Gavin Howe
The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed, but only transformed from one form to another. This principle applies to all thermodynamic processes and systems, whether open, closed, or isolated. The law distinguishes between two principal forms of energy transfer: heat and thermodynamic work. Heat is the transfer of thermal energy between two bodies at different temperatures, while work is the force used to transfer energy between a system and its surroundings. Together, heat and work enable systems to exchange energy. The first law of thermodynamics helps us understand the fundamental concepts of energy transfer and conversion, allowing for the analysis and optimization of various processes.
| Characteristics | Values |
|---|---|
| Definition | The first law of thermodynamics is a formulation of the law of conservation of energy in the context of thermodynamic processes. |
| Energy Transfer | Energy can be transferred from one thermodynamic system to another in association with the transfer of matter. |
| Energy Creation or Destruction | Energy cannot be created or destroyed, but it can be transformed from one form to another. |
| Internal Energy | The internal energy of a system increases when the heat increases and decreases when the system gives off heat or does work. |
| Heat | Heat is a form of energy that can be transferred from one location to another and converted to and from other forms of energy. |
| Work | Work is the force used to transfer energy between a system and its surroundings and is needed to create heat and transfer thermal energy. |
| Perpetual Motion Machines | Perpetual motion machines of the first kind are impossible as the work done by a system on its surroundings consumes the system's internal energy. |
| Energy Balance | Energy balance, based on the first law, helps to understand any process, facilitate design and control, and enable optimization. |
| Energy Quality | The first law states that 1 Watt of heat equals 1 Watt of work or electricity. |
| Enthalpy | The first law introduces an additional state variable, enthalpy, which is the sum of the kinetic and potential energies of the particles in a system. |
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What You'll Learn
Energy cannot be created or destroyed
The first law of thermodynamics is a formulation of the law of conservation of energy in the context of thermodynamic processes. This law states that energy cannot be created or destroyed. It can only be converted from one form to another. For instance, chemical energy is converted to kinetic energy when a stick of dynamite explodes. If one adds up all forms of energy released in the explosion, such as kinetic energy, potential energy, heat, and sound, one will get the exact decrease of chemical energy in the combustion of the dynamite.
The first law of thermodynamics distinguishes two principal forms of energy transfer: heat and thermodynamic work. The law also defines the internal energy of a system, an extensive property that accounts for the balance of heat transfer, thermodynamic work, and matter transfer into and out of the system. The internal energy of a system increases when the heat increases or when work is done on the system. Any work or heat that goes into or out of a system changes the internal energy. However, since energy is never created nor destroyed, the change in internal energy always equals zero. If energy is lost by the system, it is absorbed by the surroundings, and vice versa.
The first explicit statement of the first law of thermodynamics, made by Rudolf Clausius in 1850, referred to cyclic thermodynamic processes and the existence of a function of state of the system, the internal energy. However, the original 19th-century statements of the first law appeared in a conceptual framework that took the transfer of energy as heat as a primitive notion. This framework also took the notions of empirical temperature and thermal equilibrium as primitive. It did not presume a concept of energy in general but regarded it as derived or synthesized from the prior notions of heat and work.
The first law of thermodynamics is generally thought to be the least demanding to grasp, as it is an extension of the law of conservation of energy. This law states that the total energy of an isolated system remains constant over time. In other words, the amount of energy in the universe has always been and will always be the same. This law is also related to the concept of perpetual motion machines, stating that no system without an external energy supply can deliver an unlimited amount of energy to its surroundings.
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Energy can be transferred
The first law of thermodynamics is a formulation of the law of conservation of energy in the context of thermodynamic processes. It states that energy cannot be created or destroyed, but it can be transferred or converted from one form to another. This principle is commonly known as the conservation of energy.
The law distinguishes two principal forms of energy transfer: heat and thermodynamic work. Heat is the transfer of thermal energy between two bodies at different temperatures, and work is the force used to transfer energy between a system and its surroundings. Both work and heat allow systems to exchange energy. For example, kinetic energy, the energy of a moving object, is converted to heat energy when a driver presses the brakes to slow down a car.
In contrast, a closed system exchanges energy but not matter with its surroundings. For example, if we put a tight lid on the pot of soup, it would still radiate heat energy but would no longer emit matter in the form of steam.
An isolated system, such as soup poured into a perfectly insulated thermos bottle with a sealed lid, does not exchange energy or matter with its surroundings. However, in practice, perfectly isolated systems cannot exist, and all systems will eventually transfer energy to their environment, regardless of how well insulated they are.
