Name: gas temperature changes in adiabatic processes
Uploaded: 2023-04-30T12:32:28.000Z
Duration: 1 min 18 s
Channel: JoVE

K12 Standards

new-york-state-science-learning-standards

New York State Science Learning Standards

High School

Physical Science

HS. Energy

HS-PS3-3.

Adiabatic Processes for an Ideal Gas

תהליך תרמודינמי ללא מעבר חום בתוך המערכת או מחוצה לה נקרא אדיאבטי.
לכן, על פי החוק הראשון של התרמודינמיקה, השינוי באנרגיה הפנימית הוא השלילי של העבודה שנעשתה בתהליך אדיאבטי.
לדוגמה, לבוכנת כיבוי המורכבת מצינור מבודד תרמית יש קצה אחד סגור, ובקצה השני יש בוכנה ניידת. 
אם חתיכה קטנה של כדור צמר גפן מונחת בתוך הצינור והבוכנה מורשית לדחוף מטה במהירות בטמפרטורת החדר, כדור הצמר גפן עולה באש למרות שלא הופעל חום. זה קורה עקב דחיסה אדיאבטית.
כאן, ככל שהנפח פוחת, העבודה שנעשתה הופכת שלילית; לפיכך, שינוי האנרגיה הפנימי הוא חיובי, עם עלייה מקבילה בטמפרטורת הגז.
לעומת זאת, בעת פתיחת בקבוק משקה מוגז מקורר, הגז הכלוא בבקבוק עובר התפשטות אדיאבטית וכתוצאה מכך העבודה שנעשית חיובית.
כתוצאה מכך, האנרגיה הפנימית פוחתת עם ירידת הטמפרטורה המתאימה, ומעבה את האדים המתאדים לענן נראה.

<ol>
	<li>יאנג, ה.ד. ופרידמן, ר. (2012). פיזיקה אוניברסיטאית עם פיזיקה מודרנית. סן פרנסיסקו, קליפורניה: פירסון. עמ' 640.</li>
	<li>OpenStax. (2019). פיזיקה אוניברסיטאית כרך 2. [גרסת אינטרנט]. מקור: <a href="https://openstax.org/books/university-physics-volume-2/pages/3-6-adiabatic-processes-for-an-ideal-gas">https://openstax.org/books/university-physics-volume-2/pages/3-6-adiabatic-processes-for-an-ideal-gas</a></li>
</ol>

תהליכים אדיאבטיים לגז אידיאלי

<div style="direction:rtl">כאשר גז אידיאלי נדחס בצורה אדיאבטית, כלומר ללא הוספת חום, מתבצעת עבודה עליו והטמפרטורה שלו עולה. בהתפשטות אדיאבטית הגז אכן עובד, והטמפרטורה שלו יורדת. דחיסות אדיאבטיות מתרחשות למעשה בצילינדרים של מכונית, שם הדחיסות של תערובת הגז-אוויר מתרחשות כל כך מהר עד שאין זמן לתערובת להחליף חום עם סביבתה. עם זאת, בגלל העבודה על התערובת במהלך הדחיסה, הטמפרטורה שלה אכן עולה באופן משמעותי. למעשה, עליית הטמפרטורה יכולה להיות כה גדולה עד שהתערובת יכולה להתפוצץ ללא תוספת של ניצוץ. פיצוצים כאלה, מכיוון שהם אינם מתוזמנים, גורמים למכונית לפעול בצורה גרועה—היא בדרך כלל “נוקשת.” כאשר טמפרטורת ההצתה עולה עם האוקטן של הבנזין, השימוש בבנזין בעל אוקטן גבוה הוא אחת הדרכים להתגבר על בעיה זו. תהליך אדיאבטי מעניין נוסף הוא התפשטות חופשית של גז. תארו לעצמכם שגז מוגבל על ידי ממברנה לצד אחד של מיכל דו תאים מבודד תרמי. כאשר הממברנה מנוקבת, הגז ממהר אל הצד הריק של המיכל, ובכך מתרחב בחופשיות. כאשר הגז מתרחב “כנגד ואקום”, הלחץ יורד (p = 0), הוא אינו פועל, ומכיוון שהכלי מבודד תרמי, ההתפשטות היא אדיאבטית. עם Q = 0 ו W = 0 בחוק הראשון, dU = 0, אז האנרגיה הפנימית במצב שיווי המשקל הראשוני והסופי זהה להתפשטות החופשית. אם הגז אידיאלי, האנרגיה הפנימית תלויה רק בטמפרטורה. לכן, כאשר גז אידיאלי מתרחב בחופשיות, הטמפרטורה שלו לא משתנה.</div>

כאשר גז אידיאלי נדחס באופן אדיאבטי, כלומר, מבלי להוסיף חום, העבודה נעשית על זה, ואת הטמפרטורה שלה עולה. בהרחבה אדיאבטית ...

Education

JoVE Core

Physics

פיזיקה

Unit 1

יחידה 1

The First Law of Thermodynamics

החוק הראשון של התרמודינמיקה

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

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 the mixture can raise its temperature enough for it to explode without a spark. When that explosion happens at the wrong time, the engine knocks and runs poorly. Higher-octane gasoline can help because it has a higher ignition temperature.

The transcript also describes free expansion of a gas. In this case, a gas starts on one side of a thermally insulated two-compartment container. When the membrane is punctured, the gas rushes into the empty side and expands into a vacuum. Because the pressure outside is zero, the gas does no work. Since the container is thermally insulated, the process is adiabatic. With Q = 0 and W = 0, the first law gives dU = 0, so the internal energy stays the same before and after the expansion. For an ideal gas, internal energy depends only on temperature, so the temperature does not change during free expansion.

Gas Temperature Changes in Adiabatic Processes

gas-temperature-changes-in-adiabatic-processes

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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Path Dependence in Gas Work

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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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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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

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...

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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

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

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.

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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 adiabatic compression and free expansion affect an ideal gas. See why temperature rises in compression, drops in expansion, and stays the same in free expansion.

Gas Temperature Changes in Adiabatic Processes | JoVE K12

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Gas Temperature Changes in Adiabatic Processes

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