Principle of Energy Conservation (First Law of Thermodynamics)
The First Law of Thermodynamics—also known as the Principle of Energy Conservation—states that energy cannot be created or destroyed. It can only be transferred or transformed from one form to another.
This concept lies at the foundation of chemical engineering thermodynamics and governs every energy-related phenomenon in industrial processes, from heat exchange to mechanical work, electrical generation, chemical reactions, and fluid operations.
Modern engineering applies the First Law far beyond classical heat engines. For example, solar radiation and wind kinetic energy are converted into electrical power in solar panels and wind turbines—an everyday demonstration of the First Law in action.
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The First Law of Thermodynamics is represented by the following mathematical equation:
ΔU = Q – W
This equation expresses three essential concepts:
- It provides the general mathematical statement of energy conservation for any process.
- It establishes the existence of the internal energy function UUU.
- It formally defines heat (Q) and work (W) as energy in transit, not properties of the system.
The adjacent figure shows a classic representation of a thermodynamic system. It illustrates the sign conventions for heat (Q) and work (W) in relation to the external environment.
Sign Conventions
- Q>0: the system absorbs heat
- Q<0: the system releases heat
- W>0: the system performs work on the surroundings
- W<0: the surroundings perform work on the system
These conventions allow a consistent application of the First Law to all engineering systems.
In thermodynamics, work is considered an “ordered” form of energy because it can be fully converted into other useful forms. Heat, instead, is a “disordered” form of energy: it cannot be completely converted into work, even in an ideal machine. This fundamental distinction is what leads to the Second Law.
The Joule Experiment
(applying the Thermodynamics 1st Law)
Consider a non-cyclic transformation where a system changes from an initial state to a different final state—for example, heating 1 kg of water from 15 °C to 100 °C.
This temperature rise can be obtained in different ways:
- by directly supplying heat, or
- through mechanical work, such as the stirring mechanism in Joule’s device.
In Joule’s experiment, falling weights rotate paddles immersed in water. The mechanical motion is converted into thermal energy, increasing the water temperature.
From this experiment, Joule established the numerical equivalence: 4.186 J=1 cal.
Both produce the same measurable effect: heating 1 g of water by 1 °C.
The crucial conclusion is:
- Q and W depend on the path of the transformation.
- Their difference, Q−W, does not.
This shows that internal energy U is a state function, depending only on the initial and final states: ΔU=U2−U1.
Substituting, we return to the First Law: ΔU=Q−W
This relationship is valid for any process, irrespective of how heat or work are exchanged.
Why a System Must Operate in a Cycle to Produce Work Continuously
Because internal energy is a state function, a system cannot keep producing work indefinitely unless it returns to its initial state.
Only a cyclic process allows:
- the repetition of the same sequence of transformations,
- and therefore the continuous production of work.
Impossibility of a Perpetual Motion Machine of the First Kind (PMM1)
Based on the logical analysis of equation (1), it is possible to state the technical and conceptual impossibility of a Perpetual Motion Machine of the First Kind (PMM1)—a hypothetical device capable of delivering continuous work output without any energy input.
In fact, if a machine were to produce work without absorbing heat (i.e., with Q = 0), the First Law of Thermodynamics would give:
ΔU = –W
And if ΔU = 0, then:
0 = –W ⇒ W = 0
Therefore, no work can be produced without energy input, confirming the impossibility of PMM1.
Common Energy Transformations in Industrial Systems
Chemical Energy
Stored in fossil fuels like oil or gas. This is the energy contained in the molecular bonds of the fuel.
Thermal Energy
When the fuel is burned, its chemical energy is converted into thermal energy (heat), which is used to produce steam.
Mechanical (Kinetic) Energy
The steam spins a turbine, converting thermal energy into mechanical energy (rotational motion).
Electrical Energy
The turbine drives a generator that converts mechanical energy into electricity.
Sound Energy (or other outputs)
The electrical energy powers devices—like a guitar amplifier—that transform electricity into mechanical vibrations and ultimately into sound.
Other Fundamental Principles of Thermodynamics
After understanding the First Law, which states that energy is conserved and can change form, it is essential to briefly recall the other three laws that complete the foundation of thermodynamics.
Zeroth Law of Thermodynamics
The Zeroth Law of Thermodynamics defines the concept of temperature. It states that if two systems are in thermal equilibrium with a third system, then they are also in thermal equilibrium with each other. This logical principle forms the basis for using thermometers and comparing temperatures.
Second Law of Thermodynamics
The Second Law of Thermodynamics introduces the concept of entropy and explains the direction of natural processes. It states that in any real process, the total entropy of a system and its surroundings always increases. In practical terms, it means that not all the energy in a system can be converted into useful work. For example, heat flows spontaneously from a hot body to a cold one, never the reverse.
Third Law of Thermodynamics
The Third Law of Thermodynamics concerns the behavior of systems at extremely low temperatures. It states that as a system approaches absolute zero (0 K), the entropy of a perfect crystal tends to zero. This law defines an absolute reference point for entropy and helps to understand the limits of cooling processes.
Ing. Ivet Miranda
⬆️ Back to TopFirst Law of Thermodynamics Quiz
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FAQ
What is a Thermodynamic System?
A thermodynamic system is a defined portion of matter or space selected for analysis, where energy exchanges (in the form of heat or work) are studied. Everything outside the system is considered the surroundings.
What is Thermodynamic Equilibrium?
It’s the state in which no net flow of energy occurs within the system or between system and surroundings—mechanical, thermal, and chemical balances are all satisfied.
What is Thermodynamics in Physics?
It’s the branch of physics that studies heat, energy, and work, and how these quantities behave in different systems.
What is Thermodynamics in Biology?
Thermodynamics in biology explains how energy is produced, transferred, and used in cells—like in ATP production and metabolic pathways.
What is Thermodynamics with Example?
Thermodynamics is the science that studies how energy is transferred and transformed, especially in the form of heat and work. A classic example is Joule’s experiment, which showed the equivalence between mechanical work and heat.
To understand this in context, scroll up to the section on Joule’s experiment above.
What is a Thermodynamic Cycle?
It’s a series of processes in which a system returns to its initial state. Examples include the Carnot and Rankine cycles in engines.