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Energy can be converted to different forms
The first law of thermodynamics is a formulation of the law of conservation of energy. This law states that energy cannot be created or destroyed, but it can be transformed from one form to another. This principle is also known as energy conversion or energy transformation.
Energy conversion is the process of changing energy from one form to another. In physics, energy is a quantity that provides the capacity to perform work, such as lifting an object, or to provide heat. Energy can be converted to thermal energy from other forms with 100% efficiency. For example, when a moving object, like a car, comes to a stop, its kinetic energy is transformed into thermal energy due to friction.
Energy can also be converted from non-thermal forms with high efficiency, though some energy is always lost as heat due to friction and similar processes. An example of this is when an object falls in a vacuum, converting potential energy into kinetic energy.
Energy can also be transferred from one thermodynamic system to another in association with the transfer of matter. For example, a conventional automobile converts chemical energy in the fuel into kinetic energy through combustion, which is then converted into linear piston movement.
The first law of thermodynamics distinguishes two principal forms of energy transfer: heat and thermodynamic work. Work is the force used to transfer energy between a system and its surroundings, and it is needed to create heat and the transfer of thermal energy.
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Internal energy of a system
The first law of thermodynamics is a formulation of the law of conservation of energy in the context of thermodynamic processes. It defines the internal energy of a system, which is an extensive property that accounts for the balance of heat transfer, thermodynamic work, and matter transfer into and out of the system.
Internal energy refers to all the energy within a given system, including the kinetic energy of molecules and the energy stored in all chemical bonds between molecules. It is a state variable, like temperature or pressure, and can be converted to or stored as potential or kinetic energy. The internal energy of a system increases when heat increases, and it decreases when the system gives off heat or does work. Any work or heat that goes into or comes out of a system changes the internal energy.
However, since energy is neither created nor destroyed, the change in internal energy always equals zero. If a system loses energy, it is absorbed by the surroundings, and if the system absorbs energy, it was released by the surroundings. This is represented mathematically as:
> ΔUsystem = -ΔUsurroundings
Internal energy does not include the energy due to the motion or location of a system as a whole. It excludes any kinetic or potential energy the body may have due to its motion or location in external gravitational, electrostatic, or electromagnetic fields. However, it does include the contribution of such a field to the energy due to the coupling of the internal degrees of freedom of the system with the field.
Thermodynamics defines internal energy macroscopically for the body as a whole. In statistical mechanics, the internal energy of a body can be analyzed microscopically in terms of the kinetic energies of microscopic motion and the potential energies associated with microscopic forces, including chemical bonds.
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Heat as a form of energy
The first law of thermodynamics is a formulation of the law of conservation of energy in the context of thermodynamic processes. It distinguishes two principal forms of energy transfer: heat and thermodynamic work.
Heat is a form of energy that is transferred from one body to another as a result of a difference in temperature. If two bodies at different temperatures are brought together, heat flows from the hotter body to the cooler one. This transfer of energy usually results in an increase in the temperature of the colder body and a decrease in the temperature of the hotter body.
In the International System of Units (SI), the unit of measurement for heat is the joule (J). Heat is also measured in British thermal units (BTUs) and calories. The standard unit for the rate of heating is the watt (W), defined as one joule per second.
Heat is not to be confused with thermal energy, which is the energy stored in a body. Heat is restricted to energy being transferred. The amount of energy required to raise the temperature of a substance is called its heat capacity or specific heat.
Heat plays a crucial role in the first law of thermodynamics, which states that energy can be converted from one form to another but cannot be created or destroyed. For example, kinetic energy is converted to heat energy when a driver presses the brakes to slow down a car. Heat is also essential in converting states of matter, such as melting ice or boiling water.
In summary, heat is a form of energy that is transferred between bodies due to temperature differences, and it plays a significant role in the first law of thermodynamics by enabling energy conversion without creation or destruction.
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Frequently asked questions
The first law of thermodynamics states that energy cannot be created or destroyed but can be transferred or converted between different forms.
Energy transfer is the movement of energy from one location to another. Energy can be transferred from one thermodynamic system to another, with or without the transfer of matter.
Energy conversion is when energy is transformed from one form to another. For example, kinetic energy is converted to heat energy when a driver presses the brakes on a car to slow down.
The internal energy of a system is the sum of the kinetic and potential energies of the particles that form the system. It is a state function, meaning any change in the internal energy is equal to the difference between its initial and final values.
The first law of thermodynamics is used to understand and optimise processes, as well as to design and control systems. It also provides a foundation for the study of thermodynamics and the interaction of heat and other forms of energy.
