Keith Mack
The first law of thermodynamics is a formulation of the law of conservation of energy in the context of thermodynamic processes. It is defined as the principle that energy is conserved, meaning it cannot be created or destroyed but can be converted among different forms, while the total energy of the universe remains constant. The first law of thermodynamics is commonly called the conservation of energy. It is generally thought to be the least demanding to grasp, as it is an extension of the law of conservation of energy, meaning that energy can be neither created nor destroyed.
| Characteristics | Values |
|---|---|
| Energy | Cannot be created or destroyed, but can be converted from one form to another |
| Internal energy | A function of state |
| Heat | Can be transferred to a gas |
| Enthalpy | Sum of internal energy and pressure-volume term |
| Mass | Energy transfer is associated with mass crossing the control boundary |
| Work | Energy transfer is associated with external work |
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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. The law of conservation of energy states that energy cannot be created or destroyed. This is a fundamental concept of physics, alongside the conservation of mass and the conservation of momentum.
The total energy within a domain remains constant. Energy can be converted from one form to another, but the total energy within the domain remains fixed. For example, chemical energy is converted to kinetic energy when a stick of dynamite explodes. If one adds up all forms of energy that were released in the explosion, such as the kinetic energy and potential energy of the pieces, as well as 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 for taking account of the balance of heat transfer, thermodynamic work, and matter transfer, into and out of the system.
Energy can be transferred from one thermodynamic system to another in association with the transfer of matter. There can be pathways to other systems, spatially separate from that of the matter transfer, that allow heat and work transfer independent of and simultaneous with the matter transfer.
The first law of thermodynamics for a closed system was expressed in two ways by Clausius. Carathéodory's 1909 version of the first law of thermodynamics was stated in an axiom that refrained from defining or mentioning temperature or the quantity of heat transferred. That axiom stated that the internal energy of a phase in equilibrium is a function of state, that the sum of the internal energies of the phases is the total internal energy of the system, and that the value of the total internal energy of the system is changed by the amount of work done adiabatically on it, considering work as a form of energy.
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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 in the context of thermodynamic processes. This law of conservation of energy states that energy cannot be created or destroyed, only converted from one form to another. This principle is fundamental to physics, and it is universally accepted.
Energy conversion, also known as energy transformation, is the process of changing energy from one form to another. In physics, energy is a quantity that provides the capacity to perform work, for example, lifting an object, or providing heat. Energy can be converted to thermal energy from other forms with 100% efficiency. Conversion among non-thermal forms of energy may also occur with high efficiency, although some energy is always lost as heat due to friction and similar processes.
There are many examples of energy conversion. A roller coaster gains kinetic energy as it descends, converting potential energy to kinetic energy. A light bulb converts electrical energy into light and thermal energy. In a combustion engine, the chemical energy stored in fuel is converted into thermal energy when the fuel burns, producing heat that powers the engine. Chemical energy in the fuel of a conventional automobile is converted into the kinetic energy of expanding gas via combustion. This kinetic energy is then converted into linear piston movement, which is then converted into rotary crankshaft movement.
Energy conversion has been used for centuries, with early windmills transforming the kinetic energy of wind into mechanical energy for pumping water and grinding grain. More complex systems take raw energy from fossil fuels and nuclear fuels to generate electrical power.
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Total energy in the universe remains constant
The first law of thermodynamics is a formulation of the law of conservation of energy in the context of thermodynamic processes. It is defined as the principle that energy is conserved, meaning it cannot be created or destroyed but can be converted between different forms. This is based on the fundamental concept that the total energy within a domain remains constant.
The first law of thermodynamics is commonly referred to as the conservation of energy. It states that the total energy in the universe remains constant. This means that, however much energy existed at the start of the universe, that amount will remain at the end. The law distinguishes two principal forms of energy transfer: heat and thermodynamic work.
