Chapter 22 Lecture (1)

Chapter 22 Lecture (1) - Chapter 22 Electromagnetic Waves...

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Unformatted text preview: Chapter 22 Electromagnetic Waves Waves Units of Chapter 22 •Light as an Electromagnetic Wave and the Electromagnetic Spectrum • Measuring the Speed of Light • Energy in EM Waves EM • Momentum Transfer and Radiation Pressure • Radio and Television; Wireless Communication ConcepTest ConcepTest 22.1a A loop with an AC current produces loop with an AC current a changing magnetic field. Two loops have the same area, but one is made of plastic and the other th copper. In which of the loops is the induced voltage greater? EM Waves I 1) the plastic loop 2) the copper loop 3) voltage is same in both plastic copper ConcepTest ConcepTest 22.1a A loop with an AC current produces loop with an AC current a changing magnetic field. Two loops have the same area, but one is made of plastic and the other th copper. In which of the loops is the induced voltage greater? Faraday’s law says nothing about law says nothing about the material: ΔΦ ε = −N B Δt The change in flux is the same (and N is the same), so the induced emf is the same same. EM Waves I 1) the plastic loop 2) the copper loop 3) voltage is same in both plastic copper ConcepTest ConcepTest 22.1b In which of the loops is the induced current greater? induced current greater? EM Waves II 1) the plastic loop 2) the copper loop 3) current is same in both plastic copper ConcepTest ConcepTest 22.1b In which of the loops is the induced current greater? induced current greater? Remember that I = V / R (Ohm’s th Law), and copper has smaller resistance, resistance so the copper loop has the greater current current. EM Waves II 1) the plastic loop 2) the copper loop 3) current is same in both plastic copper ConcepTest ConcepTest 22.1c A loop with an AC current produces loop with an AC current a changing magnetic field. Consider a copper loop and next to it it, imagine a loop of air of equal size. In which of the loops will the induced electric field be greater? EM Waves III 1) the plastic loop 2) the copper loop 3) Electric field is same in both air copper ConcepTest ConcepTest 22.1c A loop with an AC current produces loop with an AC current a changing magnetic field. Consider a copper loop and next to it it, imagine a loop of air of equal size. In which of the loops will the induced electric field be greater? Just as in the example with the as in the example with the plastic loop, the induced electric field will be the same in both ! EM Waves III 1) the plastic loop 2) the copper loop 3) Electric field is same in both air copper 22.2 Production of Electromagnetic Waves The electric and magnetic waves are perpendicular to each other, and to the direction of propagation direction of propagation. ConcepTest ConcepTest 22.2 Oscillations 1) In the north-south plane. th The electric field in an EM wave traveling northeast oscillates up and down. In ill what plane does the magnetic field oscillate? 2) In the up-down plane. up3) 3) In the NE-SW plane. th NE 4) In the NW-SE plane. NW5) In the east-west plane. th ConcepTest ConcepTest 22.2 Oscillations 1) In the north-south plane. th The electric field in an EM wave traveling northeast oscillates up and down. In ill what plane does the magnetic field oscillate? 2) In the up-down plane. up3) 3) In the NE-SW plane. th NE 4) In the NW-SE plane. NW5) In the east-west plane. th The magnetic field oscillates perpendicular to BOTH the magnetic field oscillates perpendicular to BOTH electric field and the direction of the wave. Therefore the magnetic field must oscillate in the NW-SE plane. 22.2 Production of Electromagnetic Waves When Maxwell calculated the speed of propagation of electromagnetic waves, he propagation of electromagnetic waves, he found: This is the speed of light in a vacuum. 22.3 Light as an Electromagnetic Wave and the Electromagnetic Spectrum the Electromagnetic Spectrum Light was known to be a wave; after producing electromagnetic waves of other frequencies, it was known to be an electromagnetic wave as well. The frequency of an electromagnetic wave is related to its wavelength: (22-4) 22.3 Light as an Electromagnetic Wave and the Electromagnetic Spectrum the Electromagnetic Spectrum Electromagnetic waves can have any waves can have any wavelength; we have given different names to different parts of the wavelength spectrum. Calculate the wavelength of a beam of visible red light from a laser at frequency 4.74 x1014 Hz. 633 nm 633 nm 22.4 Measuring the Speed of Light The speed of light was known to be very large, although careful studies of the orbits of Jupiter’s moons showed that it is finite. One important measurement, by Michelson, used a rotating mirror: 22.4 Measuring the Speed of Light Over the years, measurements have become more and more precise; now the speed of light more and more precise; now the speed of light is defined to be: This is then used to define the meter This is then used to define the meter. 