Name: heat and phase change energy
Uploaded: 2023-04-30T12:29:47.000Z
Duration: 1 min 19 s
Channel: JoVE

K12 Standards

Minnesota Academic Standards in Science

9-12

Physics

1.2.1 9C.4.2.1.2

Phase Changes

Principalmente ci sono tre diverse fasi della materia: solida, liquida e gassosa.
Al variare della temperatura, uno stato si trasforma in un altro. Questo fenomeno è chiamato cambiamento di fase, che è di natura reversibile.
Al riscaldamento, un solido si trasforma in liquido, questo processo è chiamato fusione, mentre viceversa è chiamato congelamento.
Il calore assorbito o rilasciato in questa transizione per un materiale di massa 'm' può essere valutato utilizzando il calore di fusione, che è l'energia per unità di massa richiesta o rilasciata nel cambiamento di fase.
Allo stesso modo, se un liquido viene riscaldato, si converte in un gas e il processo è chiamato vaporizzazione o bollitura. Durante il raffreddamento, la trasformazione di un gas in un liquido è chiamata condensazione.
L'energia per unità di massa richiesta o rilasciata nel cambiamento di fase tra liquido e gas è chiamata calore di vaporizzazione, dove Q è il calore assorbito o rilasciato.
Alcuni dei materiali come il ghiaccio secco possono avere direttamente una transizione da solido a gassoso, che si chiama sublimazione.

<ol>
	<li>OpenStax. (2019). Fisica universitaria Vol. 2. [Versione web]. Estratto da <a href="https://openstax.org/details/books/university-physics-volume-2">https://openstax.org/details/books/university-physics-volume-2</a>; pagine 28-31.</li>
</ol>

cambiamenti di stato

Le transizioni di stato giocano un ruolo teorico e pratico importante nello studio del flusso di calore. Nello scioglimento o fusione, un solido diventa un liquido; il processo opposto è il congelamento. Nell'evaporazione, un liquido diventa un gas; il processo opposto è la condensazione.

Una sostanza si scioglie o congela ad una temperatura chiamata punto di scioglimento o bolle o condensa al suo punto di ebollizione. Queste temperature dipendono dalla pressione. L'alta pressione favorisce la forma più densa di una sostanza, così tipicamente, l'alta pressione alza il punto di scioglimento e di ebollizione, la bassa pressione li abbassa.

Per esempio, il punto di ebollizione dell'acqua è 100°C ad 1atm. A pressioni più elevate, il punto di ebollizione è più alto, e a pressioni più basse, è minore. L'eccezione più importante è il punto di scioglimento o congelamento dell'acqua.

L'energia richiesta in un cambiamento di stato dipende dal numero di legami o coppie forzate e dalla loro forza. Il numero di legami è proporzionale al numero di molecole e, pertanto, alla massa del campione. L'energia per unità di massa richiesta per cambiare una sostanza da uno stato solido a quello liquido, o rilasciata quando una sostanza cambia da liquida a solida, è conosciuta come il calore di fusione. L'energia per unità di massa richiesta per cambiare una sostanza dallo stato liquido a quello aeriforme è conosciuta come calore di vaporizzazione. Inoltre, il calore di fusione latente e il calore di vaporizzazione latente sono costanti materiali che possono essere sperimentalmente determinati. Queste costanti sono “latenti”, o nascoste, perché, in fase di cambiamento, l'energia entra o esce da un sistema senza causare un cambiamento di temperatura, quindi, in effetti, l'energia è nascosta.

Cambiamenti di stato

Le transizioni di fase svolgono un importante ruolo teorico e pratico nello studio del flusso di calore. Nella fusione o fusione, un solido si trasforma in un liquido; Le...

Education

Didattica

JoVE Core

Fisica

Unit 1

Unità 1

Temperature and Heat

Temperatura e Calore

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

heat and phase change energy

Heat and phase change energy explain why substances melt, freeze, boil, and condense. A solid turns into a liquid during melting, also called fusion. The reverse process is freezing. A liquid turns into a gas during evaporation. The reverse process is condensation.

Each substance changes phase at specific temperatures. The melting point is the temperature where a substance melts or freezes. The boiling point is the temperature where a substance boils or condenses. These temperatures depend on pressure. In most cases, higher pressure raises the melting point and boiling point because it favors the denser form of the substance. Lower pressure usually lowers them.

Water shows a useful example. At 1.00 atm, water boils at 100 °C. At higher pressures, its boiling point is higher. At lower pressures, its boiling point is lower. Water is also the main exception to the usual pattern for melting and freezing.

Phase changes also involve energy. The amount of energy depends on the number of bonds, or force pairs, and on their strength. Because the number of bonds depends on the number of molecules, the energy needed also depends on the mass of the sample. The energy per unit mass needed to change a solid into a liquid, or released when a liquid becomes a solid, is called the heat of fusion. The energy per unit mass needed to change a liquid into a vapor is called the heat of vaporization.

The latent heat of fusion and the latent heat of vaporization are material constants. They can be measured experimentally. These heats are called latent, or hidden, because energy enters or leaves the system during a phase change without changing the temperature. In that sense, the energy is hidden.

