per2 - Chapter 2: Electromagnetic Radiation Radiant Energy...

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15 Chapter 2: Electromagnetic Radiation – Radiant Energy I Goals of Period 2 Section 2.1: To introduce electromagnetic radiation Section 2.2: To discuss the wave model of radiant energy Section 2.3: To describe the electromagnetic spectrum Section 2.4: To discuss the quantum model of radiant energy 2.1 Introduction to Radiant Energy Electromagnetic radiation , which is the basis of this period, is one form of radiant energy. Visible light is an example of electromagnetic radiation. In Physics 103 you learned that a moving charge (an electric current) is surrounded by a magnetic field. A change in this magnetic field generates an electric field. We called this electromagnetic induction. Changing electric fields are always accompanied by a changing magnetic field and vice versa. These changing fields allow a changing current in a wire or a moving charge to produce electromagnetic radiation, which is a source of energy. The electromagnetic radiation moves outward from the source as long as the energy that causes the charge to move is present. Figure 2.1 illustrates waves of electromagnetic radiation. Figure 2.1 Electromagnetic Radiation Wave You have already seen that the electric field associated with electromagnetic radiation exerts a force on a charge. This fact is used in many devices. Almost every day we experience an example in antennas used for radio, telephone, or television. As we will discuss in Period 3, electrons in a broadcasting antenna are made to move with some frequency. Frequency describes how often something repeats a cycle . In this case, the frequency of the electromagnetic radiation being broadcast is the same as the frequency that describes how often the electrons in the broadcasting antenna vibrate per second. The electric field of the broadcast electromagnetic radiation exerts a force on the charges in the receiving antenna, causing those electrons to move with the same frequency. In other words, the current in one antenna induces a current in the other antenna, even though the antennas may be miles apart.
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16 As discussed above, the electrons in the receiving antenna move, so they must experience a force that produces the motion. Thus we know that energy is transferred from the broadcasting antenna to the receiving antenna. In order to be transferred, this energy must be associated with the electromagnetic radiation. We will use the term radiant energy to refer only to energy associated with and transferred by electromagnetic radiation. The radiant energy from the broadcasting antenna does not reach the receiving antenna instantaneously. Rather, it travels at a finite, although very fast, speed. The distances in the classroom are too small to be able to measure this effect, but you may have noticed it if you have listened to communications between people on the earth and on the space shuttle or to a live news broadcast from overseas. The speed at which radiant energy travels depends on the medium that it is passing through, but in a vacuum it is about 3 x 10 8 meters per second, or 186,000 miles per second.
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per2 - Chapter 2: Electromagnetic Radiation Radiant Energy...

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