Name: gas work in a piston cylinder
Uploaded: 2023-04-30T12:31:11.000Z
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Channel: JoVE

K12 Standards

new-jersey-student-learning-standards-for-science

New Jersey Student Learning Standards for Science

High School

Physical Science

Energy

HS-PS3-4

Work Done During Volume Change

Considera un pallone da calcio gonfiato con un volume di V. La pressione che agisce sulla sfera è la pressione atmosferica.
Supponiamo che la palla cada dalla scogliera e venga bucata; l'aria all'interno della palla si libera. Assumendo una deflazione uniforme, si riduca il suo raggio di una distanza di dr.
L'entità della forza che agisce sulla palla dipende dalla pressione atmosferica e dalla superficie della palla.
Pertanto, il lavoro svolto dall'ambiente circostante sulla palla è uguale al prodotto negativo della pressione atmosferica, della superficie della palla e della variazione del suo raggio.
Tuttavia, il prodotto della superficie e dr è la variazione del volume della palla.
Pertanto, durante una variazione finita di volume da V1 a V2, il lavoro svolto può essere espresso come l'integrale del prodotto della pressione e della variazione di volume.
Qui il segno negativo indica che il lavoro è svolto sul sistema dall'ambiente circostante. Al contrario, il lavoro svolto dal sistema è considerato positivo.

<ol>
	<li>OpenStax. (2019). Fisica universitaria Vol. 1. [Versione web]. Pp 112 Estratto da <a href="https://openstax.org/books/university-physics-volume-2/pages/3-2-work-heat-and-internal-energy">https://openstax.org/books/university-physics-volume-2/pages/3-2-work-heat-and-internal-energy</a></li>
</ol>

lavoro svolto durante un cambiamento di volume

In meccanica, il lavoro viene eseguito su un oggetto quando la forza che agisce su di esso sposta l'oggetto. In termodinamica, il lavoro compiuto su un sistema può essere stimato quando il volume del sistema cambia durante qualsiasi processo termodinamico.

Consideriamo un gas confinato in un cilindro dotato di un pistone mobile ad un'estremità. Se il gas si espande dal volume V1 al volume V2, esercita una forza sul pistone, tale che il pistone si muove di una distanza dr.

Il lavoro compiuto dal gas sul pistone può essere espresso come

<img alt="Equation1" src="/files/ftp_upload/13675/13675_Equation_PT_1.svg" style="text-align: center; height: 10px;" />

Se p è la pressione che agisce sulla faccia del pistone di area A, allora la forza che agisce sul pistone è:

<img alt="Equation2" src="/files/ftp_upload/13675/13675_Equation_PT_2.svg" style="text-align: center; height: 12px;" />

Conseguentemente, il lavoro compiuto può essere espresso come:

<img alt="Equation3" src="/files/ftp_upload/13675/13675_Equation_PT_3.svg" style="text-align: center; height: 12px;" />

La variazione del volume del gas dV è uguale a Adr. Pertanto, il lavoro compiuto dal gas sul pistone quando si espande dal volume V1 a V2 è espresso come:

<img alt="Equation4" src="/files/ftp_upload/13675/13675_Equation_PT_4.svg" style="text-align: center; height: 45px;" />

Qui, l'integrale ha significato solo per un processo quasi statico, il che significa un processo che avviene in passaggi infinitesimamente piccoli, mantenendo il sistema in equilibrio termico.

Se il pistone comprime il gas, il lavoro viene compiuto sul gas e quindi convenzionalmente è considerato negativo.

Lavoro svolto durante un cambiamento di volume

In meccanica, il lavoro viene svolto su un oggetto quando la forza che agisce su di esso sposta l'oggetto. In termodinamica, il lavoro svolto su un sistema può essere stimato quando ...

Education

Didattica

JoVE Core

Physics

Fisica

Unit 1

Unità 1

The First Law of Thermodynamics

Il Primo Principio della Termodinamica

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

gas work in a piston cylinder

Gas work in a piston cylinder describes how pressure and volume change are related in thermodynamics. In mechanics, work happens when a force moves an object. In thermodynamics, work done on a system can be estimated when the system changes volume during a process.

A gas can be trapped in a cylinder with a movable piston at one end. When the gas expands from volume V1 to volume V2, it pushes on the piston and moves it by a small distance dr. If the pressure on the piston face is p and the piston area is A, the force on the piston is based on that pressure.

The work done by the gas can then be written in terms of pressure, area, and piston motion. Because the change in volume dV equals A dr, the work can also be expressed as an integral from V1 to V2. This form is meaningful only for a quasi-static process, which means the change happens in very small steps while the system stays in thermal equilibrium.

If the piston compresses the gas, the direction of work reverses. In that case, work is done on the gas, and it is conventionally treated as negative.

Gas Work in a Piston Cylinder

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,...

work done during volume change

Gas work in a piston cylinder describes how pressure and volume change are related in thermodynamics. In mechanics, work happens when a force moves an object. In thermodynamics, work done on a system can be estimated when the system changes volume during a process. A gas can be trapped in a cylinder with a movable piston at one end. When the gas expands from volume V1 to volume V2, it pushes on the piston and moves it by a small distance dr. If the pressure on the piston face is p and the...

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Work done by an ideal gas can depend on the path it takes between two thermodynamic states. The transcript compares two routes from state A to state C for the same gas. In both cases, the gas expands from volume V1 to volume V2, but the work is not the same. Along path AC, the gas stays at constant temperature T. It expands from V1 to V2. The work done by the gas is found from the pressure-volume relation for an ideal gas. After substituting pressure as nRT/V and integrating, the work for this...

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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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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...

isochoric and isobaric processes

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

Heat capacity describes how much heat a substance absorbs for a given change in temperature. It is also called thermal capacity, and its SI unit is J/K. Specific heat capacity is the heat needed to raise the temperature of 1 kg of a substance by 1 K, and its SI unit is J/kg·K. The video also explains molar heat capacity. This is the amount of heat added or removed to change the temperature of 1 mole of a substance by 1 K. It can be measured at constant volume or constant pressure, and it is...

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...

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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...

work done in an adiabatic process

Adiabatic Compression in an Engine Cylinder

Adiabatic compression in an engine cylinder changes the pressure, temperature, and volume of a gas-air mixture. In this setup, gasoline vapor is injected into the cylinder when the piston is in its expanded position. The mixture starts at 20 °C, 1.00 x 10^5 N/m^2, and 240 cm^3. The mixture is then compressed adiabatically to 40 cm^3. Adiabatic means no heat is exchanged with the surroundings during the process. For this calculation, the compression is treated as quasi-static, even though real...

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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.

joule-thomson effect

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 gas pressure and piston motion create work during expansion or compression, and why quasi-static volume change matters in thermodynamics.

Gas Work in a Piston Cylinder | JoVE K12

physics

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HS-PS3-5

Gas Work in a Piston Cylinder

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