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

I forni a microonde utilizzano le onde elettromagnetiche per riscaldare il cibo.
La maggior parte degli alimenti contiene molecole polari come l'acqua, che si muovono avanti e indietro con il campo elettrico alternato. Durante il capovolgimento, le molecole subiscono attrito con altre molecole, di conseguenza viene generato calore.
La camera a microonde funge da cavità risonante in cui si formano onde elettromagnetiche stazionarie per riflessione dalla superficie metallica. Le due superfici metalliche sono posizionate in corrispondenza di piani nodali.
Quindi, la distanza tra i piani deve essere un multiplo intero della metà della lunghezza d'onda.
Le lunghezze d'onda che soddisfano questa condizione sono chiamate lunghezze d'onda caratteristiche e le frequenze corrispondenti sono chiamate frequenze caratteristiche.
Ogni frequenza caratteristica, forma d'onda e modello di nodo costituisce un insieme di modi normali.
Ad esempio, un tipico forno a microonde domestico funziona a 2,4 gigahertz, corrispondente a una lunghezza d'onda prontamente assorbita dalle molecole d'acqua.
Questa lunghezza d'onda ha nodi a 6,1 centimetri di distanza, il che significa che il cibo sarà freddo in questi punti e riscaldato al massimo agli antinodi.
Quindi, il piatto girevole del microonde viene ruotato per avere una distribuzione uniforme del calore.

<ol>
	<li>Young, H.D. e Freedman, R.A. (2012). Fisica universitaria con fisica moderna. San Francisco, CA: Pearson. Sezione 32.5; pagina 1071.</li>
</ol>

onde stazionarie in una cavità

Un forno a microonde domestico e i laser sono esempi di onde elettromagnetiche stazionarie in una cavità. Quando due piastre metalliche conduttive vengono posizionate parallelamente ai piani nodali, si crea una cavità dove si formano onde stazionarie. La cavità tra le due piastre è analoga a una corda tesa tenuta ai punti x=0 e x=L. Qui, la distanza 'L' tra le due piastre deve essere un multiplo intero della metà della lunghezza d'onda. Le lunghezze d'onda che soddisfano questa condizione sono date da:

<img alt="Equation1" src="/files/ftp_upload/14213/14213_Equation_1.png" style="text-align: center; height: 40px;" />

Le frequenze corrispondenti sono:

<img alt="Equation2" src="/files/ftp_upload/14213/14213_Equation_2.png" style="text-align: center; height: 40px;" />

Ogni frequenza caratteristica, forma d'onda e modello di nodi costituiscono un insieme di modi normali. La lunghezza d'onda può essere stimata misurando le posizioni dei nodi, e se la frequenza è nota, la velocità dell'onda può anche essere determinata.

Oltre alle superfici conduttrici, la riflessione delle onde elettromagnetiche può verificarsi anche all'interfaccia tra due materiali isolanti con diverse proprietà dielettriche o magnetiche. L'analogia meccanica è una giunzione di due corde con uguale tensione ma diversa densità lineare di massa. Tipicamente, un'onda incidente su una tale superficie di confine viene parzialmente trasmessa nel secondo materiale e parzialmente riflessa nel primo. Per esempio, la luce viene trasmessa attraverso una finestra di vetro, ma le sue superfici riflettono anche la luce.

Onde stazionarie in una cavità

Un forno a microonde domestico e i laser sono esempi di onde elettromagnetiche stazionarie in una cavità. Quando due piastre metalliche conduttrici sono poste parallelamente ...

Education

Didattica

JoVE Core

Physics

Fisica

Unit 1

Unità 1

Electromagnetic Waves

Onde elettromagnetiche

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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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Learn how standing electromagnetic waves form in a cavity, why only certain wavelengths fit, and how reflections create partial transmission at boundaries.

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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