Physics
How Are Electromagnetic Waves Produced
Electromagnetic waves are produced by the acceleration of charged particles. When charged particles are accelerated, they create changing electric and magnetic fields, which propagate through space as electromagnetic waves. This process occurs in various natural and human-made phenomena, such as the movement of electrons in an antenna or the oscillation of charged particles in the sun.
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12 Key excerpts on "How Are Electromagnetic Waves Produced"
eBook - PDF- Paul Peter Urone, Roger Hinrichs(Authors)
- 2012(Publication Date)
- Openstax(Publisher)
The list of the various types of electromagnetic waves, ranging from radio transmission waves to nuclear gamma-ray ( γ -ray) emissions, is interesting in itself. Even more intriguing is that all of these widely varied phenomena are different manifestations of the same thing—electromagnetic waves. (See Figure 24.2.) What are electromagnetic waves? How are they created, and how do they Chapter 24 | Electromagnetic Waves 953 travel? How can we understand and organize their widely varying properties? What is their relationship to electric and magnetic effects? These and other questions will be explored. Misconception Alert: Sound Waves vs. Radio Waves Many people confuse sound waves with radio waves, one type of electromagnetic (EM) wave. However, sound and radio waves are completely different phenomena. Sound creates pressure variations (waves) in matter, such as air or water, or your eardrum. Conversely, radio waves are electromagnetic waves, like visible light, infrared, ultraviolet, X-rays, and gamma rays. EM waves don’t need a medium in which to propagate; they can travel through a vacuum, such as outer space. A radio works because sound waves played by the D.J. at the radio station are converted into electromagnetic waves, then encoded and transmitted in the radio-frequency range. The radio in your car receives the radio waves, decodes the information, and uses a speaker to change it back into a sound wave, bringing sweet music to your ears. Discovering a New Phenomenon It is worth noting at the outset that the general phenomenon of electromagnetic waves was predicted by theory before it was realized that light is a form of electromagnetic wave. The prediction was made by James Clerk Maxwell in the mid-19th century when he formulated a single theory combining all the electric and magnetic effects known by scientists at that time. “Electromagnetic waves” was the name he gave to the phenomena his theory predicted.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Learning Press(Publisher)
Because the speed of EM waves predicted by the wave equation coincided with the measured speed of light, Maxwell concluded that light itself is an EM wave. According to Maxwell's equations, a spatially-varying electric field generates a time-varying magnetic field and vice versa . Therefore, as an oscillating electric field generates an oscillating magnetic field, the magnetic field in turn generates an oscillating electric field, and so on. These oscillating fields together form an electromagnetic wave. ________________________ WORLD TECHNOLOGIES ________________________ A quantum theory of the interaction between electromagnetic radiation and matter such as electrons is described by the theory of quantum electrodynamics. Properties Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This diagram shows a plane linearly polarized wave propagating from right to left. The electric field is in a vertical plane and the magnetic field in a horizontal plane. The physics of electromagnetic radiation is electrodynamics. Electromagnetism is the physical phenomenon associated with the theory of electrodynamics. Electric and magnetic fields obey the properties of superposition so that a field due to any particular particle or time-varying electric or magnetic field will contribute to the fields present in the same space due to other causes: as they are vector fields, all magnetic and electric field vectors add together according to vector addition. For instance, a travelling EM wave incident on an atomic structure induces oscillation in the atoms of that structure, thereby causing them to emit their own EM waves, emissions which alter the impinging ________________________ WORLD TECHNOLOGIES ________________________ wave through interference. These properties cause various phenomena including refraction and diffraction.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2015(Publication Date)
- Wiley(Publisher)
24.1 | The Nature of Electromagnetic Waves In Section 13.3 we saw that energy is transported to us from the sun via a class of waves known as electromagnetic waves. This class includes the familiar visible, ultraviolet, and infrared waves. In Sections 18.6, 21.1, and 21.2 we studied the concepts of electric and magnetic fields. It was the great Scottish physicist James Clerk Maxwell (1831–1879) who showed that these two fields fluctuating together can form a propagating electromagnetic wave. We will now bring together our knowledge of electric and magnetic fields in order to understand this important type of wave. Figure 24.1 illustrates one way to create an electromagnetic wave. The setup con- sists of two straight metal wires that are connected to the terminals of an ac generator and serve as an antenna. The potential difference between the terminals changes sinus- oidally with time t and has a period T. Part a shows the instant t 5 0 s, when there is no charge at the ends of either wire. Since there is no charge, there is no electric field at the point P just to the right of the antenna. As time passes, the top wire becomes positively charged and the bottom wire negatively charged. One-quarter of a cycle later ( t 5 1 4 T ), the charges have attained their maximum values, as part b of the drawing indicates. The corresponding electric field E B at point P is represented by the red arrow and has increased to its maximum strength in the downward direction.* Part b also shows that the electric field created at earlier times (see the black arrow in the picture) has not dis- appeared but has moved to the right. Here lies the crux of the matter: At distant points, the electric field of the charges is not felt immediately. Instead, the field is created first near the wires and then, like the effect of a pebble dropped into a pond, moves outward as a wave in all directions. Only the field moving to the right is shown in the picture for the sake of clarity.
