## First Law of Thermodynamics- Class 11 | Chapter – 12 | Physics Short Notes Series PDF for NEET & JEE

First Law of Thermodynamics: The First Law of Thermodynamics is also known as the law of conservation of energy. It states that energy cannot be created or destroyed, only transferred or converted from one form to another. In other words, the total amount of energy in a system and its surroundings remains constant.

This law has significant implications for the study of thermodynamics. It means that any energy added to a system must come from somewhere, and any energy lost by a system must go somewhere else. For example, when a gas is heated, it absorbs thermal energy from its surroundings and increases its internal energy. Similarly, when a gas is compressed, work is done on the system, and its internal energy increases.

## Formula of First Law of Thermodynamics

The equation for the first law of thermodynamics can be expressed as:

⇒ ΔU = q + W

Here,

• ΔU is the Change in the internal energy of the system.
• q is the Algebraic sum of heat transfer between the system and the surrounding
• W is the Work interaction of the system with the surrounding

The first law of thermodynamics talks about how energy can only be exchanged, it cannot be created nor can it be destroyed. Hence, the universe’s total energy always remains constant.

## Limitations of First Law of Thermodynamics

While the First Law of Thermodynamics is a fundamental principle of nature and has broad applicability, it has some limitations and assumptions that may not always hold true in practice. Here are some of the limitations of the First Law of Thermodynamics:

• It does not provide any information about the direction or reversibility of a process. The first law only tells us that energy is conserved, not whether a process can occur spontaneously or how efficient it is.
• It assumes that energy can be completely converted from one form to another without any losses. In reality, all energy conversions involve some degree of loss, usually in the form of heat, which can limit the efficiency of a process.
• It assumes that the system is closed and isolated, meaning that no energy or matter can enter or leave the system. In practice, it is often difficult to maintain a completely closed system, and energy or matter can be exchanged between the system and its surroundings.
• It assumes that the internal energy of the system is well-defined and measurable. In reality, the internal energy of a system is a complex combination of many different types of energy, such as kinetic, potential, and chemical energy, and measuring it accurately can be difficult.
• It assumes that the system is in thermal and mechanical equilibrium, meaning that the temperature and pressure are the same throughout the system. In practice, many systems may not be in perfect equilibrium, and this can affect the accuracy of the first law.

Overall, while the First Law of Thermodynamics is a powerful and widely applicable principle, it is important to recognize its limitations and make appropriate assumptions when applying it to specific physical systems.

## Perpetual Motion Machine of First Kind (PMM1)

The Perpetual Motion Machine of the First Kind (PMM1), also known as a “perpetual motion machine of the first class,” is a hypothetical machine that violates the First Law of Thermodynamics by producing energy without any input of energy from an external source. In other words, it would create an output of energy that is greater than the input of energy required to run the machine, allowing it to run indefinitely without ever stopping or requiring any additional energy.

Despite many attempts over the centuries to create such a machine, no PMM1 has ever been successfully constructed. This is because the First Law of Thermodynamics states that energy cannot be created or destroyed, only transferred or converted from one form to another. Therefore, any machine that produces more energy than it consumes would violate this fundamental law of physics.

While PMM1 is a theoretical possibility, it contradicts the laws of physics and is considered impossible to create in practice. Many attempts have been made to design such machines, but they have all failed due to various factors, such as friction, energy losses due to heat, and the impossibility of extracting more energy from a system than was put into it.

In short, the concept of a PMM1 violates the fundamental principles of thermodynamics and cannot be realized in practice.

## First Law of Thermodynamics: Closed System

The First Law of Thermodynamics applies to closed systems, which are systems that do not exchange matter with their surroundings, but can exchange energy in the form of heat or work. For a closed system, the First Law can be expressed as follows:

ΔU = Q – W

where ΔU is the change in internal energy of the system, Q is the heat added to the system, and W is the work done by the system. This equation states that the change in internal energy of a closed system is equal to the heat added to the system minus the work done by the system.

When no work is done on or by the system, and there is no change in volume, the First Law can be simplified to:

ΔU = Q

which states that the change in internal energy of the system is equal to the heat added to the system.

The First Law of Thermodynamics is a fundamental principle of physics that applies to all types of closed systems, including gases, liquids, and solids. It is an essential concept in the study of thermodynamics and is used to understand and predict the behavior of various physical systems.

Overall, the First Law of Thermodynamics for closed systems plays an important role in understanding how energy is transferred and converted within a system and how it affects the system’s internal energy.

The first law of thermodynamics for a closed system can be illustrated below:

Process Internal energy change Heat (q) Work(w) Examples Include
Adiabatic (q = 0) +/- 0 +/- Isolated system wherein heat neither enters nor leaves.
Constant Volume (ΔV) (isochoric) +/- +/- 0 A bomb calorimeter (i.e. pressure-isolated system).
Constant pressure (isobaric) +/- Enthalpy − pΔV Most of the processes take place in constant external pressure.
Isothermal 0 +/- -/+ No temperature change, such as a temperature bath.

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