What Is the First Law of Thermodynamics? 7 Fascinating Facts Explained

The first law of thermodynamics is one of the most important principles in physics. It explains how energy behaves — how it flows, transforms, and remains conserved in every physical and natural process. This law serves as the foundation for energy conservation and is essential to understanding heat engines, mechanical systems, and even biological energy use.

In simple terms, the first law of thermodynamics states that energy cannot be created or destroyed, only changed from one form to another. This single statement governs everything from how engines run to how our own bodies use food for energy.

This article focuses primarily on the physics behind the first law of thermodynamics, while briefly exploring its applications in chemistry, biology, and engineering, along with how it is covered in O-level, A-level, AP, and IB syllabi.

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Understanding What Is the First Law of Thermodynamics

In physics, the first law of thermodynamics expresses the principle of energy conservation for thermodynamic systems. It establishes a quantitative relationship between heat added to a system, the work done by the system, and the change in internal energy.

The mathematical form of this law is:

ΔU = Q – W

Where:

• ΔU = Change in internal energy of the system
• Q = Heat added to the system
• W = Work done by the system

This formula shows that when a system receives heat (Q), part of it increases the internal energy (ΔU) while the rest is used to do work (W).

For example, when gas is heated in a piston, it expands and pushes the piston outward. The gas’s internal energy increases because of the heat absorbed, and simultaneously, it performs mechanical work.

The Core Idea: Conservation of Energy

The first law of thermodynamics is a precise statement of the law of conservation of energy. Energy can move between forms — from heat to work, or from potential to kinetic — but the total energy of the universe remains constant.

Examples of energy transformations that follow this law include:

• Engines: Chemical energy of fuel turns into mechanical work and heat.
• Human Body: Chemical energy in food converts into mechanical motion and thermal energy.
• Electrical Appliances: Electrical energy changes into heat, light, or motion.

In every case, energy isn’t lost — it’s only transformed from one form to another.

Mathematical Expression and Sign Conventions

The general form of the first law of thermodynamics is:
ΔU = Q – W

Here’s how it applies depending on the direction of energy flow:

When Heat Is Added

If Q > 0, heat flows into the system, increasing its internal energy or allowing it to perform work.

When Heat Is Lost

If Q < 0, heat flows out of the system, reducing its internal energy.

When the System Does Work

If W > 0, the system performs work on its surroundings (like an expanding gas), decreasing internal energy.

When Work Is Done on the System

If W < 0, the surroundings perform work on the system (like compressing a gas), increasing internal energy.

These conventions allow physicists to analyze how energy moves and transforms in any process.

Read: What is Newton’s Third Law of Motion?

The First Law of Thermodynamics in Physics

In physics, the first law of thermodynamics connects heat, work, and internal energy, forming the backbone of thermodynamic analysis. It helps physicists study how systems behave under different conditions, such as compression, expansion, or heating.

It’s especially useful for:

• Analyzing heat engines and refrigerators
• Understanding adiabatic and isothermal processes
• Explaining energy exchange in gases and fluids

Example: Expanding Gas in a Cylinder

When a gas in a cylinder is heated, the molecules move faster, causing the gas to expand. The gas pushes the piston outward, doing work on its surroundings. The energy for this work comes from the heat added — perfectly illustrating the first law of thermodynamics.

Applications Beyond Physics

Though rooted in physics, the first law of thermodynamics also has broad applications in other fields:

In Chemistry

It helps describe energy changes during chemical reactions. The concept of enthalpy (H) is derived from this law, showing how heat changes at constant pressure.

In Biology

It explains how living organisms convert food energy into motion, heat, and biochemical reactions — conserving energy at every step.

In Engineering

Engineers use this law to design engines, turbines, power plants, and refrigeration systems by analyzing energy input, output, and loss.

These interdisciplinary applications make the first law one of the most widely applied scientific principles.

Joule’s Experiment and Energy Equivalence

The first law of thermodynamics was experimentally verified by James Prescott Joule in the 1840s.

Joule’s Paddle-Wheel Experiment

He placed a paddle wheel inside water and connected it to a falling weight. As the weight dropped, it rotated the paddles, stirring the water and increasing its temperature.

This experiment proved that mechanical work can be converted into heat energy, establishing that both are forms of the same thing — energy.

Joule’s work led to the definition of the mechanical equivalent of heat, providing experimental proof for energy conservation.

Real-World Examples of the First Law of Thermodynamics

1. Steam Engines: Heat energy from burning fuel converts to mechanical energy that moves pistons.
2. Air Conditioners: Work done by the compressor helps move heat from a cooler to a warmer region.
3. Electric Heaters: Electrical energy turns into heat to warm the surroundings.
4. Human Body: Metabolism converts chemical energy from food into motion, heat, and repair work.
5. Solar Panels: Light energy from the sun converts into electrical energy through the photovoltaic effect.

