2.4 - Department of Electrical and Computer Engineering...

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Department of Electrical and Computer Engineering ECSE 352 Electromagnetic Waves and Optics 4 Power and Energy Transport 2.4 Power and Energy Transport References: Hayt and Buck 12.3 AGK@2005 2.4-1 Overview Whenever we want to use electromagnetic waves in any sort of system, we are using them to transfer energy over a distance. This is true whether we are looking at a telephone line, a microwave oven or an optical fiber. Similarly, if we are concerned about the effects of unwanted electromagnetic waves(for example as stray reflections on a transmission line in a printed circuit board) we need to know how much power they are carrying with them, relative to the signal power. Here e ill alculate e nergy at ansported rough ace y n M ave d we will calculate the energy that is transported through space by an EM wave, and as a result obtain an expression for the power flowing in and out of a volume when an EM wave passes through it. Waves have a direction and so electromagnetic power flow is also directed. This is called the Poynting vector. Because a wave oscillates in time, the instantaneous power measured at a point is also a function of time. However, more often we are interested inthetime-averaged power rather than the instantaneous value. As a wave travels through a lossy medium it will lose power to the medium in the form of heat (this is what a microwave oven is doing). We will see that by calculating the power that emerges from the lossy aterial nd btracting om e ower at nters e aterial e n material and subtracting it from the power that enters the material, we can calculate the absorbed power. Since the human body is a lossy medium for most EM wave frequencies there are safety issues associated with maximum permitted levels of EM radiation exposure, and we will also briefly look at these. ©AGK 2005 ECSE 352 2.4-3 Learning outcomes After taking this class you should be able to: Recognize that EM waves transport energy through space • Define the Poynting vector • Calculate the Poynting vector from wave parameters alculate power absorbed by a lossy medium due to Calculate power absorbed by a lossy medium due to an EM wave • Explain the difference between instantaneous and time-averaged power density • Explain basic health and safety issues associated with EM waves. ©AGK 2005 ECSE 352 2.4-4
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Contents • The relationship between electric field amplitude and power.
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2.4 - Department of Electrical and Computer Engineering...

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