First Law of Thermodynamics

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Created by: Team Physics -, Last Updated: July 16, 2024

First Law of Thermodynamics

First Law of Thermodynamics is a fundamental concept in physics that forms the cornerstone of the laws of thermodynamics. It states that energy cannot be created or destroyed but can only change form. In other words, the total energy within a closed system remains constant. This principle, also known as the law of energy conservation, plays a vital role in understanding how heat and work interact in physical systems.

What is First Law of Thermodynamics?

The First Law of Thermodynamics asserts that a closed system maintains a constant total energy. This law specifies that energy can only transform from one form to another and cannot be created or destroyed. Adding heat to a system increases its internal energy or enables it to perform work. Therefore, the system’s internal energy changes by the amount of heat added minus the work it does on its surroundings.

First Law of Thermodynamics Formula

The First Law of Thermodynamics is expressed mathematically by the formula:

  • where:
  • Δ𝑈 represents the change in the internal energy of the system,
  • 𝑄 is the heat added to the system, and
  • 𝑊 is the work done by the system on its surroundings.

This equation encapsulates the principle that the change in a system’s internal energy is equivalent to the heat input minus the work output.

Perpetual Motion Machine of First Kind (PMM1)

A Perpetual Motion Machine of the First Kind (PMM1) aims to generate continuous energy without any input, essentially creating energy out of nothing. It tries to produce more energy than it consumes, violating the First Law of Thermodynamics, which states that energy cannot be created or destroyed but only transformed from one form to another. Despite numerous attempts

First law of Thermodynamics for a Closed System

The First Law of Thermodynamics for a closed system asserts that the work done is the product of pressure and the change in volume resulting from applied pressure.

w = −PΔV
  • where:
  • 𝑃 represents the constant external pressure exerted on the system,
  • Δ𝑉 denotes the change in the system’s volume.

This work is specifically known as “pressure-volume” work.

Interactions across the system’s boundaries increase or decrease its internal energy. When the system performs work, its internal energy decreases, but when work is done on the system, its internal energy increases. Heat exchange between the system and its surroundings also changes internal energy. The First Law of Thermodynamics dictates that the total change in internal energy remains zero because energy cannot be created or destroyed. The surroundings absorb energy lost by the system and release energy that the system gains.

This relationship is expressed as:

  • where:
  • Δ𝑈system is the change in the system’s total internal energy, and
  • Δ𝑈surroundings is the change in the surroundings’ total energy.
ProcessSign Convention for Heat (𝑞)Sign Convention for Work (𝑤)
Work done by the systemN/A
Work done on the systemN/A+
Heat extracted from the systemN/A
Heat added to the system+N/A

For a Closed Loop

ProcessInternal Energy ChangeHeat (𝑞)Work (𝑤)Example
Adiabatic (𝑞=0)±0±An isolated system in which heat neither enters nor leaves.
Constant volume (Δ𝑉) (isochoric)±±0A hard, pressure-isolated system like a bomb calorimeter.
Constant pressure (isobaric)±Enthalpy−𝑝Δ𝑉Most processes occur under constant external pressure.
Isothermal0±−/±There is no change in temperature, like in a temperature bath.

First Law of Thermodynamics Limitations

First Law of Thermodynamics Limitations
  1. Cannot Predict Energy Direction: The First Law asserts that energy cannot be created or destroyed. It does not, however, indicate the direction in which energy transformations occur. Therefore, you cannot predict the spontaneous flow of heat or determine process feasibility.
  2. Lacks Information on Efficiency: The First Law quantifies energy changes but does not account for the quality or efficiency of energy conversion. As a result, it cannot distinguish between useful work and energy lost as waste heat.
  3. No Information on Entropy: The First Law does not address entropy changes, which play a crucial role in determining process spontaneity. Consequently, it cannot explain why some processes occur naturally while others require external intervention.
  4. Does Not Apply to Open Systems: The First Law focuses on closed systems that do not exchange energy or matter. Thus, it is less applicable when analyzing open systems that continuously exchange energy and matter with their surroundings.
  5. Fails to Consider Irreversibility: The First Law describes energy conservation but overlooks irreversible processes that increase entropy and reduce the system’s ability to perform useful work. Thus, you cannot explain why practical systems never reach 100% efficiency.

Uses of First Law of Thermodynamics

Uses of First Law of Thermodynamics
  1. Heat Engines: Engineers design and analyze heat engines using the First Law, ensuring that fuel combustion efficiently converts energy into mechanical work.
  2. Refrigeration and Air Conditioning: The First Law determines the energy needed to transfer heat from cooler to warmer environments, enabling engineers to design efficient refrigerators and air conditioners.
  3. Power Generation: Power plants use the First Law to convert energy from fuels or nuclear reactions into electrical energy, optimizing the balance between heat input and electricity output.
  4. Chemical Reactions: Chemists calculate energy changes in reactions like combustion or oxidation with the First Law, helping them understand reaction efficiency and yield.
  5. Biological Systems: Biologists analyze how living organisms convert food into energy for cellular processes, gaining insights into metabolism and energy consumption.
  6. Material Sciences: Material scientists apply the First Law to understand phase changes and energy transfer in substances, developing efficient thermal insulation and energy-efficient materials.

Examples for First Law of Thermodynamics

Examples for First Law of Thermodynamics
  • Boiling Water: You heat water to add energy, increasing its internal energy and temperature until it boils.
  • Car Engines: An engine burns fuel to convert energy into mechanical work, propelling the car forward.
  • Refrigerators: The refrigerator extracts heat from inside and releases it outside, using energy to maintain a cool internal temperature.
  • Battery Operation: A battery transforms stored chemical energy into electrical energy, powering devices like flashlights or phones.
  • Human Metabolism: Your body converts food into energy to support daily activities and essential bodily functions.
  • Solar Panels: Solar panels absorb sunlight and convert it into electrical energy, demonstrating the transformation of radiant energy into electricity.
  • Air Conditioning Systems: An air conditioner extracts heat from a room and expels it outside, using work to transfer thermal energy.


How does the First Law apply to a closed system?

In a closed system, heat or work transfer causes changes in energy. The system’s internal energy changes by the amount of heat added minus the work it does.

What is internal energy?

The total energy within a system, including molecular kinetic and potential energies, defines internal energy. Heat transfer and work done cause it to change.

How is work related to the First Law of Thermodynamics?

Work represents energy transferred when a system moves against an external force. It’s subtracted from heat input to calculate the internal energy change.

What role does heat play in the First Law?

Heat is a form of energy transfer that changes a system’s internal energy, either increasing or decreasing it depending on the temperature difference.

Does the First Law explain entropy?

No, the First Law does not explain entropy. Entropy, which measures disorder, is a concept addressed by the Second Law of Thermodynamics.

How does the First Law differ from the Second Law?

The First Law focuses on energy conservation, while the Second Law introduces entropy, stating that energy transformations increase the overall disorder of a system.

Why can’t we create a Perpetual Motion Machine?

A Perpetual Motion Machine violates the First Law because it implies creating energy from nothing, which contradicts energy conservation principles.

How do heat engines demonstrate the First Law?

Heat engines convert heat into mechanical work. The energy not converted to work is expelled as waste heat, illustrating energy conservation.

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