AM I on the Radio?

Through this lesson students learn how AM radios work through basic concepts about waves and magnetic fields. Waves are first introduced by establishing the difference between transverse and longitudinal waves, as well as identifying the amplitude and frequency of a given waveform. Students then learn general concepts about magnetic fields, leading into how radio waves are created and transmitted. Several demonstrations can be performed in order to help students better understand these concepts. The goal of this lesson is for students to understand how the AM radios built during the associated activity function.

Understanding how waves and magnetic fields work are basic concepts of electricity and magnetism that all engineers should know. It is also a task of an engineer to take the concepts learned in school or other types of training and to find practical uses and applications for this knowledge, such as the AM radios for this lesson. Electromagnetic Waves AM Radio Frequency Amplitude Modulation Earlier introduction to electricity, voltage, resistance, and power is helpful. Students should be able to identify transverse and longitudinal waves Students should be able to determine the amplitude and frequency of a waveform Students should be able to describe how electromagnetic waves propagate Students should be able to describe the process by which AM radios work To introduce this lesson, students will become human wave particles. All of the students should line up, standing in a straight line, one behind the other with their hands resting on the shoulders of the person in front of them. The instructor, located at the front of the line, creates an initial disturbance of the first particle, by pulling the student forward slightly, so that his/her motion is transferred back through the line of students. This illustrates how the wave moves, as the waveform is merely individual particles displacing one another, the individual particles do not actually travel along the waveform, but rather oscillate back and forth. This example shows longitudinal waves, since the direction of displacement is in the same direction of wave propagation. To illustrate transverse wave forms, line up the students side by side, beginning in a squatting position, holding hands. The wave begins to travel as the first student stands up and then crouches back down, causing the student next to him/her to do the same. The wave travels down the line transversely (similar to "the wave" at a sporting event), because particle displacement occurs perpendicular to the direction of wave propagation. Again, point out to the students that they are doing no more than oscillating p and down, yet their motion is traveling down the line of students. The following two activities should be carried out once students have already been presented the lesson information since these activities serve to provide examples of wave and magnetism concepts. The next activity demonstrates that information can be conveyed in an electromagnetic wave in a simple manner. It also provides a link between electricity and magnetism. It requires a 9V battery, 2 iron nails, thin gage wire or magnet wire, an AM/FM radio or Walkman, a tape deck with a speaker that can be played without a tape but with the cassette door open (most old school style cassette players will work for this), wire strippers, and a male headphone jack that has been connected to a small coiled wire around an iron nail or bolt. Cut the cable of an old headphone set and strip the ends of the two wires, or buy the 1/8 inch male stereo plug from an electronics store. (Most of these items can be found, the others can be purchased at Radio Shack for a few dollars). Wrap one of the nails with the thin wire and demonstrate that it has no magnetic properties. Loop the wire at least 10 times in one direction only. Next, attach the wire to the battery and show that it now has become magnetized by holding it close to a small metal object. Explain that the electric current from the wire around the nail has generated a magnetic field. Wrap the other nail 5 times and attach it to the stripped ends of the male headphone jack. Next, play the radio or Walkman through its speaker or headphones. Remove the headphones and replace it with the modified headphone jack just constructed. With the cassette door open, place the tip of the nail close to the playing cassette driver head and listen to the signal from the other radio! This works because cassette tapes have a magnetic sensor that pulls information from the magnetized tape. As the tape moves by the sensor, the magnetic field varies and a signal is received. The same thing happens when you stick your small transmitting antenna next to the deck. Even a simplified approach to explaining radio frequency transmission through electromagnetic waves is difficult conceptually without actually showing students the process. When presenting the initial demonstration described above, try to get students to guess or come up with as many of the explanations for what is observed as possible, and then explain in full once a correct suggestion has been volunteered. If students have not been exposed to previous lessons this will be a stretch. Mention that the nail has to be close to the "receiver" of the tape deck because it is a low power signal, and because the frequency is too low to travel very far. If a person yells at the top of their lungs, it can't be heard a mile away, but a radio wave with a higher frequency can be detected for miles. Voice is low frequency, and radio broadcast is at a higher frequency. Oscilloscope demo The first concepts introduced to students are the different types of waves and important features of waves. Upon understanding fundamental concepts about waves, electromagnetic and radio waves can be discussed more specifically. Students are first introduced to transverse and longitudinal waves, the two primary types of waves. Particles in longitudinal waves are displaced in the same direction of wave propagation. Thus, if the wave is propagating horizontally, then wave particles are moving back and forth horizontally in the same direction as the entire wave. A good visual for this can be found at the website: www.kettering.edu/~drussell/Demos/waves/wavemotion.html. Ask students to follow the motion of a single particle so they can