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Applications of First Law of Thermodynamics

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  • Last Updated : 16 Dec, 2021

Energy, like matter, is always conserved, which means that it cannot be created or destroyed, but it can be converted from one form to another. Internal energy is a thermodynamic attribute of a system that refers to the energy associated with the system’s molecules and comprises both kinetic and potential energy. Whenever a system undergoes a change as a result of the interaction of heat, work, and internal energy, it is followed by a series of energy transfers and conversions. However, there is no net change in total energy throughout these exchanges. Similarly, the fundamental law of thermodynamics confirms that heat is a type of energy. This means that thermodynamic processes are guided by the concept of energy conservation. The first law of thermodynamics is often known as the Law of Energy Conservation.

First Law of Thermodynamics

A thermodynamic system in equilibrium has a state variable known as internal energy (E). The difference in internal energy between two systems is equal to the difference between heat transfer into the system and work done by the system. The energy of the universe remains constant, according to the first law of thermodynamics. It can be exchanged between the system and the environment, but it cannot be generated or destroyed. The law is primarily concerned with changes in energy states caused by work and heat transmission. It reimagines the concept of energy conservation.

Heat is a kind of energy, according to the First Law of Thermodynamics, hence thermodynamic processes are governed by the concept of energy conservation. As a result, neither heat nor cold energy can be created or destroyed. It may, however, be moved from one region to another and changed into and out of other types of energy.

The equation for the first law of thermodynamics is given as;

ΔU = q + W

where: ΔU is the change in the internal energy of the system, q is the algebraic sum of heat transfer between system and surroundings, W is the work interaction of the system with its surroundings.

  1. Energy (E) is always constant in an isolated system.
  2. Internal Energy is a system property and a point function. Internal energy is a broad (mass-dependent) characteristic, whereas specific energy is a narrow (mass-independent) attribute (independent of mass).
  3. The internal energy of an ideal gas is just a function of temperature.

Significance of First Law of Thermodynamics: The first law of thermodynamics is founded on the idea of energy conservation. This indicates that energy cannot be generated or destroyed, but it can shift into different forms with no loss of energy. When a system transitions from one state to another, both dQ and dW are affected by the nature of the process. dU, on the other hand, is the same for all operations. 

Limitations of First Law of Thermodynamics 

  • When a system goes through a thermodynamic process, it must always maintain a precise energy balance, according to the law. The first law, on the other hand, fails to provide the feasibility of the process or change of state that the system goes through. 
  • The first law, for example, does not explain why heat transfers from the hot end to the cold end when a metallic rod is heated at one end but not the other, and vice versa. 
  • The first law only quantifies the amount of energy transferred during this process. The second law of thermodynamics serves as a benchmark for the feasibility of various processes.

First law of Thermodynamics for a Closed System

Work done in a closed system is the product of pressure applied and volume change caused by applied pressure.

W = − P ΔV

Where P is the constant external pressure on the system, and ΔV is the change in volume of the system. This is specifically called pressure-volume work.

The internal energy of a system rises or falls in response to work contact that occurs across its limits. Internal energy increases when work is performed on the system and decreases when work is performed by the system. Any heat interaction that occurs in the system with its surroundings modifies the system’s internal energy. However, because energy is constant (as stated by the first law of thermodynamics), the total change in internal energy is always zero. If the system loses energy, it is absorbed by the environment. When energy is absorbed by a system, it means that the energy was released by the environment:

ΔUsystem = −ΔUsurroundings

Where ΔUsystem is the change in the total internal energy of the system, and ΔUsurroundings is the change in the total energy of the surrounding.

Applications of First Law of Thermodynamics

  • Isothermal process: The temperature of an ideal gas remains constant during an isothermal process. This means that the heat supplied to the system is utilized to do work against the environment. So,

dQ = dU + dW 

⇒ dQ = dW

  • Melting process: When a solid melts to liquid, its internal energy increases. Let m = mass of liquid and L = latent heat of the solid. Amount of heat absorbed by the system, dQ = mL.

A small amount of expansion occurs, i.e., ΔV = 0 

⇒ dW = PΔV = 0 


dQ = dU + dW 

⇒ dU = mL

Thus, internal energy increases during the melting process.

  • Heat engines: 

The heat engine is the most common practical application of the First Law. Thermal energy is converted into mechanical energy via heat engines and vice versa. The vast majority of heat engines are open systems. A heat engine’s basic idea makes use of the correlations between heat, volume, and pressure of a working fluid. This fluid is normally a gas, however, it may transition from gas to liquid and back to gas during a cycle in some instances.

When a gas is heated, it expands; nevertheless, when the same gas is confined, its pressure rises. If the confinement chamber’s bottom wall is the top of a moving piston, this pressure exerts a force on the piston’s surface, causing it to travel downward. This movement can then be used to provide work equal to the total force applied to the top of the piston multiplied by the distance traveled by the piston.  

Refrigerators, air conditioners, and heat pumps

Refrigerators and heat pumps are mechanical energy converters that convert mechanical energy to heat. The majority of these are classified as closed systems. When a gas is compressed, its temperature rises. This hot gas can then radiate heat into its surroundings. When the compressed gas is allowed to expand, its temperature drops below what it was before compression because some of its heat energy was removed during the hot cycle. After then, the cold gas can absorb heat energy from its surroundings. This is the operating principle of an air conditioner. Air conditioners do not generate cold; rather, they remove heat. A mechanical pump transports the working fluid outside, where it is compressed and heated. The heat is then transferred to the outside environment, typically via an air-cooled heat exchanger. Then it is delivered indoors to expand and cool before taking heat from the internal air via another heat exchanger.

A heat pump is basically a reverse-cycle air conditioner. The compressed working fluid’s heat is used to warm the building. It is then moved outdoors, where it expands and cools, allowing it to absorb heat from the outside air, which is normally warmer than the chilly working fluid even in winter. 

Sample Questions 

Question 1: What is the enthalpy of formation of the most stable form of an element in its standard state?


The enthalpy of formation of an element’s most stable form in its standard state is zero.

Question 2: State Hess’s law.


According to Hess’s law, the change in enthalpy of a reaction remains constant whether the reaction is carried out in one step or numerous steps.

Question 3: Define the first law of thermodynamics.


Energy cannot be created or destroyed, according to the first law of thermodynamics. The energy of an isolated system remains constant.

Question 4: Many thermodynamically feasible reactions do not occur under ordinary conditions. Why?


The average energy of the reactants may be less than the threshold energy under normal conditions. To start the reaction, they need some activation energy.

Question 5: Why is the first law of thermodynamics important to the environment?


All living species on Earth rely solely on the sun for energy. These energies received by plants are not returned to the solar system, but are instead passed on to herbivores that eat green vegetation. When herbivores die, some of the energy acquired by them is used by carnivores or transmitted to decomposers.

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