The key is that the motion of a wave is not the same as the motion of the medium. The electromagnetic wave is, in a sense, an extension of this idea. It’s a wave that has no medium whatsoever, but simply keeps itself going through its own internal mechanisms. Electromagnetic waves, then, transfer energy—what we have called radiation (see Chapter 4). These waves are created when electrical charges accelerate, but once they start moving they no longer depend on the source that emitted them. LIGHT Once Maxwell understood the connection between electromagnetism and light, his equations allowed him to draw several important conclusions. For one thing, because the velocity of the electromagnetic waves depends entirely on the nature of interactions between electrical charges and magnets, it cannot depend on the properties of the wave itself. Thus, every electromagnetic wave, regardless of its wavelength or frequency, has to move at exactly the same velocity (Figure 6-12). This velocity—the speed of light— turns out to be so important in science that we give it a special letter, c. The speed of electromagnetic waves in a vacuum is one of the fundamental constants of nature. (Light moving through solids, liquids, or gases travels at a somewhat slower speed.) For electromagnetic waves traveling in the vacuum of space, the relation among velocity, wavelength, and frequency takes on a particularly simple form: wavelength 3 frequency 5 c 5 300,000 km / s 1 5186,000 mi / s 2 In other words, if you know the wavelength of an electromagnetic wave, you can calcu- late its frequency and vice versa. THE ENERGY OF ELECTROMAGNETIC WAVES Think about how you might produce an electromagnetic wave with a simple comb. Electromagnetic waves are generated any time a charged object is accelerated, so imag- ine combing your hair on a dry winter day when the comb picks up a static charge.
No longer available |Learn more- (Author)
- 2014(Publication Date)
- Learning Press(Publisher)
Because the speed of EM waves predicted by the wave equation coincided with the measured speed of light, Maxwell concluded that light itself is an EM wave. According to Maxwell's equations, a spatially-varying electric field generates a time-varying magnetic field and vice versa . Therefore, as an oscillating electric field generates an oscillating magnetic field, the magnetic field in turn generates an oscillating electric field, and so on. These oscillating fields together form an electromagnetic wave. ____________________ WORLD TECHNOLOGIES ____________________ A quantum theory of the interaction between electromagnetic radiation and matter such as electrons is described by the theory of quantum electrodynamics. Properties Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This diagram shows a plane linearly polarized wave propagating from right to left. The electric field is in a vertical plane and the magnetic field in a horizontal plane. The physics of electromagnetic radiation is electrodynamics. Electromagnetism is the physical phenomenon associated with the theory of electrodynamics. Electric and magnetic fields obey the properties of superposition so that a field due to any particular particle or time-varying electric or magnetic field will contribute to the fields present in the same space due to other causes: as they are vector fields, all magnetic and electric field vectors add together according to vector addition. For instance, a travelling EM wave incident on an atomic structure induces oscillation in the atoms of that structure, thereby causing them to emit their own EM waves, emissions which alter the impinging wave through interference. These properties cause various phenomena including refraction and diffraction.
