Kara Sears
The first law of thermodynamics, also known as the law of conservation of energy, states that the change in internal energy of a system is equal to the difference between heat transfer into the system and the work done by the system. This can be expressed mathematically 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. This law applies to systems where heat transfer and work are the methods of energy transfer, and it relates to the changes in energy states due to work and heat transfer. The internal energy of a system can be increased by supplying heat to the system or by doing work on the system. The first law also implies that energy cannot be created or destroyed, only transferred or exchanged between the system and its surroundings.
| Characteristics | Values |
|---|---|
| First Law of Thermodynamics | ΔU = Q − W |
| Q (net heat transfer) | Sum of all heat transfers into and out of the system |
| W (net work done) | Sum of all work done on or by the system |
| ΔU (change in internal energy) | Change in total kinetic and potential energy of all atoms and molecules in a system |
| Conservation of Energy | Energy is neither created nor destroyed |
| Heat Engines | An example of the first law in action is a heat engine, where thermal energy is converted into mechanical energy |
| Isolated Systems | The energy of an isolated system always remains constant |
| Energy Balance | During any thermodynamic process, the system holds a certain energy balance |
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What You'll Learn
Work and heat are interrelated concepts
The First Law of Thermodynamics is based on the principle of conservation of energy, where heat and work are the methods of transferring energy for a system in thermal equilibrium. Heat and work are interrelated concepts that allow systems to exchange energy. Heat is the transfer of thermal energy between two bodies at different temperatures. Work, on the other hand, is the force used to transfer energy between a system and its surroundings. It is required to create heat and transfer thermal energy.
Mathematically, the First Law of Thermodynamics can be expressed as ΔU = Q − W, where ΔU represents the change in internal energy of a system, Q is the net heat transferred into the system, and W is the net work done by the system. This equation highlights the relationship between heat transfer, work done, and the change in internal energy of a system.
The internal energy of a system increases when heat is added to it. Conversely, the internal energy decreases if the system releases heat or performs work. For example, in the combustion of coal to produce electricity, the maximum amount of energy available is fixed by the energy content of the reactants and products. However, the fraction of that energy that can be converted into useful work is not fixed.
The first law also states that energy cannot be created or destroyed, only converted from one form to another. This is evident in the visual representation of the law, where the energy lost by a system is gained by its surroundings, maintaining a constant total energy in the universe. This relationship between the system and its surroundings helps determine the increase or decrease in internal energy, heat, and work.
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Energy cannot be created or destroyed
The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed. This means that the total amount of energy in any isolated system remains constant over time, even though it may be converted between different forms. For example, in a heat engine, thermal energy is converted into mechanical energy and vice versa, but the total amount of energy in the system remains the same.
Mathematically, the first law of thermodynamics can be expressed as: ΔU = Q - W. Here, ΔU represents the change in the internal energy of a system, Q is the net heat transferred into the system, and W is the net work done by the system. This equation shows that the change in internal energy of a system is equal to the difference between the heat added to the system and the work done by the system.
The first law of thermodynamics is a fundamental principle in physics and has been validated through various experiments. It is important to note that this law applies to systems in thermal equilibrium, where heat and work are the primary methods of transferring energy. By understanding and applying this law, we can analyze and predict the behaviour of energy in various systems, including those involving heat transfer and mechanical work.
While the first law of thermodynamics provides valuable insights into energy conservation, it has certain limitations. For example, it does not explain why heat flows from a hotter object to a colder one when they are in contact. This behaviour is addressed by the second law of thermodynamics, which provides additional context for understanding energy transfer and the feasibility of various processes.
In summary, the first law of thermodynamics states that energy cannot be created or destroyed. This law is based on the principle of conservation of energy and forms the foundation for understanding energy transformations in various systems, particularly those involving heat and work. By quantifying the energy transfer and changes in internal energy, this law helps us analyze and predict the behaviour of energy in the physical world.
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Energy can be converted from one form to another
The first law of thermodynamics is a law of physics that has been validated experimentally many times. It 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, but it can be converted from one form to another.
The first law of thermodynamics states that the change in internal energy of a system is equal to the net heat transfer into the system minus the net work done by the system. In other words, the change in internal energy of a system is influenced by the interaction of heat, work, and internal energy.
Mathematically, the first law of thermodynamics can be expressed as ΔU = Q − W, where ΔU is the change in internal energy U of the system, Q is the net heat transferred into the system (the sum of all heat transfers into and out of the system), and W is the net work done by the system (the sum of all work done on or by the system).
The first law of thermodynamics distinguishes two principal forms of energy transfer: heat and thermodynamic work. For example, kinetic energy, the energy of a moving object, is converted into heat energy when a driver presses the brakes to slow down a car.
The first law of thermodynamics is applicable to systems where heat transfer and work are the methods of transferring energy into and out of the system. It is important to note that the internal energy U of a system depends only on the state of the system and not on how it reached that state. This means that if we know the state of a system, we can calculate changes in its internal energy using a few macroscopic variables.
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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. It states that the change in internal energy of a system equals the net heat transfer into the system minus the net work done by the system. In other words, the change in internal energy, ΔU, is related to heat and work by the equation ΔU = Q − W, where Q is the net heat transferred into the system, and W is the net work done by 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 the chemical bonds between molecules. It is an extensive property that accounts for the balance of heat transfer, work, and matter transfer into and out of the system. The internal energy of a system can decrease if the system gives off heat or does work, and it can increase if heat or work is done on the system.
The first law of thermodynamics allows for many possible states of a system to exist, but only certain states occur. This is due to the principle that energy 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." This means that any work or heat that goes into or out of a system changes the internal energy, but the total energy within the system remains constant.
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Energy balance
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 can neither be created nor destroyed, only altered in form. This is also known as the conservation of energy principle.
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 doing work are the methods of transferring energy into and out of the system.
The change in internal energy of a system is equal to the net heat transfer into the system minus the net work done by the system. In equation form, this is expressed as ΔU = Q − W, where ΔU is the change in internal energy U of the system, Q is the net heat transferred into the system, and W is the net work done by the system.
The first law of thermodynamics has been validated experimentally many times. It allows for the conversion of energy from one form to another, but it never allows for the creation or destruction of energy in the conversion process. For example, consider a boiling kettle. Heat is transferred from the stove to the kettle, causing the water to turn to vapour. As the system gets hotter, work is done—from the evaporation of the water to the whistling of the kettle.
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Frequently asked questions
The First Law of Thermodynamics is the law of conservation of energy, which states that energy can be transferred or exchanged but cannot be created or destroyed.
The First Law states that the change in internal energy of a system is equal to the net heat transfer into the system minus the net work done by the system. Mathematically, 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.
While the First Law quantifies the energy transfer, it does not provide the feasibility of the process or the change of state that a system undergoes. For example, it fails to explain why heat flows from the hot end to the cold end of a metallic rod when heated at one end.
The First Law of Thermodynamics can be observed in everyday situations where heat and work are transferred or exchanged. For instance, when boiling water in a kettle, heat is transferred from the stove to the kettle, and work is done through the evaporation of water and the whistling of the kettle.
