Name: first law of thermodynamics: solving problems
Uploaded: 2023-09-18T17:11:16.000Z
Duration: 1 min 21 s
Channel: JoVE

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High School

Chemistry

Energy

HS-PS3-1

first-law-of-thermodynamics-problem-solving

First Law Of Thermodynamics: Problem-Solving

A primeira lei da termodinâmica afirma que o calor adicionado a um sistema é utilizado para realizar o trabalho e aumentar a energia interna do sistema.
Considere um exemplo em que 100 gramas de água a uma pressão atmosférica se convertem em vapor a 100 graus Celsius. Qual é a mudança na energia interna para este processo termodinâmico?
Este sistema termodinâmico muda do estado líquido inicial para o estado gasoso final a temperatura e pressão constantes quando a água se converte em vapor.
A transferência de calor para o sistema é o produto da massa e do calor latente da vaporização da água.
O trabalho realizado é igual ao produto da mudança de pressão e volume. Usando o valor de massa e densidade do vapor e da água, a mudança de volume e, portanto, o trabalho realizado é estimado.
Aqui, a quantidade desconhecida é a mudança na energia interna, enquanto as quantidades conhecidas são o calor fornecido e o trabalho feito pelo sistema. Assim, aplicando a primeira lei da termodinâmica, uma mudança na energia interna pode ser estimada.

<ol>
	<li>Young, H. D, e Freedman, R.A. (2012). Física Universitária com Física Moderna. São Francisco, CA: Pearson. pág. 632.</li>
	<li>OpenStax. (2019). Física Universitária Vol. 2. [Versão Web]. pp 116 Obtido de <a href="https://openstax.org/books/college-physics/pages/15-1-the-first-law-of-thermodynamicsirst-law-of-thermodynamics-and-some-simple-processes">https://openstax.org/books/college-physics/pages/15-1-the-first-law-of-thermodynamics</a></li>
</ol>

primeira lei da termodinâmica: solução de problemas

A primeira lei da termodinâmica afirma que a mudança na energia interna do sistema é igual à transferência líquida de calor para o sistema menos o trabalho líquido realizado pelo sistema. Essa equação é uma forma generalizada de conservação de energia e pode ser aplicada a qualquer processo termodinâmico.

As seguintes estratégias podem ser usadas para resolver qualquer problema envolvendo a primeira lei da termodinâmica.

<ol>
	<li>O sistema termodinâmico deve ser identificado.</li>
	<li>Os estados inicial e final do processo termodinâmico devem ser listados.</li>
	<li>As quantidades conhecidas e desconhecidas para o problema dado são identificadas. Deve-se garantir que as unidades de todas as quantidades sejam consistentes. Por exemplo, se a pressão estiver em pascals e o volume em metros cúbicos, o trabalho deve ser expresso em joules.</li>
	<li>A mudança na energia interna é independente do caminho. Como resultado, pode-se estimar para todos os caminhos possíveis se a energia interna para os estados inicial e final é conhecida.</li>
	<li>Calor, trabalho e mudança na energia interna para qualquer processo estão relacionados através da equação para a primeira lei da termodinâmica.</li>
	<li>Convencionalmente, Q é positivo quando calor é adicionado ao sistema e negativo quando calor é removido dele. Da mesma forma, W é positivo quando trabalho é realizado pelo sistema e negativo quando trabalho é realizado sobre o sistema. Isso significa que a polaridade do trabalho e do calor devem ser consideradas na primeira lei da termodinâmica.</li>
	<li>As quantidades conhecidas podem então ser inseridas na equação da primeira lei da termodinâmica para avaliar as quantidades desconhecidas.</li>
</ol>

Primeira Lei da Termodinâmica: Solução de Problemas

A primeira lei da termodinâmica afirma que a mudança na energia interna do sistema é igual à transferência líquida de calor para o sistema menos a rede ...