eBook - PDF- William Moebs, Samuel J. Ling, Jeff Sanny(Authors)
- 2016(Publication Date)
- Openstax(Publisher)
• The symmetry introduced between electric and magnetic fields through Maxwell’s displacement current explains the mechanism of electromagnetic wave propagation, in which changing magnetic fields produce changing electric fields and vice versa. • Although light was already known to be a wave, the nature of the wave was not understood before Maxwell. Maxwell’s equations also predicted electromagnetic waves with wavelengths and frequencies outside the range of light. These theoretical predictions were first confirmed experimentally by Heinrich Hertz. 16.2 Plane Electromagnetic Waves • Maxwell’s equations predict that the directions of the electric and magnetic fields of the wave, and the wave’s direction of propagation, are all mutually perpendicular. The electromagnetic wave is a transverse wave. • The strengths of the electric and magnetic parts of the wave are related by c = E/B, which implies that the magnetic field B is very weak relative to the electric field E. • Accelerating charges create electromagnetic waves (for example, an oscillating current in a wire produces electromagnetic waves with the same frequency as the oscillation). 16.3 Energy Carried by Electromagnetic Waves • The energy carried by any wave is proportional to its amplitude squared. For electromagnetic waves, this means intensity can be expressed as I = cε 0 E 0 2 2 where I is the average intensity in W/m 2 and E 0 is the maximum electric field strength of a continuous sinusoidal wave. This can also be expressed in terms of the maximum magnetic field strength B 0 as I = cB 0 2 2µ 0 and in terms of both electric and magnetic fields as 730 Chapter 16 | Electromagnetic Waves This OpenStax book is available for free at http://cnx.org/content/col12074/1.3 I = E 0 B 0 2µ 0 . The three expressions for I avg are all equivalent. 16.4 Momentum and Radiation Pressure • Electromagnetic waves carry momentum and exert radiation pressure.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2021(Publication Date)
- Wiley(Publisher)
Therefore, they exist mainly near the antenna and together are called the near field. Electric and magnetic fields do form a wave at large distances from the antenna, however. These fields arise from an effect that is different from the one that produces the near field and are referred to as the radiation field. Faraday’s law of induction provides part of the basis for the radiation field. As Section 22.4 discusses, this law describes the emf or potential difference produced by a changing mag- netic field. And, as Section 19.4 explains, a potential difference can be related to an electric field. Thus, a changing magnetic field produces an electric field. Maxwell predicted that the reverse effect also occurs—namely, that a changing electric field produces a magnetic field. The radiation field arises because the changing magnetic field creates an electric field that fluctuates in time and the changing electric field creates the magnetic field. Figure 24.3 shows the electromagnetic wave of the radiation field far from the antenna. The picture shows only the part of the wave traveling along the +x axis. The parts traveling in the other directions have been omitted for clarity. It should be clear from the drawing that an electromagnetic wave is a transverse wave because the electric and magnetic fields are both perpendicular to the direction in which the wave travels. More- over, an electromagnetic wave, unlike a wave on a string or a sound wave, does not require a medium in which to propagate. Electromagnetic waves can travel through a vacuum or a material substance, since electric and magnetic fields can exist in either one. Electromagnetic waves can be produced in situations that do not involve a wire antenna. In general, any electric charge that is accelerating emits an electromagnetic wave, whether the charge is inside a wire or not.
