The 4 Laws of Thermodynamics

Hey, everyone! Welcome to this Mometrix video over the four laws of thermodynamics.

Thermodynamics is a branch of physical science (or physics) that focuses on the correlation of heat and all other forms of energy, and, consequently, how all forms of energy relate to one another.

Well, the four laws of thermodynamics work to define the foundational physical quantities that mark thermodynamic processes when at thermal equilibrium. These foundational physical quantities include temperature, energy, and entropy.

Now, let’s take a look at how these laws are defined.

Zeroth Law of Thermodynamics

The zeroth law actually came after the first three laws of thermodynamics, but it was actually so fundamental and provided a basis for the other laws that it got the name zeroth. That way people knew the order in which they should learn or think through the laws.

So, what does the zeroth law actually state? Let’s take a look. Let’s say that you have two systems, A and B, that are connected to each other with some sort of barrier between them. Well, the heat or energy of A will transfer over to B, or vise versa, the energy of B will transfer over to A. This is assuming that the wall is diathermic, meaning that heat can be exchanged, as opposed to an adiabatic wall which would not allow for heat to pass.

Once heat or energy is transferred between two systems, such that there is no more heat being transferred (or the transference of heat stops), then the systems have reached equilibrium (more specifically thermal equilibrium since we are talking about heat).

Now, let’s say that we have another system, A and C, and they also reach equilibrium.

Well, since A is in equilibrium with B, and A is in equilibrium with C, then when you take B and C and with them together with a diathermic wall, then there will be no transference of heat. This is because the two systems are already at equilibrium together.

To say this mathematically: if \(A = B\) and \(A=C\), then \(B=C\).

That is the zeroth law of thermodynamics. For some of you this may seem like common sense, to others it may seem counterintuitive, especially if you are super familiar with Boyle’s law.

Regardless, this what the zeroth law shows us, but it gives us much more than that. The illustration you just saw tells us something. What is that? What is that physical quantity which determines that the two systems will reach equilibrium, and the transference of heat will stop from one system to the other? Well, it’s this very physical quantity that was discovered later.

There were already two laws of thermodynamics in place, the first law and the second law. Well, in these two laws the term temperature was used, which was thought of as the degree of hotness, basically. While the term “temperature” was being used in the first and second law, no one could actually define it. There wasn’t really a way to define temperature within thermodynamics. So, the zeroth law was made, so that temperature could be defined.

So, how is temperature defined?

Temperature Definition

If there are two systems in contact with each other, then the direction of heat flow tells us that one body is at a higher temperature, and the receiving body is at a lower temperature. So, temperature is the physical quantity which determines the flow or direction of heat. So, temperature decides if heat will flow from A to B, or from B to A.

Let’s review that real quick. First, the zeroth law says this, “If two systems C and B are in thermal equilibrium separately with system A, then C and B are also in thermal equilibrium with each other.” Secondly, the physical quantity defined in the zeroth law is temperature. And lastly, temperature can be defined as the physical quantity which determines the direction of flow of heat, and is the one that is equal in a state of equilibrium of two systems.

Now let’s take a look at the first law of thermodynamics.

First Law of Thermodynamics

Here is what the first law of thermodynamics states: heat energy given to a system is converted to its internal energy and work done on that system.

To put this even simpler, the first law of thermodynamics just states that energy cannot be created or destroyed, it can only be transformed.

First Law of Thermodynamics Equation

Let’s look at that mathematically. Internal energy, represented by \(ΔU\), is equal to heat energy plus work done.

\(ΔU = q + w\)


This equation is known as the first law of thermodynamics. But, I’m guessing this still doesn’t make sense to you. I don’t blame you.

Let’s look at it this way. You have a system. Everything on the outside of the system is the surroundings. Well, energy can flow either into the system, or out of the system. There are two ways in which energy can flow in and out of a system, which is through heat and work.

So, let’s say that heat flows into the system. When heat flows into the system, then the system accumulates or gains energy. That energy is called the internal energy of the system, which is where we get our symbol U. The surroundings can do work on the system.

So, heat and work are the two ways in which a system can increase its internal energy. If the system’s surroundings were to do 150 J of work on the system, that means that the system’s internal energy increases by 150 J, and since the change is positive, \(ΔU\) goes up.

And, since we know that energy cannot be created or destroyed (only transferred), the energy had to come from somewhere—the surroundings. So, if the system’s internal energy increases, then the energy of the surroundings must decrease.

Another way to think through this might be to think through the old “if you have 5 quarters and take 1 away” example. Let’s say you were selling a sticker for $1.25, and that someone is looking to buy the sticker. Well, when someone does buy the sticker, the amount of money that you have will increase by $1.25, but the amount of money that the other person has will decrease by $1.25. The money doesn’t just come from thin air, it’s coming from somewhere, or someone in this case.

In order for the amount of money that you have to increase, someone else’s money has to decrease. Now, sure, in the case of money, the government could create more, but that doesn’t really apply on a day-to-day basis when it comes to buying and spending.

Well, energy follows this same principle. It cannot be created or destroyed, it’s simply transformed or transferred from one place to another.

Second Law of Thermodynamics

Now, the second law of thermodynamics might seem like an odd law. Why? Well, firstly because the second law of thermodynamics doesn’t tell us what can happen or what is possible, it only tells us what cannot be done or what is not possible. Another reason it might seem odd is because there are several ways to state it!

The second law of Thermodynamics states that the efficiency of a process can never be one, or 100% of any machine. If a machine or process operated with 100% efficiency, then all of the heat or energy put into a system would be completely converted into work done, and there would be no heat rejected out. The Second Law tells us that this is NOT the case. The second thing that the second law of thermodynamics states is that unless you apply a pump to a system, there is no way to send heat from a cooler source to a source at a higher temperature.

Another popular way to state this law, as put by Clausius, is, “No process is possible whose sole result is the transfer of heat from a colder object to a hotter object.”

Put very simply, the second law of thermodynamics tells us that heat will never flow from a colder object to a hotter object, unless you use a pump to do so. Like how a pump is used to cool the inside of a refrigerator. In every other case, however, heat always goes from the object at a higher temperature to the object with the lower temperature. And, the other thing it states is that within a system or machine, heat or energy will never be 100% converted into work done. So, if I put 100 J into a system, and I get more than 100 J out of the system, then one of three things happen:

  1. I lied.
  2. I miscalculated either my input or output.
  3. Or three, I just turned all of physics and science on its head and defied all of it.

Third Law of Thermodynamics

The third and last law of thermodynamics defines absolute zero and brings together the concepts of entropy and temperature from the latter laws.

The third law states that the entropy of a perfect crystal approaches zero at a temperature of absolute zero. We can’t actually achieve absolute zero experimentally, or at least you probably won’t. This is because we know from the second law that heat is spontaneously moving from a warmer body to a cooler body. So, the object that you are trying to get to absolute zero will be constantly taking in heat from its surroundings.

I said that the lowest possible entropy will only occur in a perfect crystal, well a perfect crystal is a structure where all of the atoms are identical and are positioned in perfectly symmetrical ways. If there are any imperfections in the crystal, then that imperfection carries an energy with it, which does not allow for the minimization of entropy.

Another important note is that if we can’t get to perfect zero, then there will never be complete stillness in our universe, because we will always have some sort of motion due to thermal energy.

I hope that this video over the four laws of thermodynamics was helpful to you.

See you guys next time!


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by Mometrix Test Preparation | This Page Last Updated: June 28, 2022