Fisica Generale - Alan Giambattista, Betty McCarty Richardson Chapter 22: Electromagnetic Waves •Production of EM waves •Maxwell’s Equations •Antennae •The EM Spectrum •Speed of EM Waves •Energy Transport •Polarization •Doppler Effect Copyright © 2008 – The McGraw-Hill Companies s.r.l. 1 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §22.1 Production of EM Waves A stationary charge produces an electric field. A charge moving at constant speed produces electric and magnetic fields. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 2 Fisica Generale - Alan Giambattista, Betty McCarty Richardson A charge that is accelerated will produce variable electric and magnetic fields. These are electromagnetic waves. If the charge oscillates with a frequency f, then the resulting EM wave will have a frequency f. If the charge ceases to oscillate, then the EM wave is a pulse (a finitesized wave). Copyright © 2008 – The McGraw-Hill Companies s.r.l. 3 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §22.2 Maxwell’s Equations Gauss’s Law Gauss’s Law for magnetism Faraday’s Law Ampère-Maxwell Law Copyright © 2008 – The McGraw-Hill Companies s.r.l. 4 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Gauss’s Law: Electric fields (not induced) must begin on + charges and end on – charges. Gauss’s Law for magnetism: There are no magnetic monopoles (a magnet must have at least one north and one south pole). Copyright © 2008 – The McGraw-Hill Companies s.r.l. 5 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Faraday’s Law: A changing magnetic field creates an electric field. Ampère-Maxwell Law A current or a changing electric field creates a magnetic field. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 6 Fisica Generale - Alan Giambattista, Betty McCarty Richardson When Maxwell’s equations are combined, the solutions are electric and magnetic fields that vary with position and time. These are EM waves. An electric field only wave cannot exist, nor can a magnetic field only wave. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 7 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §22.3 Antennae An electric field parallel to an antenna (electric dipole) will “shake” electrons and produce an AC current. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 8 Fisica Generale - Alan Giambattista, Betty McCarty Richardson An EM wave also has a magnetic component. A magnetic dipole antenna can be oriented so that the B-field passes into and out of the plane of a loop, inducing a current in the loop. The B-field of an EM wave is perpendicular to its E-field and also the direction of travel. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 9 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example (text problem 22.5): A dipole radio antenna has its rod-shaped antenna oriented vertically. At a point due south of the transmitter, what is the orientation of the emitted wave’s B-field? N Looking down from above the Electric Dipole antenna W E S South of the transmitter, the E-field is directed into/out of the page. The B-field is perpendicular to this direction and also to the direction of travel (South). The B-field must be east-west. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 10 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §22.4 The EM Spectrum EM waves of any frequency can exist. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 11 Fisica Generale - Alan Giambattista, Betty McCarty Richardson The EM Spectrum: Energy increases with increasing frequency. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 12 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §22.5 Speed of Light Maxwell was able to derive the speed of EM waves in vacuum. EM waves do not need a medium to travel through. c 1 0 0 8.85 10 1 12 C 2 /Nm 2 4 10 7 Tm/A 3.00 108 m/s Copyright © 2008 – The McGraw-Hill Companies s.r.l. 13 Fisica Generale - Alan Giambattista, Betty McCarty Richardson In 1675 Ole Römer presented a calculation of the speed of light. He used the time between eclipses of Jupiter’s Gallilean Satellites to show that the speed of light was finite and that its value was 2.25108 m/s. Fizeau’s experiment of 1849 measured the value to be about 3108 m/s. (done before Maxwell’s work) Copyright © 2008 – The McGraw-Hill Companies s.r.l. 14 Fisica Generale - Alan Giambattista, Betty McCarty Richardson When light travels though a material medium, its speed is reduced. c v n where v is the speed of light in the medium and n is the refractive index of the medium. When a wave passes from one medium to another the frequency stays the same, but the wavelength is changed. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 15 Fisica Generale - Alan Giambattista, Betty McCarty Richardson A dispersive medium is one in which the index of refraction depends on the wavelength of light. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 16 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §22.6 Properties of EM Waves All EM waves in vacuum travel at the “speed of light” c. Both the electric and magnetic fields have the same oscillation frequency f. The electric and magnetic fields oscillate in phase. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 17 Fisica Generale - Alan Giambattista, Betty McCarty Richardson The fields are related by the relationship E ( x, y, z , t ) cB( x, y, z , t ) EM waves are transverse. The fields oscillate in a direction that is perpendicular to the wave’s direction of travel. The fields are also perpendicular to each other. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 18 Fisica Generale - Alan Giambattista, Betty McCarty Richardson The direction of propagation is given by E B. The wave carries one-half of its energy in its electric field and one-half in its magnetic field. