Law of Thermodynamics: Definition, Process, Formula, Formulation, and Its Application

Law of Thermodynamics – Sinaumed’s must have known that heat energy can move from objects with high temperatures to objects with low temperatures. Well, the transfer process can be studied in the Science of Thermodynamics. In general, thermodynamics is a science that studies energy which specifically discusses the relationship between heat energy and its work processes. Even the technology that we often use today, call it AC ( Air Conditioner ) and rice cooker , is also a concrete form of applying the Laws of Thermodynamics.

The existence of the Laws of Thermodynamics is an important part of physics and has three legal studies. Then, what is thermodynamics ? How does this law of thermodynamics sound? When it comes to physics, does the Law of Thermodynamics then have its own formula? How does the law of thermodynamics apply to rice cooker technology? So, so that Sinaumed’s understands these things, let’s look at the following review!

Definition of Thermodynamics

Basically, thermodynamics is a branch of physical science and engineering. If in the field of science, experts will try to study the basic behavior of the physical and chemical properties of a number of materials in a state of rest (at rest) by using this thermodynamic principle. While in engineering, experts (engineers) will usually use the principles of thermodynamics to study systems and their interactions with the environment. Then, what is the definition of thermodynamics so that its working principle can be used in two different branches of science?

Thermodynamics is the science of energy, which specifically discusses the relationship between heat energy and how it works. This energy can change from one form to another, either naturally or through technological engineering. The workings of most technological systems can be explained by means of thermodynamics. In fact, it is often mentioned that thermodynamics is the main capital of an engineering graduate to design thermal pumps, rocket motors, rice cookers, air conditioners, to chemical distillers.

In short, thermodynamics is a branch of theoretical physics concerned with the laws of the movement of heat and the transformation of heat into other forms of energy. The term thermodynamics comes from the Greek, namely ” Therme ” which means ‘ heat ‘ and ” dynamis ” which means ‘ force ‘. The existence of this thermodynamics will not be separated from heat.

What Are Calories?

Heat (Q) is energy that is transferred from one object to another due to a temperature difference. When it is related to the system and the environment, it can be said that heat is energy that is transferred from the system to the environment or energy that is transferred from the environment to the system due to a difference in temperature. If the system temperature is higher than the ambient temperature, then heat will flow from the system to the surroundings. Conversely, if the ambient temperature is higher than the system temperature, then heat will flow from the environment to the system.

Well, if the existence of Heat (Q) is related to the transfer of energy due to differences in temperature, then Work (W) is related to the transfer of energy that occurs through mechanical means (mechanical related to motion). For example, if the system does work on the environment, energy will automatically move from the system to the environment. Conversely, if the environment does work on the system, energy will move from the environment to the system.

Thermodynamic System

In a thermodynamic system, it has certain terms, namely:

  • System Boundary is an imaginary line that limits the system to its environment.
  • A closed system is when the system and its environment do not exchange energy or mass, in other words, energy or mass does not cross the boundaries of the system.
  • Open system is when energy and mass can cross or pass the system boundaries. System with its environment there is interaction.

What Are the Sounds of the Laws of Thermodynamics?

The existence of this Law of Thermodynamics actually has three forms, all of which originate from the same foundation, namely the Initial Law or the Zeroth Law. In the Initial Law of Thermodynamics it states that: “If two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other”. Well, here is the explanation of the Laws of Thermodynamics I, II, and III.

Law of Thermodynamics I

The First Law of Thermodynamics states that “Energy cannot be created or destroyed, but can only be changed in form.” As it sounds, the energy provided by heat must be the same as the external work done, plus the internal energy gain due to an increase in temperature. Indirectly, the First Law of Thermodynamics is related to the conservation of energy.

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If heat is given to the system, then the volume and temperature of the system will certainly increase (shown by expanding and increasing the heat of the system). Conversely, if heat is taken from the system, the volume and temperature of the system will decrease (it can be seen that the system will shrink and feel colder). This principle is a natural law and the form of the law of the conservation of energy is in line with the First Law of Thermodynamics. A system that has undergone a change in volume will later do work. Meanwhile, a system that experiences a change in temperature will tend to experience a change in internal energy. So, the presence of heat given to the system can cause the system to do work and experience a change in internal energy.

Processes in Thermodynamics I

In the Law of Thermodynamics I will experience 4 processes, namely:

1. Isothermal Process (Constant Temperature)

A system can undergo a thermodynamic process, where changes occur within the system. A thermodynamic process that takes place mainly at constant temperature is called an isothermal process. Since the process takes place at a constant temperature, there is no change in internal energy. This isothermal process can be proven in everyday activities, for example popcorn in a pot.

Well, if you refer to the Law of Thermodynamics I, then the heat supplied will be equal to the work done by the system (Q = W). Please note that this process can also be enforced by Boyle’s Law, namely:

Well, since the temperature is constant, in this isothermal process there will be no change in energy ∆U=O. While the business can still be calculated from the area under the curve, with the formula:

2. Isochoric Process (Constant Volume)

When a gas carries out a thermodynamic process in a constant volume, it is in an isochoric process. This is because the gas is in a constant volume (∆ V=0), so the gas does not do work (W=0) and the heat supplied will also be equal to the change in energy inside. The heat in this process can be expressed as the heat of gas at constant volume Q V . This process has a formula in the form of:

W = P dV = P.0 = 0

While the graph of an isochoric process will form:

