connect this pair of heated conductors together in a circuit, then the heating effect will cause a direct current (DC) to flow through the circuit. In this situation, we have electrons in a lower state of entropy in a cold conductor reaching a higher state of entropy through the application of heat, and so disorder increases.

Image 7: The Seebeck Effect uses heat to generate a direct current. (Image source)

The Third Law of Thermodynamics

The Third Law of Thermodynamics says that a perfect crystalline structure at absolute zero temperatures will have zero disorder or entropy. However, if there is even the smallest hint of imperfection in this crystalline structure, then there will also be a minimal amount of entropy. This law gets a little strange though, because even at zero Kelvin there is still some atomic movement happening, so it’s a bit theoretical. Regardless, this law allows us to understand that as the entropy of a system approaches a temperature of absolute zero, the entropy present within a system decreases.

Image 8: The Third Law of Thermodynamics. (Image source)

The Zeroth Law of Thermodynamics

The Zeroth Law of Thermodynamics says that if two systems are in thermal equilibrium with a third system, then the first two systems are also in thermal equilibrium with one another. Using our good old Transitive Property of Equality:

• If System A is in balance with System C

• And System B is in balance with System C

• Then System A and System B are also in balance with each other.

This law allows you to define the direction of heat flow between systems. If you know the temperature of a set of connected systems, then you’ll know which direction heat will travel based on the fundamentals of thermal equilibrium.

Image 9: Thermal equilibrium established between systems. (Image source)

Note that while we’re covering the Zeroth Law last, it actually comes first. In the 18th century when the Laws of Thermodynamics were defined, only the first three were included. However, scientists realized that they needed a fourth law that defined the movement of temperature. Rather than renumber all of the existing laws and add confusion to existing literature, English scientist Robert Fowler came up with the name Zeroth Law.

Who Discovered These Laws?

The Laws of Thermodynamics were not discovered by one person. The development dates back as far as the 1600s when the basic idea of heat and temperature were first being formulated. In 1824, French physicist Sadi Carnot was the first to define the basic principles of thermodynamics in his discussions on the efficiency of an ideal machine. Sadi originally used the caloric system for describing the heat that is lost during the motion of an engine, which was later replaced with entropy in the Second Law of Thermodynamics.

Image 10: The Father of Thermodynamics, Sadi Carnot. (Image source)

In 1850, German physicist Rudolf Clausius developed the Clausius statement, which said that “Heat generally cannot flow spontaneously from a material at a lower temperature to a material at a higher temperature.” Around the same time, William Thomson (Lord Kelvin), developed the Kelvin statement which said that “It is impossible to convert heat completely in a cyclic process [without losing energy].” Both of these statements went on to form the foundation of the First and Second Laws of Thermodynamics. The Third Law of Thermodynamics was later developed by German Chemist Walther Nernst and is often referred to as Nernst’s Theorem.

Image 11: Lord Kelvin, one of the great minds behind the Laws of Thermodynamics. (Image source)

Thank You for reading ! 😇