Heat and Phase Change Energy

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Heat and temperature are key ideas in thermodynamics and in everyday life. People often connect temperature with hot and cold, but science gives it a more precise meaning. Temperature is the quantity measured with a thermometer. It is also linked to the average translational kinetic energy of particles. Heat is different from temperature. Heat is energy that moves from one place or material to another because there is a difference in temperature. That difference in temperature keeps heat...

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The zeroth law of thermodynamics explains thermal equilibrium between objects. If object A is in equilibrium with object B, and object B is in equilibrium with object C, then object A is also in equilibrium with object C. This transitive relationship is the key idea behind the law. A simple example uses metal blocks and a metal plate at room temperature. A cold metal block and a hot metal block are both placed on the plate. Over time, the cold block and the plate reach thermal equilibrium, and...

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thermal stress

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Thermal stress is the stress that builds up when an object changes temperature but cannot expand or contract freely. If the object would expand without restraint, the stress is compressive. If it would contract, the stress is tensile. This stress from heating or cooling can become large enough to cause damage. Engineers often reduce it by designing parts so they can move as temperature changes. Highway blocks, for example, are separated by gaps so thermal stress does not build up. When gaps...

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Heat is energy transferred because of a temperature difference, and it can change an object’s temperature. Since heat is energy, its SI unit is the joule (J). Heat transfer is the movement of energy from one place or material to another, and it plays a role in home heating, cooking, and many industrial processes. Two common units for heat are the calorie and the kilocalorie. A calorie (cal) is the energy needed to change the temperature of 1 g of water by 1 °C, specifically between 14.5 °C and...

phase changes

Heat and phase change energy explain why substances melt, freeze, boil, and condense. A solid turns into a liquid during melting, also called fusion. The reverse process is freezing. A liquid turns into a gas during evaporation. The reverse process is condensation. Each substance changes phase at specific temperatures. The melting point is the temperature where a substance melts or freezes. The boiling point is the temperature where a substance boils or condenses. These temperatures depend on...

calorimetry

Heat Exchange in an Insulated System

Calorimetry studies heat exchange in an insulated system. A calorimeter is a container that prevents heat transfer in or out. Scientists use calorimetry to measure heat and specific heat capacity. A calorimetry problem is any situation where the objects are thermally isolated from their surroundings. The key idea comes from conservation of energy. During heat transfer, the heat gained by the colder object must equal the heat lost by the hotter object. A simple example is water poured into a...

mechanisms of heat transfer i

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Heat transfer happens when there is a temperature difference. It can move fast, like through a cooking pan, or slowly, like through the walls of a picnic ice box. Heat transfer appears in many everyday situations, so it is hard to find a place where it does not occur. There are only three ways heat is transferred: conduction, convection, and radiation. A fireplace shows all three at once. Radiation carries most of the heat into the room. Conduction also moves heat into the room, but much more...

mechanisms of heat transfer ii

Convection and Radiation in Heat Transfer

Convection and radiation are two major ways heat moves. In convection, thermal energy is carried by the large-scale flow of matter. Warm air and water rise because they are less dense, so ocean currents and large-scale atmospheric circulation move heat from the tropics toward the poles and move cold air from the poles toward the tropics. Earth’s rotation changes these moving air currents. It interacts with the flow and helps produce the eastward movement of air in the temperate zones.

conduction, convection and radiation: problem solving

conduction-convection-and-radiation-problem-solving

Heat Transfer Calculations with Conduction, Convection

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radiation: applications

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Earth’s energy balance and the greenhouse effect help explain its average temperature. Earth is in radiative contact with both the Sun and dark space. It receives almost all of its energy from solar radiation, and it reflects some of that energy back into outer space. Dark space is very cold, about 3 K, so Earth also radiates energy into it. Heat can move quickly from soil and grasses, and frost can form on clear summer evenings, even in warm latitudes. In a first approximation, Earth’s...

absorption of radiation

Radiation Heat Transfer and Thermographs

Radiation heat transfer depends on temperature, surface area, and emissivity. The Stefan-Boltzmann law describes the rate of heat transfer by emitted radiation. In this law, σ is the Stefan-Boltzmann constant, A is the object’s surface area, and T is its temperature in kelvins. The fourth-power dependence on absolute temperature makes radiation highly sensitive to small temperature changes. Even a slight rise in temperature can change the emitted heat a lot. That sensitivity is why radiation...

key terms and concepts

Explore melting, freezing, boiling, and condensation, plus how pressure changes melting and boiling points and why latent heat stays hidden.

Heat and Phase Change Energy | JoVE K12

physics

JoVE Core Physics introduces the theory and application of fundamental physics topics, including kinematics, dynamics, fluid mechanics, statics, waves, and acoustics, with concise and easy-to-understand animated videos. Additionally, the scientist-in-action videos demonstrate the application of related concepts in classical and original research experiments performed in today's laboratories worldwide.

Arizona Science Standards

High School

Physical Science

HS+Phy.P4

HS+Phy.P4U1.6

Texas Essential Knowledge and Skills

Chemistry

CHEM.13

CHEM.13.D

Alabama Course of Study Science

Earth and Space Science

Earth's Systems

9-12.ESS.2.7

9-12.ESS.2.7.a

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Heat and Phase Change Energy

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