3. Isobaric Process (Constant Pressure)

When a gas undergoes a thermodynamic process to keep the pressure constant, the gas is undergoing an isobaric process. An example of applying this isobaric process is boiling water at constant pressure. This is because the gas is under constant pressure, while the gas is doing work (( W = pV ). The existence of heat in this process is expressed as the heat of the gas at constant pressure ( Q p ). Well, if this isobaric process, if it is based on the Laws of Thermodynamics I, then the formula will apply:

While the gas work graph in an isobaric process can be expressed as:

4. Adiabatic Process (Fixed Heat)

An adiabatic process is a thermodynamic process in which pure gases work by changing their internal energy. No energy enters or leaves during this process. An example of applying this adiabatic process is the use of a motorcycle pump. If based on the Law of Thermodynamics I, it will be: the change in the internal energy of the gas (dU) is the amount of heat energy supplied (Q) minus the work done by the gas (P.dV). If Sinaumed’s is confused by this description, here is the formula in brief:

dU = Q – P.dV = – P dV

PV ƴ = K (constant)

While the gas work graph in an adiabatic process can be expressed as:

Formula of the Law of Thermodynamics I 


Q = heat/heat received/released (J)
W = energy/work (J)
∆U = change in energy (J)

Second Law of Thermodynamics

In the Second Law of Thermodynamics it is related to entropy and the tendency over time, differences in temperature, pressure, and chemical potentials balance in isolated physical systems. Please note , Sinaumed’s , entropy is a thermodynamic balance, especially regarding energy changes whose law is called the Second Law of Thermodynamics. In the Law of Thermodynamics II it states that: “Heat flows spontaneously from high temperature objects to low temperature objects and does not flow spontaneously in the opposite direction.”

In fact, the First Law of Thermodynamics is considered unable to explain whether a process may or may not occur. Therefore, the Second Law of Thermodynamics emerged which was compiled inseparable from efforts to find the nature or magnitude of the existing system.

From the experimental results, the experts concluded that it is impossible to build a heat engine that converts heat entirely into work, namely a machine with 100% thermal efficiency. This impossibility is the basis of one statement of the second law of thermodynamics as follows:

“It is impossible for any system to undergo a process in which the system absorbs heat from a reservoir at a single temperature and converts the heat entirely to mechanical work, with the system ending up in the same state as it started.”

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Formulation in the Law of Thermodynamics II

In the Second Law of Thermodynamics there are two formulations that are useful for understanding the conversion of heat energy to mechanical energy, namely:

1. Kelvin-Planck formulation

This first formulation states that ” It is impossible to make a heat engine that works in a cycle that simply converts the heat energy obtained from a source at a certain temperature entirely into mechanical work”. In other words, this formulation reveals that there really is no way to extract thermal energy from the oceans. So it is better to use this energy to run electric generators without causing further effects, such as warming the atmosphere. Therefore, every tool or machine must have a certain efficiency value. This efficiency will state the comparison value of the mechanical effort that has been obtained with the heat energy from the highest temperature source.

2. Clausius Formulation

In this formulation it states that “It is impossible to make a heat engine that works in a cycle that solely transfers heat energy from a cold object to a hot object” . In other words, one cannot take energy from a cold source (which has a low temperature) and transfer it all to a hot source (which has a high temperature) without giving energy to the pump to do work.

Law of Thermodynamics III

In the Law of Thermodynamics III it relates to absolute zero temperature. This law also states that when a system reaches absolute zero temperature, all processes will stop and the entropy of the system will approach the minimum value”. 

Application of the Laws of Thermodynamics in Rice Cookers

The law of thermodynamics can be applied to technology that helps people with daily activities, one of which is the rice cooker. A rice cooker is a rice cooker which can also be used to boil vegetables, steam gravy, and so on. Of course rice cookers use electricity, starting from 300 watts, 500 watts, 800 watts, and so on. In a rice cooker, it has heating elements located on the bottom, sides and top.

The working principle of a rice cooker is that when the switch is connected to the main heating element, the electric current will go directly to the main element and the lights on the rice cooker will turn on. When the heating temperature has reached its maximum and the rice is cooked, the trip thermostat will immediately move the lever so that the switch position changes to flow electricity to the rice heating element through the thermostat.

In the warmer position, when the thermostat temperature is maximum, the current leading to the heating element will be cut off automatically. Likewise, when the temperature on the thermostat decreases, the current to the heating element will automatically be reconnected automatically. This process will take place continuously. If it is based on the Law of Thermodynamics, then how the rice cooker works will be in the form of:

“Rice, which was originally rice and has a hard texture, when it is given water and heats it, the texture will turn soft and easy to eat. Well, a liquid will evaporate if the vapor pressure of the gas coming from the liquid is the same as the pressure from the liquid to the surroundings (P vapor = P liquid). So, the boiling point of a liquid can actually be manipulated by increasing the pressure outside the liquid (external pressure).

In an ordinary rice cooker, water will boil with the usual external pressure, which is 101 kPa and boil at the normal boiling point, which is 100°C (373 K). Meanwhile, in a rice cooker that manipulates pressure ( pressure cooker , or electric pressure cooker ) if the steam vent lid is opened, the pressure cooker will work like a normal rice cooker, because the external pressure is the same as the outside air pressure.

However, if the steam vent cover (usually a valve) is closed, there will be a change in the air pressure in the chamber inside the pressure cooker and the boiling point of the liquid will change. Unlike when the valve is closed, the condition of the system changes because the water vapor can only be in the pressure cooker chamber . Since there is additional mass, the pressure will be higher and the equilibrium point between the phases (in this case, between the liquid phase and the vapor phase) changes to a higher temperature, and a new boiling point is formed.

The following are the components in a rice cooker:

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