EA3_elecpart1 - EA3: Systems Dynamics Electrical Systems...

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EA3: Systems Dynamics Electrical Systems Sridhar Krishnaswamy 1 VIII. SYSTEMS DYNAMICS – Electrical Domain Let us now delve into some simple electrical systems. Once again, we will adopt a systems approach here and infer a lot about the behavior of electrical systems by analogy with their counterparts in the mechanics domain. VIII.1 Electrical Systems Basics VIII.1.1 Dynamic Variables: The physical entities of interest in the electrical domain are charge, current, and voltage. While a complete physical understanding of these concepts must be deferred to another course (PhysA35-2, or other courses on electromagnetism), I will attempt to briefly highlight some of the essential concepts here. Charge : The building blocks of matter (elementary particles) have a property associated with them called electric charge , symbolically denoted by q . At one level of understanding, charge can be thought of as an analog of another property of elementary particles -- mass. Matter is discrete (quantized) and therefore the mass of any measured amount of matter must be some multiple of the masses of all the elementary particles making up that amount of matter. When we are concerned with things at the atomic scale, it makes sense to consider the unit of mass to be the mass of an electron at rest: m e . The mass of a proton is 1836.15m e , and that of a neutron is 1836.68m e . However, when large amounts of matter are under consideration, it is simpler to consider matter as being continuous and to use a better known (and larger) unit of mass, the kilogram. (Note: 1m e = 9.11x10 -31 kg.) This is what we have been doing all along. Analogous to mass, charges in nature are also quantized; that is, they occur only in discrete multiples of the elementary charge , which is denoted by e . However, unlike mass, charges come in two types: positive or negative. The charge of an electron is labeled negative and the charge of a proton is labeled positive. Some elementary particles such as the neutron are neutral with no charge. Once again, when large amounts of charges are considered, it makes sense to ignore the fact that they come only in discrete multiples, and to use a larger unit of charge, the Coulomb (C). The charge of an electron is –e; the charge of a proton is +e, where e=1.60x10 -19 C. In nature, positive and negative charges are found in great abundance, but they are almost always perfectly This is not the primary analogy that we will use in this course. But it is important to realize that different analogies can be drawn between different physical entities depending on the context. The purpose of an analogy (or ‘similarity’ to use the standard terminology) is to leverage our understanding of one aspect of nature to another area where we observe similar phenomena.
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EA3: Systems Dynamics Electrical Systems Sridhar Krishnaswamy 2 balanced against each other so that the net charge of any macro-object is usually zero. Of course, we (or nature) can at times do something to disturb this charge balance, and that
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This note was uploaded on 04/17/2008 for the course GEN_ENG 203 taught by Professor Krishnaswamy during the Fall '08 term at Northwestern.

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EA3_elecpart1 - EA3: Systems Dynamics Electrical Systems...

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