per3 - Chapter 3 Electromagnetic Waves Radiant Energy II...

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27 Chapter 3: Electromagnetic Waves – Radiant Energy II Goals of Period 3 Section 3.1: To discuss some properties of radiant energy Section 3.2: To describe the transfer of energy and information Section 3.3: To discuss radio and television signal transmission 3.1 Radiant Energy from the Sun The sun emits 3.3 x 10 31 joules of radiant energy each day. Because the earth is so far away from the sun, only about 2 x 10 –9 of the sun's radiation reaches the earth's upper atmosphere. Some of this energy is absorbed or reflected by clouds and gases in the atmosphere, so that only about one half of this amount reaches the earth's surface. Nevertheless, the radiant energy reaching the surface of the earth each day is about 15,000 times the total energy used by the earth's population each day. The maximum, or peak, power falling on the roof of a house of average size in an average location on a clear sunny day is almost 100 kW. If radiant energy could be converted into electrical energy and stored with 100% efficiency, an average family could run their household electrical appliances (excluding heating) on two sunny days a month. The interior of the sun emits radiant energy at all wavelengths. The matter in the outer layers of the sun (or any other star) absorbs radiation of particular wavelengths. Most of the radiation reaching the Earth’s surface is in the visible region. If we view the light from a star through a device that spreads out the wavelengths, such as a prism, we see that some wavelengths are missing, producing black lines in the pattern seen with the prism. Electromagnetic radiation spread out over a range of wavelengths is called a spectrum. The gaps in the spectrum due to the missing, or absorbed, wavelengths are called absorption bands. The absorption bands in the spectra of stars help us to identify the elements in the stars. You will see examples of absorption bands in class. Solar Cells and the Photoelectric Effect Radiant energy from the sun can be converted into electrical energy by solar cells, which are sometimes called photoelectric cells or photovoltaic (PV) cells, which make use of the photoelectric effect. The photoelectric effect can be explained using the quantum model of radiant energy, but not the wave model. What happens is that the absorption of a photon by an atom may cause an electron to escape from the atom, provided that the energy of the photon is large enough. Any energy of the photon that is in excess of the energy required to free the electron appears as energy of motion of the electron. If the photon does not have sufficient energy to release an electron, the photoelectric effect does not take place. It was Albert Einstein’s explanation of the photoelectric effect in 1905 that later earned him a Nobel Prize. Solar cells are used to provide power for earth satellites, where it is important to
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This note was uploaded on 06/11/2011 for the course PHYSICS 104 taught by Professor Staff during the Winter '11 term at Ohio State.

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per3 - Chapter 3 Electromagnetic Waves Radiant Energy II...

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