Name: electromagnetic wave reflection at conductors
Uploaded: 2023-09-18T17:11:17.000Z
Duration: 1 min 15 s
Channel: JoVE

K12 Standards

Virginia  Standards of Learning

HIgh School

Physical Science

PS.7

PS.7.a

Standing Electromagnetic Waves

היזכרו בגלים העומדים שנוצרו במיתר הדוק. 
גלים עומדים הם שילוב של שני גלים הנעים בכיוון ההפוך אך בעלי אותה משרעת ותדירות.
חשבו על גיליון מוליך מושלם הממוקם במישור y-z. גל אלקטרומגנטי מקוטב ליניארי פוגע ביריעה, תוך כדי נסיעה בכיוון x שלילי.
גל האירוע משרה את הזרם על פני השטח של המוליך, וכתוצאה מכך שדות חשמליים נוספים, המייצרים גל מוחזר באותה תדירות ומשרעת כמו גל האירוע.
הסופרפוזיציה של שני גלים אלה יוצרת את הגל העומד. פישוט ביטויים אלה מספק את הנקודות על הגל שבהן סדרי הגודל של השדה החשמלי והשדה המגנטי הם אפס. אלה נקראים הצמתים או מישורי הצמתים.
אנטי-צומת הוא נקודה שבה המשרעת עוברת מהתזוזה החיובית המקסימלית לתזוזה השלילית המקסימלית.
הצמתים של השדה החשמלי חופפים לאנטי-צמתים של השדות המגנטיים, ולהיפך. לפיכך, הם 90 מעלות מחוץ לפאזה בכל נקודה.

<ol>
	<li>יאנג, ה.ד. ופרידמן, ר. (2012). פיזיקה אוניברסיטאית עם פיזיקה מודרנית. סן פרנסיסקו, קליפורניה: פירסון. סעיף 32.5; עמודים 1069–1070.</li>
</ol>

גלים אלקטרומגנטיים עומדים

<div style="direction:rtl">
	
	גלים אלקטרומגנטיים יכולים להשתקף; פני השטח של מוליך או דיאלקטרי יכולים לשמש כמשקף. כפי ששדות חשמליים ומגנטיים מצייתים לעקרון הסופרפוזיציה, כך גם גלים אלקטרומגנטיים מצייתים. הסופרפוזיציה של גל בולט וגל אלקטרומגנטי מוחזר מייצרת גל עומד בדומה לגלים העומדים שנוצרו על מיתר מתוח. 

	נניח שגיליון של מוליך מושלם ממוקם במישור yz, וגל אלקטרומגנטי מקוטב ליניארי הנוסע בכיוון x השלילי פוגע בו. מכיוון שלשדה החשמלי לא יכול להיות רכיב מקביל למשטח מוליך מושלם, השדה החשמלי חייב להיות אפס בכל מקום במישור הyz. השדה החשמלי של הגל האלקטרומגנטי המתרחש אינו אפס בכל עת במישור הyz, אך גל זה משרה זרמים מתנודדים על פני המוליך, מה שמוביל לשדה חשמלי נוסף . השדה החשמלי נטו, שהוא הסכום הווקטורי של שדה זה ושל שדה האירוע, הוא אפס בכל מקום בתוך ועל פני המוליך. הזרמים המושרים על פני המוליך מייצרים גם גל מוחזר שעובר החוצה מהמטוס. עקרון הסופרפוזיציה קובע כי השדה הכולל בכל נקודה הוא הסכום הווקטורי של השדות החשמליים של התרחשות והגלים המשתקפים, בדומה לשדה המגנטי. הסופרפוזיציה של גלים מקריים וגלים מוחזרים מייצרים גלים עומדים.
	<img alt="Equation1" src="/files/ftp_upload/13829/13829_Equation_1.png" style="text-align: center; height: 24px;"> 
	<img alt="Equation2" src="/files/ftp_upload/13829/13829_Equation_2.png" style="text-align: center; height: 22px;"> 
	
	פישוט הביטויים הללו מספק את הנקודות על הגל שבהן גודל השדה החשמלי והשדה המגנטי הם אפס. אלה נקראים צמתים או מישורי צמתים. באמצע הדרך בין כל שני מישורים צמתים סמוכים נמצאים המישורים בעלי המשרעת המקסימלית; אלו הם המישורים האנטי-נודליים. 

	השדה החשמלי הכולל הוא פונקציית סינוס, והשדה המגנטי הכולל הוא פונקציית קוסינוס. לכן, הווריאציות הסינוסואידאליות של שני השדות אינן בפאזה בכל נקודה. צמתי השדה החשמלי חופפים לאנטי-הצמתים של השדות המגנטיים ולהיפך. לפיכך, הם 90° מחוץ לפאזה בכל נקודה. זאת בניגוד לגל הנעים בכיוון אחד, שעבורו הווריאציות הסינוסואידאליות של השדות החשמליים והמגנטיים נמצאים בשלב בכל נקודה מסוימת.
 </div>

גלים אלקטרומגנטיים יכולים להשתקף; פני השטח של מוליך או דיאלקטרי יכולים לשמש כרפלקטור. כמו שדות חשמליים ומגנטיים לציית ...

Education

JoVE Core

Physics

פיזיקה

Unit 1

יחידה 1

Electromagnetic Waves

גלי אלקטרומגנטיים

KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

electromagnetic wave reflection at conductors

Electromagnetic waves can reflect from a conductor or a dielectric. When the reflected wave combines with the incoming wave, the result is a standing electromagnetic wave. This is similar to the standing waves formed on a stretched string.

To see how this happens, consider a perfect conductor placed in the yz-plane. A linearly polarized electromagnetic wave moving in the negative x-direction strikes the surface. The electric field cannot have a component parallel to the surface of a perfect conductor, so the electric field must be zero everywhere in the yz-plane.

The incident wave is not zero at the surface, but it induces oscillating currents on the conductor. These currents create an additional electric field. The net electric field is the vector sum of the incident and induced fields, and it is zero everywhere inside and on the conductor's surface. The same surface currents also produce a reflected wave that moves away from the plane.

The total field at any point is the vector sum of the incident and reflected electric fields, and the same idea applies to the magnetic field. This superposition creates standing waves. The places where the electric or magnetic field magnitude is zero are called nodes, or nodal planes. The planes halfway between adjacent nodes have maximum amplitude and are called antinodal planes.

For these standing waves, the total electric field follows a sine function, while the total magnetic field follows a cosine function. The two fields are therefore 90° out of phase at each point. Electric field nodes line up with magnetic field antinodes, and magnetic field nodes line up with electric field antinodes. This is different from a wave traveling in one direction, where the electric and magnetic fields are in phase at a given point.

Electromagnetic Wave Reflection at Conductors

electromagnetic-wave-reflection-at-conductors

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Explore how reflected electromagnetic waves form standing patterns at conductors, with nodes, antinodes, and electric and magnetic fields out of phase.

Electromagnetic Wave Reflection at Conductors | JoVE K12

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PS.7.b

Electromagnetic Wave Reflection at Conductors

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