Ch. 22 Electromagnetic Waves

Transcription

1 Ch. 22 Electromagnetic Waves James Clerk Maxwell From his studies on electromagnetic induction, Maxwell realized: Accelerating Charges give rise to Electromagnetic Waves Maxwell's equations demonstrate that electricity, magnetism and light are all manifestations of the same phenomenon, namely the electromagnetic field.

2 Units of Chapter 22 Changing Electric Fields Produce Magnetic Fields; Maxwell s Equations Production of Electromagnetic Waves Light as an Electromagnetic Wave and the Electromagnetic Spectrum Measuring the Speed of Light Energy in EM Waves Momentum Transfer and Radiation Pressure Radio and Television; Wireless Communication

3 22.1 Changing Electric Fields Produce Magnetic Fields; Maxwell s Equations Maxwell s equations are the basic equations of electromagnetism. They involve calculus; here is a summary: 1. Gauss s law relates electric field to charge 2. A law stating there are no magnetic charges 3. A changing electric field produces a magnetic field 4. A magnetic field is produced by an electric current, and also by a changing electric field

4 22.2 Production of Electromagnetic Waves And A changing electric field produces a magnetic field A changing magnetic field produces an electric field Once sinusoidal fields are created they can propagate on their own. These propagating fields are called electromagnetic waves.

5 22.2 Production of Electromagnetic Waves Maxwell invented the field called Dimensional Analysis Realized that the ratio of Electric to Magnetic Field had the dimensions of a velocity (m/s), and this term appeared in an expression for the velocity at which EM waves should travel E = v = c B This is the speed of light in a vacuum.

6 22.2 Production of Electromagnetic Waves Oscillating charges will produce electromagnetic waves Far from the source, the waves are plane waves

7 22.2 Production of Electromagnetic Waves The electric and magnetic waves are perpendicular to each other, and to the direction of propagation.

8 22.3 Light as an Electromagnetic Wave and the Electromagnetic Spectrum Light was known to be a wave, and experiments demonstrated that EM waves share all of the same behaviors. The frequency of an electromagnetic wave is related to its wavelength: c = 3x10 8 m/s This wave-speed equation is the same one that applies to sound, and any other waves. A few years after Maxwell s breakthrough, Tesla invented (and patented) radio technology (1897) Marconi commercialized it and Wireless communication revolutionized the world within a few short years. (First TransAt: 1901)

9 Electromagnetic waves can have any wavelength; we have given different names to different parts of the wavelength spectrum.

10 Electromagnetic Spectrum

11 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:

12 Measuring the Speed of Light Over the years, measurements have become more and more precise; now the speed of light is defined to be: This is then used to create a stable definition for the meter.

13 22.7 Radio, TV, Wireless Communication This figure illustrates the process by which a radio station transmits information. The audio signal is combined with a carrier wave: Remember those Capacitor Time Constants? Radio Transmitters contain a special circuit containing a coil and a capacitor, designed to oscillate at khz - MHz frequencies. This circuit creates the carrier wave. The audio signal is added by simply connecting the microphone output into the circuit.

14 Amplitude Modulation The mixing of signal and carrier can be done two ways. First, by using the signal to modify the amplitude of the carrier (AM): Note the audio signal is only approximated by the AM envelope. The closer together the carrier peaks the more faithfully the signal can be reproduced.

15 FM results in much better signal reproduction, partly because the signal strength is constant, and partly because it is achieved at higher frequencies (more bits of information per second) FM - Frequency Modulation Second, by using the signal to modify the frequency of the carrier (FM):

16 At the receiving end, the wave is received, demodulated, amplified, and sent to a loudspeaker: The receiving antenna is bathed in waves of many frequencies; a tuner is used to select the desired one:

17 Why use a carrier wave at all? -Why not just broadcast the raw audio signal as an electromagnetic wave? To reduce the wavelength for efficient transmission and reception (the optimum antenna size is ½ or ¼ of a wavelength). A typical audio frequency of 3000 Hz will have a wavelength of 100 km and So would need an effective antenna length of 25 km! (this goes for both reception and transmission) By comparison, a typical carrier for FM is 100 MHz, with a wavelength of 3m, and could use an antenna only 80 cm long. The energy Density in very long wavelength wave is so small that interference would be overwhelming. Long wavelengths do not behave in a very directional way (which for certain very specialized applications is extremely useful) Digital Radio is even more efficient than FM, it uses a very high frequency, and basically switches the signal on and off rapidly to transmit 1 s and zero s. Many more stations can occupy the the radio spectrum

18 Summary of Chapter 22 Maxwell s equations are the basic equations of electromagnetism Electromagnetic waves are produced by accelerating charges; the propagation speed is given by: The fields are perpendicular to each other and to the direction of propagation.

19 The simplest radio consists of something to rectify the signal (a diode in this case), and a tiny speaker A Diode is a one-way valve for electric current. It isolates the positive side of the radio waveform. Earpiece Diodes today use germanium or silicon (semiconductors) In the past Galena crystals were used, Even rusty razor blades and pencil leads can do the job in a pinch Diode