The first law of thermodynamics applies the conservation of energy principle to systems where heat transfer and work are the methods of transferring energy into and out of the system. The change in internal energy is equal to the heat transfer minus the work done. This is expressed as: ΔU = Q - W, where ΔU is the change in internal energy, Q is the net heat transfer, and W is the net work done.
The first law of thermodynamics evolved from the experimental demonstration that heat and mechanical work are interchangeable forms of energy. It is often stated as: "Energy can neither be created nor destroyed in a system of constant mass, although it may be converted from one form to another." This law accurately describes the beginning and ending points of complex processes, such as metabolism and photosynthesis, without regard to the complications in between.
The first law of thermodynamics is considered the least demanding to grasp as it is an extension of the law of conservation of energy. It is a fundamental concept of physics, along with the conservation of mass and the conservation of momentum. This law also forms the basis for understanding the thermodynamic properties of any system, through the use of five thermodynamic functions and basic knowledge of calculus and differential equations.
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Energy transfer associated with mass transfer
The first law of thermodynamics is a conservation law, which means that energy in a system of constant mass can be converted from one form to another but cannot be created or destroyed. This is often stated as:
> Energy can neither be created nor destroyed in a system of constant mass, although it may be converted from one form to another.
The first law of thermodynamics relates changes in internal energy to heat added to a system and the work done by a system. It distinguishes two principal forms of energy transfer: heat and thermodynamic work. The internal energy of a system is directly proportional to temperature.
Energy can be transferred from one thermodynamic system to another in association with the transfer of matter. When there is a transfer of energy associated with matter transfer, work and heat transfers can be distinguished only when they pass through walls physically separate from those for matter transfer. There can be pathways to other systems, spatially separate from that of the matter transfer, that allow heat and work transfer independent of and simultaneous with the matter transfer.
An example of energy transfer associated with mass transfer is a gas expanding so quickly that no heat can be transferred. The expansion does work, and the temperature drops. This is exactly what happens with a carbon dioxide fire extinguisher, with the gas coming out at high pressure and cooling as it expands at atmospheric pressure.
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Energy conservation in complex processes
The first law of thermodynamics, also known as the conservation of energy principle, states that energy cannot be created or destroyed in a system of constant mass. Instead, energy is converted from one form to another. For instance, chemical energy is converted to kinetic energy when a stick of dynamite explodes. The total kinetic, potential, internal, and "flow" energies within the system remain constant.
This principle is encapsulated in the first law of thermodynamics, which defines the internal energy of a system. The change in internal energy is equal to the heat transfer minus the work done on the system. This law is based on the work of Joule, who experimentally demonstrated that heat and mechanical work are interchangeable forms of energy. This is also referred to as the "mechanical equivalent of heat".
The conservation of energy is a fundamental concept in physics, and it applies to various complex processes and systems. For example, in a closed system, the total amount of energy within the system can only change if energy enters or leaves the system. This is in contrast to an isolated system, where the total energy remains constant.
Energy conservation efforts in complex systems aim to reduce wasteful energy consumption. This can be achieved through efficient energy use, such as utilizing renewable energy sources and improving energy efficiency through technological upgrades and behavioural changes. Energy audits, for instance, can help identify specific opportunities for energy conservation in existing buildings.
Furthermore, understanding user behaviour and profiling energy usage patterns can lead to substantial energy conservation. By modifying user habit loops and providing recommendations for improving habits, significant reductions in energy demand can be achieved. This involves considering various factors, such as household income, education, and attitudes towards environmentally friendly practices.
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Frequently asked questions
The first law of thermodynamics is defined as the principle that energy is conserved, meaning it cannot be created or destroyed but can be converted among different forms, while the total energy of the universe remains constant.
The conservation of energy is a fundamental concept of physics that states that within some problem domain, the amount of energy remains constant and energy is neither created nor destroyed.
The first law of thermodynamics applies the conservation of energy principle to systems where heat transfer and doing work are the methods of transferring energy into and out of the system.
