Introduction
Thermodynamics deals with the transfer of energy from one form to another. You might be sitting in an air-conditioned room, or traveling in a vehicle, and the laws of thermodynamics are always applicable. Some of the most fundamental physical parameters such as heat, work, energy, temperature, and entropy, which characterize a thermodynamic system, are defined by these laws. Although the three laws of thermodynamics are quite well known and well established, there’s a fourth one that holds up all the other three laws, called— the zeroth law of thermodynamics.
Amongst many ways to state the zeroth law, the simplest way is that: systems in thermal equilibrium exist at equal temperatures. When you dip a mercury thermometer into a cup of boiling water (say, cup A), the mercury inside it gets warmed up until the thermometer reads 100℃. The thermometer is then said to be in thermal equilibrium with cup A. If we transfer the same thermometer to another cup of boiling water (say, cup), it will continue to read 100°C, meaning it is in thermal equilibrium with cup B also. According to the zeroth law, you can conclude that both cups A and B are in thermal equilibrium with each other.
What Is The Zeroth Law Of Thermodynamics?
The zeroth law of thermodynamics states that if two bodies are individually in equilibrium with a third body, then the first two bodies are also in thermal equilibrium with each other.
Zeroth Law of Thermodynamics—
the double arrow represents the resultant thermal equilibrium within each of the three systems
Consider three systems– A, B, and C. If A and C are at equal temperatures (i.e, in thermal equilibrium), and A and B are also at equal temperatures, then automatically, B and C are also in thermal equilibrium. This may seem trivial and obvious, but it is an important observation that shapes the laws of thermodynamics. It means that, if two objects are at the same temperature, there will be no heat flow between them when they are in contact. The zeroth law established as such, the transitive property (A=B=C) of thermodynamics.
Systems are said to be in “thermal equilibrium'' if they cease to transfer heat between each other even if they are in a position to do so. For a practical instance, why do you think your hot cup of beverage does not remain hot long enough? It is because the coffee in your cup continues to lose its heat to the surrounding cool air until it reaches a state of equilibrium with your room’s temperature. We will discuss thermal equilibrium further in this article.
Why Is It Called The “Zeroth” Law?
Sir Ralph Howard Fowler
The zeroth law of thermodynamics has established the role of temperature as a measurable property of matter.
It was observed that the physical properties of matter can change with temperature. Liquids can expand or contract when heated or cooled or the conductivity of different wires may change as a function of temperature. Old fashioned thermometers use the expansion of mercury or other liquids for the measurement of temperature. Modern electronic thermometers commonly use thermoreceptors, where the resistance of the material changes with a change in temperature.
When the zeroth law was originally conceived in the 1800s, the other laws of thermodynamics already existed. But this new law presented a more superior and pivotal approach to defining temperature as a quantity. This is how the zeroth law rightly earned its place before the other two existing laws. Having considered that renumbering or reassigning the laws would cause a great deal of confusion, Ralph H. Fowler came up with a solution to the dilemma. He called the new law the “zeroth law” of thermodynamics.
Thermal Equilibrium
Thermal Equilibrium—Two bodies at different temperatures when in contact, approaches the same temperature
Thermal Equilibrium—Two bodies at different temperatures when in contact, approaches the same temperature
Like various other concepts in physics, the concept of thermal equilibrium is also quite intuitively understandable. We all know what happens to a hot cup of water when refrigerated—it cools down! We probably also know that the water continues to cool down till it freezes and it cannot get any colder. This is when the temperature of the water has become the same as the temperature of the air in the freezer. This state is called thermal equilibrium and can be defined as a state in which two bodies in thermal contact do not have any heat transfer between them.
In other words, heat will flow from one system to another only as long as there is a temperature difference between them. When heat transfer ceases to occur, the two systems are said to be in thermal equilibrium with each other.
Zeroth Law And The Definition Of Temperature
Temperature is the measure of heat in a body. Here, temperature (T) is considered to be a thermodynamic variable whose value remains constant for two systems in thermal equilibrium.
Let us consider temperature of body A is greater than body B
Let us consider the temperature of body A is greater than body B
So, if T(A)=T(B) and T(A)=T(C), then T(B)=T(C)
Consider two bodies A and B kept side by side, are separated by an adiabatic wall, so no energy is exchanged between them. The wall separating the two bodies from the third is made up of a conducting or a diathermic material.
Assume that the temperature of body A exceeds body B [T(A)>T(B)]. Then it is observed that body A begins to lose heat to the colder body B.
At the moment, the two bodies are not in thermal equilibrium; what happens instead is that molecules of body A vibrate more than body B due to its high kinetic energy. When the molecules of the two bodies interact, energy is transferred from the higher potential to the lower potential of energy.
Eventually, both their kinetic energies equalise, temperatures equalise, indicating that heat transmission has now ceased to occur.
No heat is transferred between two objects at thermal equilibrium. Therefore they are at the same temperature.
No heat is transferred between two objects at thermal equilibrium. Therefore they are at the same temperature.
Again, let us assume that we want to measure the temperature of objects A and B, placed in two different geographical locations. Here enters a third object C, which is a standard mercury thermometer. At both locations, the height of the rise in mercury was measured and found to be equal for both objects A and B. The expansion of mercury in the thermometric device is a representation of the thermal equilibrium reached between the object and the thermometer itself. Therefore, it is possible to assume that the two objects (A and B), physically distant from each other, are indeed in thermal equilibrium with each other.
In fact, as soon as they are not in equilibrium anymore, the zeroth law becomes inapplicable and the second law takes over– according to which heat flows in the direction of increasing entropy, when exiting the hotter object. This is why the zeroth law is the basis for all accurate temperature measurements.
Applications Of The Zeroth Law
The thermometer is one of the most common applicators of the zeroth law of thermodynamics. If one desires to measure the degree of hotness or coolness of a substance accurately, a third reference body and a variable feature (w.r.t temperature) of that body is required. A simple mercury thermometer is THAT reference body, which operates on the principle of liquid expansion with an increase in temperature.
If the thermostat in your room shows 72℉, it means that the thermostat is in thermal equilibrium with the air in your room. Automatically, all other objects (furniture, books, etc) are also at 72℉ according to the Zeroth law of thermodynamics.
All the food in your refrigerator kept overnight, will achieve the same lower temperature as the air inside the fridge. Hence, we can say that all the vegetables in your refrigerator are in thermal equilibrium with each other.
The temperature sensor in your refrigerator and air conditioners, that makes it stop and start at intervals, also works on the basis of the zeroth law of thermodynamics.
In food preservation, pyrometers are used to measure the temperature of substances by subjecting them to infrared radiation. Such pyrometers also work on the basis of the zeroth law of thermodynamics.
Some physicians recommend consuming hot soups and beverages on a hot summer day instead of a cold ice cream sundae. However torturous that may sound, it is backed up scientifically. Consumption of hot food will increase your core body temperature, and will result in your body trying to cool down fast, by losing heat in the form of sweat. The process of regulation and maintenance of your body temperature (homeostasis) also, quite inevitably, follows the laws of thermodynamics.
Significance Of The Zeroth Law
Although it might seem trivial, or even obvious, the zeroth law as mentioned already, validates the truth of the first and second laws of thermodynamics. It is required in order to define the concept of “temperature” as a property of systems in thermodynamic equilibrium. It formalizes the definition of temperature and is the basis for the construction of thermometers. In conclusion, the zeroth law helps us to actualize thermodynamics into a quantitative/measurable science in the first place.