Education

JoVE Core

Physics

Física

Unit 1

Unidade 1

The First Law of Thermodynamics

A Primeira Lei da Termodinâmica

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

first law of thermodynamics: solving problems

The first law of thermodynamics links internal energy, heat, and work. 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. This is a general form of energy conservation, and it can be used for any thermodynamic process.

To solve a first-law problem, start by identifying the thermodynamic system. Then list the initial and final states of the process. Next, identify the known and unknown quantities. It is also important to keep the units consistent. For example, if pressure is in Pascals and volume is in cubic meters, then work should be expressed in Joules.

The change in internal energy is path independent. That means it can be estimated for any possible path if the internal energy of the initial and final states is known. Heat, work, and internal energy are then related through the first-law equation. In this sign convention, Q is positive when heat is added to the system and negative when heat is removed. W is positive when work is done by the system and negative when work is done on the system.

Once the signs and units are set, the known quantities can be substituted into the first-law equation. The unknown quantity can then be calculated from the relationship between heat, work, and internal energy. This method gives a clear way to solve thermodynamics problems step by step.

First Law of Thermodynamics: Solving Problems

first-law-of-thermodynamics-solving-problems

thermodynamic systems

Open, Closed, and Isolated Systems

A thermodynamic system is the part of the world that scientists study for its heat and energy behavior. It sits inside a surrounding environment, or surroundings. The system and the surroundings can exchange heat and do work across the boundary that separates them. The immediate surroundings affect the system most strongly because they interact with it directly. A kettle of boiling tea shows how this works in everyday life. The tea and the kettle together form the system. If the lid is open,...

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Gas Work in a Piston Cylinder

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How Internal Energy Changes in Gas Systems

Internal energy is the energy stored inside a thermodynamic system. It is the sum of the kinetic and potential energies of the molecules or other particles in the system. Molecular kinetic energy includes translational motion, rotation, and vibration. Molecular potential energy comes from interactions between molecules. A system’s location and overall motion do not affect its internal energy. For an ideal monatomic gas, each particle is a single atom. That means there is no rotational or...

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Heat, Work, and Internal Energy

Heat, work, and internal energy are linked by the first law of thermodynamics. This law is a generalized form of energy conservation. It explains how energy changes when heat is added to or removed from a system, and when the system does work or has work done on it. The change in internal energy equals the heat transferred into the system minus the work done by the system. Added heat increases internal energy. For that reason, heat is taken as positive when it enters the system and negative...

first law of thermodynamics: problem-solving

The first law of thermodynamics links internal energy, heat, and work. 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. This is a general form of energy conservation, and it can be used for any thermodynamic process. To solve a first-law problem, start by identifying the thermodynamic system. Then list the initial and final states of the process. Next, identify the known and unknown quantities. It is...

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Net Work in a Cyclic Thermodynamic Process

A cyclic thermodynamic process returns a system to its initial state. In a cycle, the change in internal energy is zero. An isolated system is different because it has zero heat exchange and zero work, so its internal energy stays constant. For a non-isolated system, the internal energy change is zero only when the process is cyclic. The first law of thermodynamics relates internal energy, heat transfer, and work. It states that the change in internal energy is the difference between the net...

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Isothermal Expansion and Heat Transfer

An isothermal process is a thermodynamic process that happens at constant temperature. During the process, heat slowly flows into the system or out of the system so thermal equilibrium is maintained. Phase changes are common examples of isothermal processes. Water can evaporate into steam at a constant temperature, and water can also freeze into ice at a constant temperature. An ideal gas can also undergo isothermal expansion or compression. Here, an ideal gas in an isolated cylinder expands...

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Constant Volume and Pressure in Thermodynamics

Thermodynamic processes can happen at constant volume or at constant pressure. An isochoric process is a constant-volume process. In this case, the volume does not change, so the work done by the system is zero. The first law of thermodynamics still applies, so any heat added or removed changes the internal energy of the system only. A simple example is heating 1000 g of water from 40 degrees celsius to 70 degrees celsius at constant volume. Because the process is isochoric, no work is done by...

