Keith Mack
The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed. In other words, the total energy in a closed system remains constant, though it can be transferred or transformed. This law has no known exceptions and applies to all systems, from the smallest scale of rubbing one's hands together to generate heat, to the largest scale of oceans, planets, and solar systems. While there are special cases, such as open systems where the concept of heat flow does not apply, the principle of energy conservation still holds true. The first law also has implications for the impossibility of perpetual motion machines, as all machines require an external energy source to counteract the effects of friction.
| Characteristics | Values |
|---|---|
| First Law of Thermodynamics | Energy cannot be created or destroyed |
| Conservation of Energy | Energy can be transferred between systems through the transfer of heat or by the performance of mechanical work |
| Open System | A system connected to its surroundings through a single permeable wall, but otherwise isolated |
| Closed System | No transfer of matter into or out of the system |
| Internal Energy | The energy of a system that is not kinetic or potential |
| Perpetual Motion Machine | A machine that can continue to move forever without an external energy source |
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What You'll Learn
Energy transfer without matter transfer
The first law of thermodynamics is a version of the law of conservation of energy, which states that energy can be transformed from one form to another but cannot be created or destroyed. This law applies to closed systems, where there is no transfer of matter into or out of the system. In such systems, the change in internal energy is equal to the heat supplied to the system and the work done by the system on its surroundings.
While the first law of thermodynamics typically involves the transfer of matter, there are exceptions where energy transfer occurs without matter transfer. This concept was proposed by Max Born in 1921 and 1949, building on the work of Constantin Carathéodory in 1909. Born's definition focused on energy transfers without the transfer of matter, and it has been widely adopted in textbooks.
Born's work highlights that when there is a transfer of matter between two systems, it is accompanied by a transfer of internal energy that cannot be solely attributed to heat and work components. However, there can be pathways to other systems, spatially separate from the matter transfer, that facilitate simultaneous heat and work transfers. In these cases, energy is conserved without the movement of matter.
One example of energy transfer without matter transfer is through waves. Waves can transmit energy and information without physically displacing matter. For instance, sunlight warms the Earth without transferring any physical substance. Similarly, sound energy is generated by the vibration of sound waves and travels from its source to another body without transferring matter.
Another example is radiation, where energy is transferred from one object to another as electromagnetic waves or high-energy particles without direct contact. This can be observed when the sun's energy warms the Earth, or in the case of a hot air balloon rising due to differences in air density. These examples demonstrate energy transfer without the physical exchange of matter, adhering to the first law of thermodynamics.
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Perpetual motion machines
The first law of thermodynamics is a version of the law of conservation of energy, which states that energy can be transformed from one form to another but can neither be created nor destroyed. In other words, the total energy of an isolated system remains constant.
A perpetual motion machine is a hypothetical machine that can do work indefinitely without an external energy source. This kind of machine is impossible, as its existence would violate the first and/or second laws of thermodynamics. These laws apply regardless of the system's size, meaning that machines extracting energy from finite sources cannot operate indefinitely because they are driven by the energy stored in the source, which will eventually be exhausted.
The history of perpetual motion machines dates back to the Middle Ages, with attempts to create such machines continuing into modern times. Indian engineers in the 12th century, for example, proposed a machine based on the principle of an unbalanced wheel, with spokes weighted by mercury that would shift as the wheel rotated, forcing the axle to continue turning. In 1588, Agostino Ramelli designed a perpetual motion device, though he knew it was futile to present a serious design for such a device. More recently, David Jones created a perpetual motion machine that has been displayed at the Royal Society since July.
A perpetual motion machine of the third kind is defined as one that completely eliminates friction and other dissipative forces, maintaining motion forever due to its mass inertia. However, it is impossible to create such a machine as dissipation can never be completely eliminated in a mechanical system.
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Open vs closed systems
The first law of thermodynamics is a version of the law of conservation of energy, which states that the total energy of an isolated system remains constant; energy can change forms but cannot be created or destroyed. This law is typically discussed in the context of closed systems, where there is no transfer of matter or energy into or out of the system. In closed systems, the change in internal energy is equal to the heat supplied minus the work done by the system.
