A Quick Guide to the 3 Laws of Thermodynamics 

The laws of thermodynamics have been around for a while – since the 19th century, in fact. However, just because they’re well-established doesn’t mean they can’t get confusing. Part of understanding these three laws is untangling them from each other and piecing the individual facts together.

Thermodynamics is a branch of science that involves relationships between heat and all forms of energy, such as mechanical, electrical and chemical. It’s a physical concept, so it deals with tangible systems or machines. With this information acting as a foundation, researchers and scientists began to develop the three laws we know today.

The Emergence of Thermodynamic Laws

During the Industrial Revolution, the laws of thermodynamics started to emerge. There was the notion that a machine should be able to function off of its own heat, providing a means of operation for an indefinite amount of time.

Engineers later ruled that idea out due to the nature of a working machine. There will always be heat loss or energy conversion during any process. From there, the laws of thermodynamics slowly came into play and helped experts understand the properties of energy and heat.

The First Law of Thermodynamics

The first law of thermodynamics focuses on the nature of energy and heat. It is perhaps the most common law – you may hear people refer to it or remember it from school. It states that energy cannot be created nor destroyed. Even on a subatomic particle level, this law remains. However, you can conserve or transfer energy. This concept led scientists to develop a simple equation, E2 – E1 = Q – W.

E represents the internal energy of a system. Q represents the transfer of heat into a system, and W is the work the system executes. This equation represents how the change in the internal energy of a system equals the difference in heat transfer and work. Where Q and W depend on the system and process, E does not. Instead, it acts as the internal energy, which is something like kinetic or potential energy. Therefore, a system can store or change this energy depending on the process.

As part of this law, the idea that heat flow depends on the stages of a process is essential. This concept may seem obvious, but it can get more complicated. The initial and final states, as well as the processes, influence the amount of heat flow, and thus the outcome of the final state.

For instance, replace the equation with parts of an engine. The first law proves that the output of work from the engine added to the change in internal energy will equal its heat input.

The Second Law of Thermodynamics

The second law of thermodynamics complicates matters of the first by adding entropy, the degree of disorder and randomness in a system. It represents the unavailable or unusable energy within a system, and as it increases, usable energy decreases.

To help better understand how entropy operates within thermodynamic systems, scientists developed another equation, ∆S = ∆Q/T. Here, Q again represents the heat transfer into a system. T represents the temperature, and S represents the level of entropy.

Entropy involves the breakdown of a system, which is a natural occurrence and part of why they can’t run indefinitely. Therefore, energy will, at some point, dissipate. Particles will continuously want to expand, but entropy will eventually take over.

The second law takes the example of the engine from the first law and adds entropy into it. Yet what does that look like? First, keep in mind that work output can never overtake energy input. For instance, the engine cannot transfer 100% of its energy into work output, because some will become heat and sound. Next, entropy is present in every system, so as energy transfers, a portion becomes unusable. This idea is the root of entropy, which will eventually spread more and more.

To what extent can entropy slow down movement or temperature? The third law expands on that idea.

The Third Law of Thermodynamics

The third law of thermodynamics incorporates the idea of absolute zero. Absolute zero is the lowest temperature theoretically possible where particle motion becomes the slowest it can possibly be. For perspective, absolute zero is zero Kelvin. It’s equivalent to -459.67˚F or -273.15˚C. If that seems unbelievably cold to you, you’re right, because absolute zero is impossible to achieve. At this temperature, no heat energy remains in a system.

Because entropy is always increasing, nothing can reach absolute zero. Energy cannot be created nor destroyed and is always stored or transferred. At absolute zero, entropy would have to reach zero as well. The equation that best represents this law incorporates many of the elements from the other laws, but differently, limT -> 0, S = 0.

This equation may seem more complicated, but the variables remain the same. T represents the temperature, and S represents the level of entropy. Yet what exactly does it mean? As the temperature reaches zero, entropy would have to equal zero as well. Again, however, absolute zero is impossible, and unfortunately, will remain that way.

All matter – solids, liquids and gasses – are always in motion at a molecular level. Absolute zero would require those molecules and atoms to reach zero, too. As entropy continuously expands, though, absolute zero is impossible.

The Zeroth Law of Thermodynamics

Another less common law of thermodynamics is the zeroth one. This law is simpler and more consolidated than the others and is more of a rule. It states that if A=B and A=C, then B=C. In other words, if system B equals system A and system C also equals system A, then systems B and C will equal each other as well.

This law helped scientists identify temperature in equations. As a result, if the temperature of A is equal to B and equal to C, then B and C must also have the same temperature when nearby and thermal equilibrium with each other. Two systems are in thermal equilibrium when they exchange no heat, despite being in contact with each other.

Because of their contact, they no longer affect the temperature of the other. Think of adding milk to a cup of coffee. The two are in thermal equilibrium with each other and will eventually be the sample temperature. If a third system comes into play, it will also become the same temperature as the other two. Thus, this law helped researchers understand and derive the variable for temperature when putting it into an equation.

Importance of Thermodynamics

Thermodynamics and its laws have a significant bearing in chemistry, physics and engineering. They help with things like developing proper systems for energy use, understanding chemical processes, natural reactions, temperature conversions and more. They also help with everyday life. Refrigeration, air conditioning, boilers, radiation and more are all based on thermodynamics and heat transfer properties. Smaller things, too, like laptops and tablets, use heat and energy laws.

Essentially, thermodynamics serves as a basis for everything. It involves atoms, particles, energy and entropy, which are part of every tangible thing. Without these laws, scientists wouldn’t understand how energy and heat work and how they make up systems, things, people and everything else.