see that the particle oscillates in the same direction as the wave. Next students are shown a transverse wave, which can be drawn as the commonly seen sinusoidal wave. Individual wave particles for this type of wave move in a direction perpendicular to the direction of wave propagation. Thus, if the wave is propagating horizontally, the particle will be displaced vertically, moving up and down. A visual of this can be found on the same website as the visual for the longitudinal waves. Transverse waves will continue to be explored, as this waveform is used in radio signal transmission. If a sinusoidal waveform is drawn on the board, the two major components which should be identified are the signal's amplitude and the frequency. The amplitude of the wave is defined as the distance from the midpoint of the vertical component of the wave to its peak, or half the distance form its maximum and minimum values. The amplitude of the signal shown below is labeled. Another important component of the wave is its frequency. Frequency is defined as the number of cycles the wave completes per second; it has units of Hertz (Hz), where one Hz is equivalent to a second-1. One cycle of the wave is the distance the wave travels until it reaches the same vertical position as where it started. Once cycle of the wave is also shown on the figure above. Next, electromagnetic waves and their relationship to AM radios is explained. As shown with the nail activity, current through the wire wrapped around the nail generates a magnetic field around the nail. The same is true for antennas used to broadcast radio signals. As current enters the antenna, a magnetic field is created around the antenna. Magnetic fields will also induce an electric field in an antenna or wire placed close to the first wire, also shown by the nail activity. While there is not another wire coil close to radio wave transmitters, the magnetic field around the antenna actually induces an electric field in the open space surrounding it. This electric field in turn, creates another magnetic field in the space surrounding it. This change between electric and magnetic fields propagates the wave through space, creating an electromagnetic wave. A radio wave is just a type of electromagnetic wave, having a large wavelength and high frequency. A sound which is to be transmitted is created and converted into an electrical signal. Since this signal is not very strong, the signal is amplified with an amplifier. The signal now has a greater amplitude, making it stronger. This observation can also be correlated with the use of the oscilloscope. A wave is generated and then a modulator changes the amplitude of the carrier signal (the signal being broadcast via radio waves) mimicking changes in the original sound's amplitude. The signal then travels to the antenna. Once the electromagnetic wave is emitted from the antenna, (known as the transmitter) it is received by the antenna of your radio. This consists of a wire or metal stick (as was used in the AM radio kits for the AM I on the Radio? Activity). The specific station on the AM radio denotes the frequency a wave must have in order to be played by your radio. The frequency is specified with a tuner; the antenna receives waves of many different frequencies, so the tuner finds the signal of the desired frequency. The signal again is very weak and must be amplified, via an amplifier. A demodulator is then used to cut the radio signal in half, as both halves provide the same information. This is carried out with a diode (students should be familiar with this circuit component from the previous circuits lesson). The carrier wave originally assigned to the wave in order to transmit it is removed by a filter, producing the original electrical signal, which is then transferred to the speaker, creating the original sound. Visuals help greatly for the explanation of how an AM radio works, two websites with good visuals are the following: 1) How Stuff Works and 2) PBS. wave particles are displaced in the same direction as wave propagation wave particle are displaced in a perpendicular direction to wave propagation the distance from a wave's mean position to one of its extremes Number of cycles per second for a signal - higher frequency signals travel farther generally than lower frequency signals, so AM radio waves which have a frequency in the range of a few hundred thousand cycles per second go farther than sound waves, which are in the 20-20,000 cycles per second range radiation consisting of waves of energy associated with electric and magnetic fields resulting from the acceleration of an electric charge Varies the frequency, amplitude, phase, or other characteristic of an electromagnetic wave Device which extracts modulation from a radio carrier wave An electrical device used to reject signals of certain frequencies while allowing others to pass. A device that increases the signal power, voltage, or current of an electrical signal AM I on the radio? Students should be able to identify the difference between transverse and longitudinal waves, as well as which type is used in AM radio transmission. If given a sinusoidal wave, they should be able to determine the amplitude and frequency of the signal. Students should also be able to explain the process by which AM radio waves are transmitted. Have students find the amplitude and frequency of different waves Have students diagram the process by which radio waves are transmitted and received Integrate more knowledge of circuit components into the discussion of how a radio works (i.e. diode used as demodulator, components that comprise a filer, etc) to help them understand why they are soldering each component in their AM radio kits How Stuff Works: Radio Website PBS Radio Website Acoustics Animations by Dr. Dan Russell at Kettering University Applied Physics PBS radio transmission activity PBS radio transmission activity, http://www.pbs.org/wgbh/aso/tryit/radio/#, 06/23/04 PBS Radio Transmission Website How Stuff Works How Stuff Works: electronics.howstuffworks.com/radio.htm How Stuff Works Acoustics Animations Dr. Dan Russell Kettering University Applied Physics Acoustics Animations by Dr. Dan Russell at Kettering University Applied Physics http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html Acoustics Animations Dr. Dan Russell Kettering University Applied Physics