Kara Sears
The first law of thermodynamics, also known as the conservation of energy principle, states that energy cannot be created or destroyed, only transformed from one form to another. This law can be expressed mathematically as ΔU = q - w, where ΔU represents the change in internal energy, q is the heat transfer into the system, and w is the work done by the system on its surroundings. This equation highlights the relationship between heat transfer, work done, and the change in internal energy of a system. The first law of thermodynamics provides a foundation for understanding the exchanges of external work and heat in a system, helping us analyze the energy balance and conservation within the system.
| Characteristics | Values |
|---|---|
| Equation | \(\Delta E = q + w\) or $∆U = q - w |
| \(ΔE\) | Change in internal energy of a system |
| \(q\) | Net heat transfer (the sum of all heat transfer into and out of the system) |
| \(w\) | Net work done (the sum of all work done on or by the system) |
| Law | Conservation of energy |
| Energy | Cannot be created or destroyed, but can be transformed from one form to another |
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What You'll Learn
The first law of thermodynamics is a conservation law
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. The internal energy of a system depends only on the state of the system and not on how that state was reached. The first law gives the relationship between heat transfer, work done, and the change in internal energy of a system.
The first law of thermodynamics is commonly called the conservation of energy. In elementary physics courses, the study of the conservation of energy emphasizes changes in mechanical kinetic and potential energy and their relationship to work. A more general form of conservation of energy includes the effects of heat transfer and internal energy changes. This more general form is usually referred to as the first law of thermodynamics. Other forms of energy may also be included, such as electrostatic, magnetic, strain, and surface energy.
The first law of thermodynamics is given as ΔE = q + w, where ΔE is the change in internal energy of a system, q is the net heat transfer (the sum of all heat transfers into and out of the system), and w is the net work done (the sum of all work done on or by the system). The first law can be used to categorise the performance of cyclic conversion systems like fossil-fired, steam power cycles, or geothermal cycles.
The first law of thermodynamics evolved from the experimental demonstration that heat and mechanical work are interchangeable forms of energy. The law 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.
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Energy can be transformed into various forms
The First Law of Thermodynamics is a conservation law, which means that energy in the universe can be transformed into various forms but cannot be created or destroyed. This law applies to systems with constant mass, where energy may be converted from one form to another. The internal energy of a system, also known as entropy, is a state function that does not depend on the path taken to reach that state.
The First Law of Thermodynamics can be expressed mathematically as ΔU = Q - W, where ΔU represents the change in internal energy of the system, Q is the heat supplied to the system, and W is the work done by the system on its surroundings. This equation highlights the relationship between heat transfer, work done, and the resulting change in internal energy.
The law is particularly useful in understanding the behaviour of heat engines, which are open systems where thermal energy is converted into mechanical energy and vice versa. In such systems, the balance of energy is maintained, as all energy entering the system equals the energy leaving it, plus any change in stored energy.
The First Law also provides insights into the concept of internal energy, which includes kinetic energy and potential energy. For example, when a driver applies brakes to slow down a moving car, the kinetic energy of the car is converted into heat energy. This demonstrates how energy can be transformed from one form to another, as outlined by the First Law.
Additionally, the First Law serves as a foundation for understanding the various states of a system. By considering the five thermodynamic functions (entropy, enthalpy, Helmholtz free energy, internal energy, and Gibbs free energy) and applying basic calculus and differential equations, we can analyse the thermodynamic properties of any system.
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The 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. The first law of thermodynamics states that energy can neither be created nor destroyed in a system of constant mass, though it may be converted from one form to another. This is often referred to as the conservation of energy principle.
Mathematically, the first law of thermodynamics can be expressed as:
ΔU = q + w
Where ΔU is the change in internal energy of the system, q is the net heat transfer, and w is the net work done. This equation represents the balance of heat transfer and work done on or by the system, with any changes in internal energy being a result of these interactions.
The first law of thermodynamics helps us understand the relationship between heat transfer, work done, and the change in internal energy of a system. It is important to note that the first law does not account for the feasibility of the process or the change of state that a system undergoes, as this is addressed by the second law of thermodynamics.
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The relationship between heat transfer, work done, and internal energy
The first law of thermodynamics is a conservation law, which means that the energy in the universe can neither be created nor destroyed. This law relates to the changes in energy states due to work and heat transfer. 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.
Mathematically, this is represented as ΔU = q + w, where ΔU is the change in internal energy of a system, q is the net heat transfer (the sum of all heat transfers into and out of the system), and w is the net work done (the sum of all work done on or by the system).
Heat and work are interrelated concepts. 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, and it is required to create heat and transfer thermal energy. Both work and heat allow systems to exchange energy.
The first law of thermodynamics applies to both machinery and living systems. For example, in a heat engine, thermal energy is converted into mechanical energy, and the process can also occur in reverse. Heat engines are usually categorised as open systems, which allow mass, heat, and external work to cross the control boundary.
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The first law can be written in terms of heat and work
The first law of thermodynamics is a conservation law, which means that the energy in the universe can neither be created nor destroyed. It states that energy can only be converted from one form to another. The law relates to changes in energy states due to work and heat transfer. It can be written in terms of heat and work as follows:
$$$\Delta E = q + w$$
Here, $\Delta E$ is the change in internal energy of a system, $q$ is the net heat transfer (the sum of all heat transfer into and out of the system), and $w$ is the net work done (the sum of all work done on or by the system). Both $q$ and $w$ are energy in transit, while only $\Delta E$ represents an independent quantity capable of being stored. The internal energy $E$ of a system depends only on the state of the system and not on how that state was reached.
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 is useful for categorising the performance of cyclic conversion systems like fossil-fired, steam power cycles, or geothermal cycles.
The first explicit statement of the first law of thermodynamics, by Rudolf Clausius in 1850, referred to cyclic thermodynamic processes and the existence of a function of state of the system, the internal energy. He expressed it in terms of a differential equation for the increments of a thermodynamic process. The first law can be used to explain the efficiency of a heat engine, which is a machine that converts thermal energy into mechanical energy. Heat engines are mostly categorised as open systems, which allow mass, heat, and external work to cross the control boundary.
The first law of thermodynamics is commonly called the conservation of energy. In elementary physics courses, the study of the conservation of energy emphasises changes in mechanical kinetic and potential energy and their relationship to work. A more general form of this law includes the effects of heat transfer and internal energy changes. This more general form is usually referred to as the first law of thermodynamics.
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