Each example follows the first law’s rule — energy changes form but never disappears.

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The First Law in Cyclic and Isolated Systems

Cyclic Processes

In a cyclic process, such as that in an engine, the system returns to its original state after one complete cycle. Thus,
ΔU = 0
and therefore,
Q = W
This means all the heat added to the system is converted into work.

Isolated Systems

For isolated systems, no heat or work exchange occurs with surroundings. Hence,
ΔU = 0
The internal energy remains constant — another reflection of energy conservation.

Limitations of the First Law of Thermodynamics

While the first law of thermodynamics is powerful, it has limitations:

1. It doesn’t specify the direction of energy flow — that’s governed by the second law.
2. It doesn’t distinguish between useful work and wasted heat.
3. It doesn’t account for energy degradation (loss of quality).

Thus, while it tells us energy is conserved, it doesn’t tell us how efficiently it can be used.

Importance of the First Law of Thermodynamics

The importance of understanding what is the first law of thermodynamics lies in its universal application. It helps scientists and engineers:

• Analyze and design energy-efficient systems
• Understand natural phenomena like weather and biological metabolism
• Predict energy exchanges in physical and chemical processes
• Maintain energy balance in engineering systems

Without this law, it would be impossible to quantify how energy moves through systems or how much work can be produced from a given amount of heat.

Read: What Is Thermal Energy?

The First Law of Thermodynamics in Different Syllabi

Let’s explore how the first law of thermodynamics is covered across popular educational programs.

O-Level Syllabus

• Focuses on energy conservation and heat transfer.
• Introduces simple practical examples (heating, expansion, and calorimetry).
• Qualitative understanding: energy cannot be created or destroyed.

A-Level Syllabus

• Introduces the full mathematical form ΔU = Q – W.
• Covers adiabatic and isothermal processes.
• Teaches numerical calculations of work, heat, and internal energy.
• Links to enthalpy and energy flow in chemical reactions.

AP (Advanced Placement)

• AP Physics: Focuses on thermodynamic systems, energy transfer, and PV diagrams.
• AP Chemistry: Uses the law to define enthalpy and calorimetry equations (Q = mCΔT).
• Links the macroscopic first law to microscopic kinetic theory.

Read: Is AP Physics Hard?

IB (International Baccalaureate)

• Emphasizes energy conservation and experimental verification.
• Encourages real-world applications and lab experiments.
• Physics: Focus on work-energy relationships and thermodynamic cycles.
• Chemistry: Focus on enthalpy, energy transfer, and calorimetry.

Connection Between Physics and Other Subjects

Although fundamentally a physics law, the first law of thermodynamics acts as a bridge between different scientific disciplines:

• In Chemistry: It forms the foundation of thermochemistry and reaction energetics.
• In Biology: It governs how organisms maintain energy balance and survive.
• In Engineering: It guides the design of systems that efficiently convert one form of energy into another.

This makes the first law not just a rule of physics but a universal truth across all sciences.

Summary

The first law of thermodynamics is the cornerstone of energy science. It states that energy can neither be created nor destroyed, only transformed. Whether in engines, chemical reactions, or living systems, it governs how energy flows and ensures balance in every process.

Understanding what is the first law of thermodynamics is essential not just for physics students but for anyone exploring how energy drives the world around us — from the smallest atom to the largest star.

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FAQ

What does the first law of thermodynamics state?

It states that energy cannot be created or destroyed, only transformed from one form to another. The total energy of an isolated system remains constant.

Who discovered the first law of thermodynamics?

It was developed through the contributions of James Joule, Rudolf Clausius, and Hermann von Helmholtz during the 19th century.

What is the mathematical formula of the first law of thermodynamics?

ΔU = Q – W
Where ΔU is the change in internal energy, Q is the heat added, and W is the work done by the system.

What is an example of the first law of thermodynamics in real life?

Heating a gas in a piston — the added heat increases internal energy and allows the gas to do work by pushing the piston.

How is the first law applied in physics?

It’s used to analyze energy exchange in thermodynamic systems, like engines, turbines, and heat pumps.

Why is the first law of thermodynamics important?

It forms the foundation of all energy studies, ensuring that energy is always conserved, no matter how it changes form.

Image Suggestions:

1. Diagram of a piston showing energy flow (heat, work, internal energy).
2. Illustration of Joule’s paddle-wheel experiment showing conversion of work into heat.