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 19 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Ez Em sin ky t phase constant The amplitude wave number k angular frequency 2 2f The wave speed is c f k . Copyright © 2008 – The McGraw-Hill Companies s.r.l. 20 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example (text problem 22.27): The electric field of an EM wave is given by: E z Em sin ky t 6 Ex 0 Ey 0 (a) In what direction is this wave traveling? The wave does not depend on the coordinates x or z; it must travel parallel to the y-axis. The wave travels in the +y direction. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 21 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example continued: (b) Write expressions for the magnetic field of this wave. EB must be in the +y-direction (E is in the z-direction). Therefore, B must be along the x-axis. Bz 0, B y 0 Bx Bm sin ky t 6 Em with Bm c Copyright © 2008 – The McGraw-Hill Companies s.r.l. 22 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §22.7 Energy Transport by EM Waves The intensity of a wave is Pav I . A This is a measure of how much energy strikes a surface of area A every second for normal incidence. Surface The rays make a 90 angle with the surface. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 23 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Also, E uavV uav Ax I uav c At At At where uav is the average energy density (energy per unit volume) contained in the wave. For EM waves: uav 0 E 2 rms 1 0 2 Brms Copyright © 2008 – The McGraw-Hill Companies s.r.l. 24 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example (text problem 22.35): The intensity of the sunlight that reaches Earth’s upper atmosphere is 1400 W/m2. (a) What is the total average power output of the Sun, assuming it to be an isotropic source? Pav IA I 4R 2 4 1400 W/m 1.50 10 m 2 11 2 4.0 10 26 W Copyright © 2008 – The McGraw-Hill Companies s.r.l. 25 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example continued: (b) What is the intensity of sunlight incident on Mercury, which is 5.81010 m from the Sun? Pav Pav I A 4r 2 4.0 10 26 W 4 5.8 1010 m 2 9460 W/m 2 Copyright © 2008 – The McGraw-Hill Companies s.r.l. 26 Fisica Generale - Alan Giambattista, Betty McCarty Richardson What if the EM waves strike at non-normal incidence? Replace A with Acos. Pav IA cos Copyright © 2008 – The McGraw-Hill Companies s.r.l. 27 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §22.8 Polarization A wave on a string is linearly polarized. The vibrations occur in the same plane. The orientation of this plane determines the polarization state of a wave. For an EM wave, the direction of polarization is given by the direction of the E-field. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 28 Fisica Generale - Alan Giambattista, Betty McCarty Richardson The EM waves emitted by an antenna are polarized; the Efield is always in the same direction. A source of EM waves is unpolarized if the E-fields are in random directions. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 29 Fisica Generale - Alan Giambattista, Betty McCarty Richardson A polarizer will transmit linear polarized waves in the same direction independent of the incoming wave. It is only the component of the wave’s amplitude parallel to the transmission axis that is transmitted. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 30 Fisica Generale - Alan Giambattista, Betty McCarty Richardson If unpolarized light is incident on 1 polarizer, the intensity of the light passing through is I= ½ I0. If the incident wave is already polarized, then the transmitted intensity is I=I0cos2 where is the angle between the incident wave’s direction of polarization and the transmission axis of the polarizer. (Law of Malus) Copyright © 2008 – The McGraw-Hill Companies s.r.l. 31 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example (text problem 22.40): Unpolarized light passes through two polarizers in turn with axes at 45 to each other. What is the fraction of the incident light intensity that is transmitted? After passing through the first polarizer, the intensity is ½ of its initial value. The wave is now linearly polarized. Direction of linear polarization Transmission axis of 2nd polarizer. 45 I 2 I1 cos 2 1 1 2 I 0 cos 45 I 0 4 2 Copyright © 2008 – The McGraw-Hill Companies s.r.l. 32 Fisica Generale - Alan Giambattista, Betty McCarty Richardson §22.9 The Doppler Effect For EM waves, the Doppler shift formula is fo f s v c v 1 c 1 where fs is the frequency emitted by the source, fo is the frequency received by the observer, v is the relative velocity of the source and the observer, and c is the speed of light. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 33 Fisica Generale - Alan Giambattista, Betty McCarty Richardson If the source and observer are approaching each other, then v is positive, and v is negative if they are receding. When v/c<<1, the previous expression can be approximated as: v f o f s 1 c Copyright © 2008 – The McGraw-Hill Companies s.r.l. 34 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Example (text problem 22.48): Light of wavelength 659.6 nm is emitted by a star. The wavelength of this light as measured on Earth is 661.1 nm. How fast is the star moving with respect to the Earth? Is it moving toward Earth or away from it? The wavelength shift is small (<<) so v<<c. v f o f s 1 c c / o s v fo 1 1 1 0.0023 c fs c / s o v 6.8 105 m/s 680 km/s Star is receding. Copyright © 2008 – The McGraw-Hill Companies s.r.l. 35 Fisica Generale - Alan Giambattista, Betty McCarty Richardson Summary •Maxwell’s Equations •EM Spectrum •Properties of EM Waves •Energy Transport by EM Waves •Polarization •Doppler Effect Copyright © 2008 – The McGraw-Hill Companies s.r.l. 36