eBook - PDF- John D. Cutnell, Kenneth W. Johnson, David Young, Shane Stadler(Authors)
- 2018(Publication Date)
- Wiley(Publisher)
LEARNING OBJECTIVES After reading this module, you should be able to... 24.1 Describe the nature of electromagnetic waves. 24.2 Calculate speed, frequency, and wavelength for electromagnetic waves. 24.3 Relate the speed of light to electromagnetic quantities. 24.4 Calculate energy, power, and intensity for electromagnetic waves. 24.5 Solve problems involving the Doppler effect for electromagnetic waves. 24.6 Solve polarization problems using Malus’ law. Terje Rakke/Getty Images CHAPTER 24 Electromagnetic Waves Each of the colors on the sails of these boats corresponds to a different wavelength in the visible region of the spectrum of electromagnetic waves. As we will see in this chapter, however, the visible wavelengths comprise only a small part of the total electromagnetic spectrum. 24.1 The Nature of Electromagnetic Waves In Section 13.3 we saw that energy is transported to us from the sun via a class of waves known as electromagnetic waves. This class includes the familiar visible, ultra- violet, and infrared waves. In Sections 18.6, 21.1, and 21.2 we studied the concepts of electric and magnetic fields. It was the great Scottish physicist James Clerk Maxwell (1831–1879) who showed that these two fields fluctuating together can form a propagating electromagnetic wave. We will now bring together our knowledge of electric and magnetic fields in order to understand this important type of wave. Animated Figure 24.1 illustrates one way to create an electromagnetic wave. The setup consists of two straight metal wires that are connected to the terminals of an ac generator and serve as an antenna. The potential difference between the terminals changes sinusoidally with time t and has a period T. Part a shows the instant t = 0 s, when there is no charge at the ends of either wire. Since there is no charge, there is no electric field at the point P just to the right of the antenna.
- Ian Poole(Author)
- 2003(Publication Date)
- Newnes(Publisher)
Radio waves As already mentioned radio signals are a form of electromagnetic wave. They consist of the same basic type of radiation as light, ultraviolet and infrared rays, differing from them in their wavelength and frequency. These waves are quite complicated in their make-up, having both electric and magnetic components that are inseparable. The planes of these fields are at right angles to one another and to the direction of motion of the wave. These waves can be visualized as shown in Figure 2.4. The electric field results from the voltage changes occurring in the antenna which is radiating the signal, and the magnetic field changes result from the current flow. It is also found that the lines of force in the electric field run along the same axis as the antenna, but spreading out as they move away from it. This electric field is measured in terms of the Figure 2.4 An electromagnetic wave The wavelength is the length from a point on one wave to the identical point on the next The best point to take is usually the peak Radio waves and propagation 21 change of potential over a given distance, e.g. volts per metre, and this is known as the field strength. There are a number of properties of a wave. The first is its wavelength. This is the distance between a point on one wave to the identical point on the next as shown in Figure 2.5. One of the most obvious points to choose is the peak as this can be easily identified although any point is acceptable. The second property of the electromagnetic wave is its frequency. This is the number of times a particular point on the wave moves up and down in a given time (normally a second). The unit of frequency is the hertz and it is equal to one cycle per second. This unit is named after the German scientist who discovered radio waves. The frequencies used in radio are usually very high. Accordingly the prefixes kilo, mega, and giga are often seen.
