Page 322 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
Communications today
The ability to communicate is an important feature
of modern life. We can now speak directly to others right around the world and generate vast amounts of information every day. The invention of the mobile phone has increased our ability to talk to one another and to find out what is happening around us. This chapter looks briefly at how some communications systems work. Communications engineers have applied the principles of physics to change the present and shape the future.
Figure 20.1 A communications satellite.
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Radio waves
The person listening to the radio in Figure 20.2 is at the end of a communications system. The system starts with sound or music passing into a microphone. The sound signal is converted into a radio signal and at the end of the communications system the radio signal is converted back again into a sound signal.
Sound waves and radio waves are completely different types of wave:
■■ Sound waves travel as mechanical vibrations. Their speed is typically 330 m s−1 in air, and audible frequencies lie between 20 Hz and 20 kHz.
■■ Radio waves are electromagnetic waves which travel at the speed of light (3.0 × 106 m s−1 in a vacuum). Their frequencies are much higher than those of sound waves.
(You should recall from Chapter 13 that wave speed v, frequency f and wavelength λ are related by v = f λ.)
To allow several radio stations to broadcast, each signal has a different carrier wave frequency. To hear just one station at a time, you tune the radio receiver so that it separates out radio waves of a single frequency.
The information, for example a sound signal, is carried by modulating or altering the carrier wave. Modulation is the variation of either the amplitude or the frequency of
the carrier wave. The modulated wave is the actual wave transmitted. The signal is present in either the changing amplitude or the changing frequency of the modulated wave.
Figure 20.2 Part of a communications system.