Cameron Miranda
The first law of thermodynamics is a conservation law that states that energy in a closed system can be converted from one form to another but cannot be created or destroyed. This law distinguishes two principal forms of energy transfer: heat and thermodynamic work. 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. The first law of thermodynamics is foundational to our understanding of energy and has been applied in various fields, including the development of heat engines and other machines that can perform work.
| Characteristics | Values |
|---|---|
| Definition | The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes. |
| Energy | Energy can be converted from one form to another with the interaction of heat, work and internal energy, but it cannot be created or destroyed. |
| Work | Work is the primary foundation of thermodynamics and in particular of the first law. |
| Heat | Heat is a form of energy. |
| Perpetual motion | The first law implies that perpetual motion machines of the first kind are impossible. |
| Isolated system | In an externally isolated system, with internal changes, the sum of all forms of energy is constant. |
| Closed system | In a closed system, there is no transfer of matter into or out of the system. |
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What You'll Learn
Energy transfer as heat
The first law of thermodynamics is a conservation law, which means that energy in the universe can neither be created nor destroyed. It states that energy can be converted from one form to another with the interaction of heat, work, and internal 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. It is defined as energy transferred by thermal contact with a reservoir, which has a temperature and is generally so large that the addition and removal of heat do not alter its temperature. The internal energy of a system increases when heat is added to it, and decreases when the system gives off heat.
Work is the force used to transfer energy between a system and its surroundings, and it is needed to create heat and transfer thermal energy. Work is equal to the force multiplied by the distance moved in the direction of the force. For example, a gas confined by a piston in a cylinder will expand and do work on the piston if heated.
The first law of thermodynamics relates changes in internal energy to heat added to a system and the work done by a system. In an adiabatic process, no heat is added or removed from a system, and the change in internal energy is equal to the negative work done. In an isothermal process, the temperature stays constant, and pressure and volume are inversely proportional.
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Energy transfer as work
The first law of thermodynamics states that energy can be converted from one form to another, but it cannot be created or destroyed. This law defines the internal energy of a system, taking into account the balance of heat transfer, thermodynamic work, and matter transfer into and out of the system.
Work is the force used to transfer energy between a system and its surroundings. It is the primary foundation of thermodynamics, particularly the first law. Work is motion against an opposing force, such as raising a weight against the force of gravity. The magnitude of work depends on the mass of the object, the strength of the gravitational force acting on it, and the height through which it is raised.
In the context of the first law of thermodynamics, work is one of the principal forms of energy transfer, along with heat. Work is done on a system when energy is transferred to it, and work is done by the system when energy is transferred from it. This is represented mathematically as work being equal to the negative external pressure on the system multiplied by the change in volume.
The first law of thermodynamics applies to various scenarios, including machinery and living systems. For example, in the human body, heat transferred out of the body (Q) and work done by the body (W) remove internal energy, while food intake replaces it. Similarly, plants absorb solar energy and convert it into chemical energy through photosynthesis.
The first law of thermodynamics also introduces the concept of enthalpy, an additional state variable. Enthalpy accounts for the internal energy of a system, as well as the pressure and volume of the system. This allows for a more comprehensive understanding of the energy transfers and conversions occurring within a thermodynamic system.
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Energy cannot be created or destroyed
The first law of thermodynamics, originally formulated by Rudolf Clausius in 1850, is a conservation law. This means that energy in the universe can neither be created nor destroyed. The law defines two principal forms of energy transfer: heat and thermodynamic work. It also 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.
The first law of thermodynamics states that the total energy of a system remains constant, even if it is converted from one form to another. For example, kinetic energy—the energy that an object possesses when it moves—is converted to heat energy when a driver presses the brakes on a car to slow it down. The first law of thermodynamics relates the various forms of kinetic and potential energy in a system to the work the system can perform and the transfer of heat.
The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic processes. In general, the conservation law states that the total energy of an isolated system is constant; energy can be transformed from one form to another but can be neither created nor destroyed. 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 heat due to the work done by the system.
The first law of thermodynamics evolved from the experimental demonstration that heat and mechanical work are interchangeable forms of energy. Work is the primary foundation of thermodynamics and, in particular, of the first law. Work is a process of transferring energy to or from a system in ways that can be described by macroscopic mechanical forces acting between the system and its surroundings. The work done by the system can come from its overall kinetic energy, its overall potential energy, or its internal energy. For example, when a machine lifts a weight, energy is transferred from the machine to the system.
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Internal energy
The first law of thermodynamics, also known as the law of conservation of energy, states that energy can be converted from one form to another with the interaction of heat, work, and internal energy, but it cannot be created or destroyed. This law applies to both closed and isolated systems.
Mathematically, the first law of thermodynamics can be expressed as:
\[ \Delta U_{univ}=ΔU_{sys}+ΔU_{surr}=0 \]
Where the subscripts univ, sys, and surr refer to the universe, the system, and the surroundings, respectively. This equation demonstrates that the change in energy of a system is equal in magnitude but opposite in sign to the change in energy of its surroundings.
For a closed system, the change in internal energy is equal to the sum of the heat transferred and the work done. This can be represented by the equation:
\[ ΔU = q + w \]
Where \(ΔU\) is the change in internal energy, \(q\) is the heat transferred, and \(w\) is the work done.
The internal energy of systems that are more complex than an ideal gas cannot be measured directly. However, it is proportional to its temperature, so changes in internal energy can be monitored by observing changes in temperature.
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Adiabatic and non-adiabatic systems
The first law of thermodynamics is a conservation law, which means that energy in a system can be converted from one form to another but cannot be created or destroyed. This law distinguishes two principal forms of energy transfer: heat and thermodynamic work.
Non-adiabatic systems, on the other hand, do not have such independence. In a non-adiabatic process, heat supplied is defined as the residual change in internal energy after work has been taken into account. This is in contrast to an adiabatic process, where there is no transfer of heat to or from the system. The assumption of adiabatic isolation is often used in calculations to obtain a good first approximation of a system's behaviour. For example, when sound travels through a gas, there is no time for heat conduction, so the propagation of sound is considered adiabatic.
In summary, adiabatic and non-adiabatic systems refer to the transfer of energy as heat between a system and its surroundings. Adiabatic systems are those where there is no heat transfer, while non-adiabatic systems involve heat transfer and a change in the internal energy of the system. The distinction between these two types of systems is important in understanding the behaviour of thermodynamic processes and the conservation of energy.
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Frequently asked questions
The first law of thermodynamics is a conservation law, which means that the energy in the universe can be converted from one form to another but can neither be created nor destroyed.
The first law of thermodynamics states that the total energy of a system remains constant, even if it is converted from one form to another. It distinguishes two principal forms of energy transfer: heat and thermodynamic work.
The first law of thermodynamics relates the various forms of kinetic and potential energy in a system to the work a system can perform and to the transfer of heat. For example, kinetic energy is converted to heat energy when a driver presses the brakes to slow down a car.