However, the law also applies to open systems, where there is a transfer of matter or energy. An open system is connected to its surroundings and can exchange matter and energy with them. For example, evaporation is a process that occurs in an open system. While the first law of thermodynamics for closed systems focuses on the distinction between transfers of energy as work and heat, open systems primarily consider the transfer of internal energy. This distinction is crucial because, in open systems, there may be cross-effects between distinct transfers, where the transfer of one substance influences the transfer of another.
The first law of thermodynamics for open systems can be understood through the concept of composite isolated systems. By combining two isolated systems with internal energies U1 and U2, a new system with internal energy U is formed. This can be expressed as U = U1 + U2, indicating that the internal energy of the new system is the sum of the internal energies of the initial two systems. This principle demonstrates the law of conservation of energy, where the total energy of the composite system remains constant.
It is important to note that the first law of thermodynamics for closed and open systems is not interchangeable. The closed system view relies on the concepts of adiabatic enclosures and walls that prevent the passage of matter and internal energy. In contrast, open systems allow the transfer of matter and energy through permeable walls. While some processes in open systems can be treated as closed systems for specific purposes, the underlying principles and considerations differ between the two types of systems.
In summary, the first law of thermodynamics applies to both closed and open systems, but the specific considerations and distinctions between transfers of energy as work, heat, and internal energy vary between the two types of systems. The law ensures that energy is conserved in all thermodynamic processes, whether in isolated or interacting systems.
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Internal energy
The first law of thermodynamics is commonly called the conservation of energy. It states that energy can be transferred or transformed from one form to another but cannot be created or destroyed. This law can be expressed mathematically as:
> The change in the internal energy of a system is equal to the sum of the heat gained or lost by the system and the work done by or on the system.
The internal energy of a system can change when energy is transferred into or out of the system in the form of substance, heat, or thermodynamic work. These processes are measured by changes in the system's properties, such as temperature, entropy, volume, electric polarisation, and molar constitution. For example, when a machine lifts a system upwards, energy is transferred from the machine to the system, increasing its internal energy.
In a closed system, the change in internal energy (ΔU) is equal to the difference between the heat supplied to the system (Q) and the work done by the system on its surroundings (W). This can be expressed as:
> ΔU = Q - W
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Heat and work
The first law of thermodynamics is a version of the law of conservation of energy, which states that energy can be transferred between a system and its surroundings through the transfer of heat or by the performance of mechanical work. This law is based on the principle that energy cannot be created or destroyed, only transformed from one form to another.
In a closed system, the first law states that the change in internal energy of the system is equal to the difference between the heat supplied to the system and the work done by the system on its surroundings. This can be understood through the example of a burning log in a fireplace. The log's total energy and mass before burning remain the same as the mass and energy of the resulting soot, ash, smoke, heat, and light afterward. The chemical potential energy of the wood in the log is released when burned, appearing in the form of heat and light.
Work, in the context of thermodynamics, refers to the process of transferring energy to or from a system through macroscopic mechanical forces. This work can be derived from the system's overall kinetic energy, potential energy, or internal energy. For instance, when a machine lifts a system upwards, energy is transferred from the machine to the system.
The first law of thermodynamics has implications for the concept of perpetual motion. According to the law, it is not possible to construct a machine that will perpetually output work without an equal amount of energy input. This is because machines inevitably encounter friction, which demands energy from the system, and any gain in energy by the system corresponds to a loss in energy from its surroundings.
While the first law of thermodynamics provides a foundational framework for understanding energy transfer, it is important to note that there are complexities and exceptions to consider. For example, in open systems, the concept of "heat flow" does not apply, and only the transfer of internal energy is considered. Additionally, in certain cases, the transfer of energy associated with matter transfer cannot be distinctly resolved into work and heat components.
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Frequently asked questions
The first law of thermodynamics states that energy cannot be created or destroyed. It is also known as the law of conservation of energy.
No exceptions or contradictions to the first law of thermodynamics have ever been observed.
In an open system, there is no transfer of matter, and transfer of energy as heat is not defined. Instead, there is a transfer of internal energy.
No, they are impossible according to the first law. Any machine requires some form of input to continue moving, and this input must come from the total energy of the system.
