Name: kelvin temperature and gas thermometer standards
Uploaded: 2023-04-30T12:29:16.000Z
Duration: 1 min 22 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry

CHEM.13

CHEM.13.D

Gas Thermometers and the Kelvin Scale

Um termômetro de gás avalia a temperatura com base nas mudanças na pressão ou no volume do gás preenchido.
Existem dois tipos de termômetros de gás: volume constante e pressão constante, e o termômetro de gás de volume constante é o mais utilizado.
O princípio deste termômetro é que, em volume constante, a pressão de um gás aumenta à medida que a temperatura aumenta.
Para calibrar o termômetro de gás, a pressão a 0 e 100 graus Celsius é medida.
Esses valores são plotados em linha reta, que é extrapolada para encontrar a temperatura de pressão zero.
Esta temperatura é o ponto zero da escala Kelvin. Esta é a escala padrão para temperatura, pois a unidade SI de temperatura é Kelvin.
O zero absoluto é a temperatura mais baixa possível que reflete idealmente a total ausência de energia térmica.
Na escala Kelvin, o ponto de congelamento da água é 273,15, enquanto o ponto de ebulição é 373,15. A diferença é a mesma da escala Celsius; apenas o ponto zero é diferente.
Assim, a relação entre as escalas Kelvin e Celsius pode ser facilmente estabelecida.

<ol>
	<li>OpenStax. (2019). Física Universitária Vol. 2. [Versão Web]. Obtido de <a href="https://openstax.org/books/university-physics-volume-2/pages/1-2-thermometers-and-temperature-scales">https://openstax.org/books/university-physics-volume-2/pages/1-2-thermometers-and-temperature-scales</a></li>
</ol>

termômetros de gás e a escala kelvin

A definição de temperatura em termos de movimento molecular sugere que deve existir uma temperatura mínima possível, onde a energia cinética média das moléculas é zero (ou o mínimo permitido pela mecânica quântica). Experimentos confirmam a existência dessa temperatura, chamada de zero absoluto. Uma escala de temperatura absoluta é aquela cujo ponto zero é o zero absoluto. Tais escalas são convenientes na ciência porque várias quantidades físicas, como o volume de um gás ideal, estão diretamente relacionadas à temperatura absoluta.

A escala Kelvin é a escala de temperatura absoluta comumente usada na ciência.

A escala Kelvin faz parte do sistema internacional de unidades, sendo assim, sua definição atual é mais complicada do que a mencionada acima. Primeiramente, ela não é definida em termos dos pontos de congelamento e ebulição da água, mas sim em termos do ponto triplo. O ponto triplo é a combinação única de temperatura e pressão na qual gelo, água líquida e vapor de água podem coexistir de forma estável. A temperatura do ponto triplo é definida como 273.16 K. Essa definição tem a vantagem de que, embora a temperatura de congelamento e de ebulição da água dependam da pressão, há apenas uma temperatura de ponto triplo.

Mesmo com pontos bem definidos na escala, diferentes termômetros fornecem resultados um pouco diferentes para outras temperaturas. Portanto, é necessário um termômetro padrão. Os metrologistas escolheram o termômetro de gás de volume constante para esse propósito. Um recipiente de volume constante cheio de gás é submetido a mudanças de temperatura, e a temperatura medida é proporcional à mudança de pressão. Os resultados dependem um pouco da escolha do gás, mas quanto menos denso for o gás no bulbo, melhores serão os resultados para diferentes gases. Se os resultados forem extrapolados para uma densidade zero, os resultados concordam muito bem, onde uma pressão zero corresponde a uma temperatura de zero absoluto. Os termômetros de gás de volume constante são grandes e alcançam o equilíbrio lentamente, portanto, eles são usados principalmente como padrões para calibrar outros termômetros.

Termômetros de gás e a Escala Kelvin

A definição de temperatura em termos de movimento molecular sugere que deve haver uma temperatura mais baixa possível, onde a energia cinética média ...

Education

JoVE Core

Physics

Física

Unit 1

Unidade 1

Temperature and Heat

Temperatura e Calor

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

kelvin temperature and gas thermometer standards

Kelvin temperature is the absolute scale used in science, and it starts at absolute zero. That zero point matches the lowest possible temperature, where molecular motion reaches zero or the minimum allowed by quantum mechanics. Scientists use this scale because some physical quantities, such as the volume of an ideal gas, depend directly on absolute temperature.

The Kelvin scale is part of the SI system of units, so its definition is more precise than simply using water’s freezing and boiling points. Instead, it is tied to the triple point of water. The triple point is the only temperature and pressure combination where ice, liquid water, and water vapor can all coexist. Its temperature is defined as 273.16 K, which is useful because it does not change with pressure the way water’s freezing and boiling points do.

Even with fixed reference points, different thermometers can give slightly different readings for other temperatures. For that reason, scientists need a standard thermometer. Metrologists use the constant-volume gas thermometer for this job. In this device, a gas in a sealed vessel stays at the same volume while temperature changes, and the temperature is found from the change in pressure.

The choice of gas can affect the results a little. However, the less dense the gas in the bulb, the better different gases agree. When the results are extrapolated to zero density, they match well, and zero pressure corresponds to absolute zero. Constant-volume gas thermometers are large and slow to reach equilibrium, so they are mainly used as standards to calibrate other thermometers.

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Kelvin Temperature and Gas Thermometer Standards

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