22.5 Energy in EM Waves Energy is stored in both electric and magnetic fields giving the total energy density of an fields, giving the total energy density of an electromagnetic wave: (22-5) Each field contributes half the total energy density. (Use c=E/B) (U (22-6a) 22.5 Energy in EM Waves This energy is transported by the wave. 22.5 Energy in EM Waves The energy transported through a unit area per unit time is called the intensity: per unit time is called the intensity: (22-7) Its average value is given by: 22.6 Momentum Transfer and Radiation Pressure Radiation Pressure In addition to carrying energy, electromagnetic waves also carry momentum This means that waves also carry momentum. This means that a force will be exerted by the wave. Th The radiation pressure is related to the average th intensity. It is a minimum if the wave is fully absorbed: absorbed: And a maximum if it is fully reflected: 22.7 Radio and Television; Wireless Communication Wireless Communication This figure illustrates the process by which a radio station transmits information. The audio signal is combined with a carrier wave: 22.7 Radio and Television; Wireless Communication Wireless Communication The mixing of signal and carrier can be done two ways. First, by using the signal to modify the amplitude of the carrier (AM): 22.7 Radio and Television; Wireless Communication Wireless Communication Second, by using the signal to modify the by using the signal to modify the frequency of the carrier (FM): 22.7 Radio and Television; Wireless Communication Wireless Communication At the receiving end, the wave is received, demodulated, amplified, and sent to a loudspeaker: 22.7 Radio and Television; Wireless Communication Wireless Communication The receiving antenna is bathed in waves of receiving antenna is bathed in waves of many frequencies; a tuner is used to select the desired one: ConcepTest ConcepTest 22.3 Before the days of cable, the days of cable, televisions often had two antennae on them, one straight, and one circular. Which antenna Whi picked up the magnetic oscillations? TV Antennas 1) the circular one 2) the straight one 3) both equally, they were th straight and circular for different reasons. ConcepTest ConcepTest 22.3 Before the days of cable, the days of cable, televisions often had two antennae on them, one straight, and one circular. Which antenna Whi picked up the magnetic oscillations? The varying B field in the loop means the flux is changing and therefore an EMF is induced. TV Antennas 1) the circular one 2) the straight one 3) both equally, they were th straight and circular for different reasons. ConcepTest ConcepTest 22.4 If a radio transmitter has a vertical antenna, should a receiver’s antenna be vertical or horizontal to obtain the best reception? Radio Antennas 1) vertical 2) horizontal horizontal 3) doesn’t matter ConcepTest ConcepTest 22.4 If a radio transmitter has a vertical antenna, should a receiver’s antenna be vertical or horizontal to obtain the best reception? Radio Antennas 1) vertical 2) horizontal horizontal 3) doesn’t matter If a wave is sent out from a vertical antenna, antenna the electric field oscillates up and down. Thus, the receiver’s up and down Thus the receiver’s E field fi antenna antenna should also be vertical so of wave that the arriving electric field can set the charges in motion. E field of wave ConcepTest ConcepTest 22.5 Heat Insulation Imagine you are an alien from another planet with infrared eyes Imagine you are an alien from another planet with infrared eyes. What What do you see when you look around the room? 1) Bright spots where the bodies are and dark elsewhere. 2) Dark spots where the bodies are and bright elsewhere. 3) The same as what we see, only everything looks red. 4) The same as what we see, except that red is invisible. ConcepTest ConcepTest 22.5 Heat Insulation Imagine you are an alien from another planet with infrared eyes Imagine you are an alien from another planet with infrared eyes. What What do you see when you look around the room? 1) Bright spots where the bodies are and dark elsewhere. 2) Dark spots where the bodies are and bright elsewhere. 3) The same as what we see, only everything looks red. 4) The same as what we see, except that red is invisible. Bodies are sources of heat and are sources of heat and therefore emit infrared radiation radiation. An alien with an instrument to detect infrared would see these detect infrared would see these sources sources as bright spots spots. Infrared photo of a building to check photo of building to check the heat insulation – where are the problem spots in this case? ConcepTest ConcepTest 22.6 Since Superman is from the planet Since Superman is from the planet Krypton Krypton his eyes are sensitive to the entire electromagnetic spectrum. Does that mean he can use x-ray th xvision to see that Lois Lane is being kidnapped in the other room? Superman 1) Yes, no problem 2) Nope, he can’t 3) Need more information ConcepTest ConcepTest 22.6 Since Superman is from the planet Since Superman is from the planet Krypton Krypton his eyes are sensitive to the entire electromagnetic spectrum. Does that mean he can use x-ray th xvision to see that Lois Lane is being Superman 1) Yes, no problem 2) Nope, he can’t 3) Need more information kidnapped in the other room? X-ray vision means that Superman’s eyes can receive x-rays, but not send them! receive send So what would have to happen for him to So what would have to happen for him to see Lois Lane being kidnapped? Summary of Chapter 22 • Maxwell’s equations are the basic equations of electromagnetism • Electromagnetic waves are produced by accelerating charges; the propagation speed accelerating charges; the propagation speed is given by: • The fields are perpendicular to each other fields are perpendicular to each other and to the direction of propagation. Summary of Chapter 22 • The wavelength and frequency of EM waves are related: • The electromagnetic spectrum includes electromagnetic spectrum includes all wavelengths, from radio waves through visible light to gamma rays. ...
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This note was uploaded on 09/08/2011 for the course PHYS 1403 taught by Professor Glab during the Summer '08 term at Texas Tech.

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