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Ideal Gas Heat Capacity at Constant Pressure

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heat capacities of an ideal gas ii

Ideal Gas Heat Capacity Differences

Ideal gas heat capacities depend on whether the gas is heated at constant volume or constant pressure. At constant volume, the heat absorbed only raises the internal energy of the system. No work is done in this case. At constant pressure, the situation is different. The heat absorbed increases the internal energy and also does work. Molar heat capacity is the heat needed to raise the temperature of 1 mole of gas by 1 unit. Because some of the added heat goes into work, more heat is needed at...

heat capacities of an ideal gas iii

Ideal Gas Heat Capacity and Degrees of Freedom

Ideal gas heat capacity depends on degrees of freedom, which are the independent ways a gas molecule can move, rotate, and vibrate. If d is the number of degrees of freedom, the molar heat capacity at constant volume can be written in terms of d. The molar heat capacity at constant pressure can also be written using the relationship between the two heat capacities. The ratio of molar heat capacity at constant pressure to molar heat capacity at constant volume is called the ratio of heat...

adiabatic processes for an ideal gas

Gas Temperature Changes in Adiabatic Processes

Adiabatic processes change the temperature of an ideal gas without heat transfer. In an adiabatic compression, no heat is added, but work is done on the gas. That work raises the gas temperature. In an adiabatic expansion, the gas does work and its temperature drops. A common example is the compression of a gas-air mixture inside a car engine cylinder. The compression happens so quickly that there is no time for heat to move between the mixture and its surroundings. Even so, the work done on...

pressure and volume in an adiabatic process

pressure-and-volume-in-an-adiabatic-process

Free Expansion: Pressure, Volume, and Temperature

Free expansion shows how pressure, volume, and temperature behave when a gas expands without doing work. It is an adiabatic process, but it is not the same as adiabatic expansion. In free expansion, no work is done and the internal energy does not change. In adiabatic expansion, work is done and the internal energy changes. For an adiabatic process, the pressure-volume relation comes from the adiabatic condition. That relation does not apply to free expansion, even though the expansion is...

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Adiabatic Compression in an Engine Cylinder

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Energy Forms in Thermodynamics

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Using Maxwell's Relations in Thermodynamics

Maxwell's thermodynamic relations help solve problems in thermodynamics. They connect partial differentials for quantities that are hard to measure in the lab with quantities that are easier to measure. This makes them useful for working with temperature, pressure, volume, and entropy. These relations come from thermodynamic potentials and the symmetry of second derivatives. Thermodynamic potentials are exact differentials, so the order of second differentiation does not change the result.

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Gas Cooling and Heating in Throttling

Gas cooling and heating in throttling is explained by the Joule-Thomson effect, also called the Joule-Kelvin effect. It describes how a fluid changes temperature when it passes through a valve or porous plug in a thermally insulated system. This type of flow is called a throttling process. In the experiment, a high-pressure gas is forced through a throttle valve or porous plug into a lower-pressure region. The gas expands as it moves into the larger volume. Work is done on the gas as it is...

key terms and concepts

Learn how to solve first law of thermodynamics problems by identifying the system, tracking heat and work, checking units, and using sign conventions.

First Law of Thermodynamics: Solving Problems | JoVE K12

physics

JoVE Core - Fisica apresenta a teoria e a aplicação de tópicos fundamentais da física, incluindo cinemática, dinâmica, mecânica dos fluidos, estática, ondas e acústica, com vídeos animados concisos e de fácil compreensão. Além disso, os vídeos de cientistas em ação demonstram a aplicação de conceitos relacionados em experimentos de pesquisa clássica e original realizados nos laboratórios de todo o mundo hoje em dia.

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First Law of Thermodynamics: Solving Problems

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