Available until 23 Dec |Learn more- Page Truitt(Author)
- 2021(Publication Date)
- Page Publishing, Inc.(Publisher)
CHAPTER 8 Production Of The Electromagnetic FieldUp to this point in the discussion, except for magnetic fields surrounding a conductor, electric and magnetic fields have been attached to particles. The fields have been in motion when the particles have been in motion. According to the books, however, accelerating fields can become detached from particles and move through space unattached to anything. Examples are light waves, radio waves, microwaves, x-rays, and many other forms of electromagnetic energy fields.I spent a large amount of time thinking about how an accelerating electron could release an electromagnetic field and failed miserably. I finally concluded that acceleration alone is inadequate to produce light if the production of light requires some portion of the electron’s field to be separated from the main body of the electron. Even when no separation is necessary, an oscillating motion is probably required to produce the alternating wave characteristics usually described in the literature. It is possible that the oscillation of the particle simply creates a wave of energy that somehow travels through the medium of space. It is, of course, possible that an accelerating electron can, in some unknown way, produce a light wave without any portion of the electron’s field becoming detached. The detachment hypothesis will be explored first with the other hypotheses being noted as the discussion progresses.Hypothesis number one: The detachment of the electromagnetic field from an electron requires the rapid oscillating motion of the particle.Since this hypothesis is the one most likely to be confirmed, it will be discussed first. Electromagnetic theory to date states simply that electromagnetic wave energy is released when a particle is accelerating . There appears to be no mechanism whereby the detachment of some portion of a particle’s electromagnetic field can occur with acceleration only; therefore this theoretical position is open to question. Acceleration is
- Bruce Hapke(Author)
- 2012(Publication Date)
- Cambridge University Press(Publisher)
2 Electromagnetic wave propagation 2.1 Maxwell’s equations Because reflectance spectroscopy uses electromagnetic radiation to probe matter, this book begins with Maxwell’s electromagnetic equations and the solutions to them that describe propagating plane waves. For those whose knowledge of vectors may be a bit rusty, a brief review of vector notation is provided in Appendix A.1. In their general form, Maxwell’s equations can be written as follows: div D e = ρ e , (2.1) div B m = 0, (2.2) curl E e = −∂ B m /∂t, (2.3) curl H m = j e + ∂ D e /∂t. (2.4) In these equations, E e is the electric field, D e is the electric displacement, B m is the magnetic-induction field, H m is the magnetic intensity, ρ e is the electric-charge density, j e is the electric current density, t is the time, and “div” and “curl” are, respectively, the vector divergence and curl operators. The reader is referred to the many excellent textbooks on electromagnetic theory for detailed derivations and more rigorous discussions of these equations, includ- ing Stratton (1941), Panofsky and Phillips (1962), Marion (1965), Elliott (1966), Landau and Lifschitz (1975), and Jackson (1999). Equation (2.1) states that electric charges can generate electric fields and that the field lines diverge from or converge toward the charges. Equation (2.2) states that there are no sources of magnetic fields that are analogous to electric charges; that is, magnetic monopoles do not exist. According to equation (2.3), electric fields can also be generated by magnetic fields that change with time, and electric fields generated in this manner tend to coil or curl around the magnetic-field lines. Simi- larly, according to equation (2.4), magnetic fields can be generated by both electric 5 6 Electromagnetic wave propagation currents and time-varying electric fields, and the magnetic lines of force tend to curl around these sources. Equations (2.1) – (2.4) are called the field equations.
eBook - PDF- Stephen McKnight, Christos Zahopoulos(Authors)
- 2015(Publication Date)
- Cambridge University Press(Publisher)
15.1 Maxwell’s equations and electromagnetic waves in free space We will begin our treatment with Maxwell’s equations in differential form, as intro- duced in the last chapter. In fact, the derivation of electromagnetic waves is one of the strongest motivations for studying the differential form of Maxwell’s equation. We recall that the differential Maxwell’s equations are local equations, connecting the spatial and time derivatives of the electric and magnetic fields at the same point r ! D ! ¼ ρ r ! E ! ¼ ∂ B ! ∂t r ! B ! ¼ 0 r ! H ! ¼ J ! þ ∂D ! ∂t : (15.1) As we pointed out in the last chapter, the relations between D ! and E ! , between H ! and B ! , and between J ! and E ! depend on the properties of the materials that the fields are in. The 317 electromagnetic properties of materials are completely determined by the three consti- tutive relations: D ! ¼ εE ! B ! ¼ μH ! J ! ¼ σ E ! (15.2) where the permittivity ε, the permeability μ, and the conductivity σ completely describe the electromagnetic properties of materials. Along with the Lorentz force F ! ¼ qE ! þ q v ! B ! , these equations and materials functions are sufficient to describe all of classical electro- magnetic, radio-frequency, and optical phenomena. We will begin by considering electromagnetic waves in free space (vacuum), where the permittivity and permeability are scalar quantities, ε ¼ ϵ o ¼ 8.8543 10 12 F/m, μ ¼ μ o ¼ 4π 10 7 H/m, with ρ ¼ 0 and σ ¼ 0, since there are no charges or currents in total vacuum.
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