(1) Electromagnetic wave emission process ① Principle: Radio and TV stations undertake the work of transmitting sound and image signals by electromagnetic waves. The oscillator in the radio station first generates a high-frequency oscillation current, and at the same time, it turns the sound signal into an electrical signal, which is loaded on the radio wave by the modulator. Radio waves are emitted into the air through the antenna of the transmitter. ② Oscillator: a device that generates high-frequency oscillation current. ③ Modulator: a device that converts high-frequency oscillation current into high-frequency oscillation current with signal. (2) The receiving process of electromagnetic waves. ① Principle: The reception of sound is completed by the radio. The receiving antenna of the radio receives electromagnetic waves carrying signals in the air. After being processed by the tuner, the electrical signal of the sound is taken out by the detector and restored to sound through the speaker. (2) Tuner: A device that selects electromagnetic waves of a certain frequency that we need from numerous electromagnetic waves and turns them into current. (3) Detector: a device for extracting sound signals from high-frequency oscillating current. Simple experiment: transmission and reception of electromagnetic waves (1) Turn on the radio switch and turn the tuning knob to the position where the radio cannot be received. Then pull the cable switch of the indoor light, and you will hear a "click" sound in the radio speaker. This is because when the switch of the lamp is about to be turned on, the high-frequency electromagnetic wave generated by spark discharge is received by the radio. Turn on the switch of the fluorescent lamp, and the effect is more obvious. This is because when the bimetal of the starter in the fluorescent lamp circuit is turned off, the ballast produces higher self-induced electromotive force, which makes the discharge more intense. In summer thunderstorm weather, electromagnetic waves generated by lightning in the distance can also be received by the radio. You can hear a series of "clicks", which appear earlier than the thunder you hear. This is because the propagation speed of electromagnetic waves in air is close to the speed of light in vacuum and much larger than the speed of sound. You can use your watch to measure the time difference between the sound and the thunder on the radio, and calculate how far the lightning is from you. (2) Open the back cover of the radio. Turn on the power supply and turn the tuning knob, and you can see that the knob is linked with the movable piece of variable capacitance in the tuning circuit. When it is adjusted to a certain position, you can receive the broadcast of a certain station. If you receive broadcasts from other radio stations, you must change the angle of the rotor of the variable capacitor. This can explain the function of tuning: adjusting the natural frequency of LC circuit to make it the same as the frequency of external electromagnetic wave. So that the induced voltage reaches the maximum value. If you can borrow a teaching signal source (or a student signal source), you can further deepen the content of the experiment. A single ring (fixed on an insulating seat) bent by thick copper wire (or aluminum wire) is connected to the high-frequency output end of the signal source as a transmitting antenna. In the experiment, the radio is placed next to the antenna, so that the axis of the magnetic rod in the radio is in a straight line with the axis of the circular antenna. As shown in figure 2.6- 1. If there is no copper ring, two ends of a wire can be connected to the high-frequency output end of the signal source, and then the wire can be sheathed on the magnetic rod antenna of the radio. First, put the signal source in the high-frequency constant-amplitude output section (switch in the "constant-amplitude" position), put the frequency selection switch in the "frequency 1" position, and turn on the radio switch. No matter how you tune it, you can't get the audio signal. If the "AM, Equal Happiness" switch of the signal source is placed in the "AM" position and the low-frequency selector switch is placed in the "1kHZ" position, the signal source will output high-frequency modulated waves, and the audio signal of 1 kHz can be received by adjusting the tuning knob of the radio. If you turn the frequency selector switch of the signal source, the modulation signal will become 500 Hz and 2 kHz. The frequency of the sound emitted by the radio will also change, which can explain the role of modulation. If you can find the detector diode on the bottom plate of the radio and short-circuit the diode with a clip wire, you can hear the rustling in the radio speaker, but you can hardly hear the broadcast signal. This shows that the detector is an indispensable part of the radio. Its function is to detect audio signals from intermediate frequency signals. If the probe of the ohmmeter touches between the moving plate of the volume potentiometer and the ground, or directly strokes between the moving plate of the volume potentiometer and the ground with a small screwdriver (touching the metal bar), you can hear the "click" sound from the speaker, indicating that there is an audio amplifier behind the detector. Through low frequency amplification and power amplification, the signal current has enough power to push the speaker to make enough sound.