Name: electromagnetic modes in a microwave cavity
Uploaded: 2023-09-18T17:11:21.000Z
Duration: 1 min 28 s
Channel: JoVE

K12 Standards

Virginia  Standards of Learning

HIgh School

Physical Science

PS.7

PS.7.a

Standing Waves in a Cavity

Os fornos de microondas usam ondas eletromagnéticas para aquecer os alimentos.
A maioria dos alimentos contém moléculas polares como a água, que giram para frente e para trás com o campo elétrico alternado. Ao inverter, as moléculas sofrem atrito com outras moléculas, como resultado, o calor é gerado.
A câmara de microondas atua como uma cavidade ressonante onde as ondas eletromagnéticas estacionárias são formadas pela reflexão da superfície do metal. As duas superfícies metálicas são colocadas em planos nodais.
Portanto, a distância entre os planos deve ser um múltiplo integral de metade do comprimento de onda.
Os comprimentos de onda que satisfazem essa condição são chamados de comprimentos de onda característicos, e as frequências correspondentes são chamadas de frequências características.
Cada frequência característica, forma de onda e padrão de nó constitui um conjunto de modos normais.
Por exemplo, uma micro-ondas doméstica típica funciona a 2,4 gigahertz, correspondendo a um comprimento de onda prontamente absorvido pelas moléculas de água.
Esse comprimento de onda tem nós a 6,1 centímetros de distância, o que significa que a comida estará fria nesses pontos e aquecida ao máximo nos antinós.
Assim, o toca-discos do micro-ondas é girado para ter uma distribuição uniforme de calor.

<ol>
	<li>Young, H.D. e Freedman, R.A. (2012). Física Universitária com Física Moderna. São Francisco, CA: Pearson. Seção 32.5; página 1071.</li>
</ol>

ondas estacionárias em uma cavidade

Um forno de micro-ondas doméstico e lasers são exemplos de ondas eletromagnéticas estacionárias em uma cavidade. Quando duas placas de metal condutoras são colocadas em paralelo nos planos nodais, cria-se uma cavidade onde são formadas ondas estacionárias. A cavidade entre as duas placas é análoga a uma corda esticada, presa nos pontos x = 0 e x = L. Aqui, a distância 'L' entre as duas placas deve ser um múltiplo inteiro de metade do comprimento de onda. Os comprimentos de onda que satisfazem essa condição são dados por:

<img alt="Equation1" src="/files/ftp_upload/14213/14213_Equation_1.png" style="text-align: center; height: 40px;" />

E as frequências correspondentes são:

<img alt="Equation2" src="/files/ftp_upload/14213/14213_Equation_2.png" style="text-align: center; height: 40px;" />

Cada frequência característica, formato de onda e padrão de nó constituem um conjunto de modos normais. O comprimento de onda pode ser estimado medindo as posições dos nós e, se a frequência for conhecida, a velocidade da onda também pode ser determinada.

Além das superfícies condutoras, a reflexão das ondas eletromagnéticas também pode ocorrer em uma interface entre dois materiais isolantes com diferentes propriedades dielétricas ou magnéticas. O análogo mecânico é uma junção de duas cordas com tensão igual, mas densidade linear de massa diferente. Tipicamente, uma onda incidente em tal superfície limítrofe é parcialmente transmitida para o segundo material e parcialmente refletida de volta para o primeiro. Por exemplo, a luz é transmitida através de uma janela de vidro, mas suas superfícies também refletem a luz.

Ondas estacionárias em uma cavidade

Um micro-ondas doméstico e lasers são exemplos de ondas eletromagnéticas estacionárias em uma cavidade. Quando duas placas de metal condutoras são colocadas paralelamente no ...

Education

JoVE Core

Physics

Física

Unit 1

Unidade 1

Electromagnetic Waves

Ondas Eletromagnéticas

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

electromagnetic modes in a microwave cavity

Electromagnetic waves can form standing patterns inside a cavity. Household microwaves and lasers are common examples of this effect. When two conducting metal plates are placed parallel at the nodal planes, they create a space where standing waves can exist.

The cavity between the plates works like a stretched string fixed at x = 0 and x = L. The distance L must be an integer multiple of half a wavelength. The wavelengths that meet this condition have matching frequencies, and each frequency, wave shape, and node pattern is a normal mode.

Node positions can be measured to estimate the wavelength. If the frequency is known, the wave speed can also be found. Electromagnetic waves can also reflect at the boundary between two insulating materials with different dielectric or magnetic properties.

This kind of boundary acts like a junction between two strings with the same tension but different linear mass density. A wave that reaches the boundary is partly transmitted into the second material and partly reflected back. Light passing through a glass window is one example, since the surfaces transmit some light and also reflect some of it.

Electromagnetic Modes in a Microwave Cavity

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Electromagnetic waves can form standing patterns inside a cavity. Household microwaves and lasers are common examples of this effect. When two conducting metal plates are placed parallel at the nodal planes, they create a space where standing waves can exist. The cavity between the plates works like a stretched string fixed at x = 0 and x = L. The distance L must be an integer multiple of half a wavelength. The wavelengths that meet this condition have matching frequencies, and each frequency,...

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Electromagnetic Modes in a Microwave Cavity | JoVE K12

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PS.7.b

Electromagnetic Modes in a Microwave Cavity

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