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

Les fours à micro-ondes utilisent des ondes électromagnétiques pour chauffer les aliments.
La plupart des aliments contiennent des molécules polaires comme l’eau, qui se retournent d’avant en arrière avec le champ électrique alternatif. Lors du retournement, les molécules subissent des frottements avec d’autres molécules, ce qui génère de la chaleur.
La chambre à micro-ondes agit comme une cavité de résonance où les ondes électromagnétiques stationnaires sont formées par réflexion sur la surface métallique. Les deux surfaces métalliques sont placées sur des plans nodaux.
Ainsi, la distance entre les plans doit être un multiple intégral de la moitié de la longueur d’onde.
Les longueurs d’onde qui satisfont à cette condition sont appelées longueurs d’onde caractéristiques, et les fréquences correspondantes sont appelées fréquences caractéristiques.
Chaque fréquence caractéristique, forme d’onde et motif de nœud constitue un ensemble de modes normaux.
Par exemple, un micro-ondes domestique typique fonctionne à 2,4 gigahertz, ce qui correspond à une longueur d’onde facilement absorbée par les molécules d’eau.
Cette longueur d’onde a des nœuds à 6,1 centimètres de distance, ce qui signifie que la nourriture sera froide à ces points et chauffée au maximum aux antinœuds.
Ainsi, le plateau tournant du micro-ondes est tourné pour avoir une distribution uniforme de la chaleur.

<ol>
	<li>Young, H.D. et Freedman, R.A. (2012). La physique universitaire avec la physique moderne. San Francisco, Californie : Pearson. l’article 32.5 ; page 1071.</li>
</ol>

ondes stationnaires dans une cavité

Un four à micro-ondes domestique et des lasers sont des exemples d'ondes électromagnétiques stationnaires dans une cavité. Lorsque deux plaques métalliques conductrices sont placées en parallèle aux plans nodaux, cela crée une cavité où des ondes stationnaires se forment. La cavité entre les deux plans est analogue à une corde tendue tenue aux points x = 0 et x = L. Ici, la distance 'L' entre les deux plans doit être un multiple entier de la moitié de la longueur d'onde. Les longueurs d'onde qui satisfont à cette condition sont données par :

<img alt="Equation1" src="/files/ftp_upload/14213/14213_Equation_1.png" style="text-align: center; height: 40px;" />

Les fréquences correspondantes sont :

<img alt="Equation2" src="/files/ftp_upload/14213/14213_Equation_2.png" style="text-align: center; height: 40px;" />

Chaque fréquence caractéristique, forme d'onde et schéma de nœuds constituent un ensemble de modes normaux. La longueur d'onde peut être estimée en mesurant les positions des nœuds et, si la fréquence est connue, la vélocité de l'onde peut également être déterminée.

En plus des surfaces conductrices, la réflexion des ondes électromagnétiques peut également se produire à une interface entre deux matériaux isolants avec différentes propriétés diélectriques ou magnétiques. L'analogie mécanique est une jonction de deux cordes avec une tension égale mais une densité de masse linéaire différente. Généralement, une onde incidente sur une telle surface limite est partiellement transmise dans le deuxième matériau et partiellement réfléchie dans le premier. Par exemple, la lumière est transmise à travers une fenêtre en verre, mais ses surfaces réfléchissent également la lumière.

Ondes stationnaires dans une cavité

Un micro-ondes domestique et des lasers sont des exemples d’ondes électromagnétiques stationnaires dans une cavité. Lorsque deux plaques métalliques conductrices sont placées parallèlement au niveau ...

Education

Enseignement

JoVE Core

Physics

Physique

Unit 1

Unité 1

Electromagnetic Waves

Ondes électromagnétiques

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

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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