2008 Ch 03_2 - Making sense of Electrical Engineering 2nd...

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Making sense of Electrical Engineering // 2 nd edition Chapter 3: Ohm’s Law // The Big Picture © 2008 Alexander Ganago The Big Picture: Ohm’s law, Sources, and Switches Ohm’s law is NOT among the fundamental laws of this Universe. It is merely a statement relating the current I through a circuit element to the voltage V across this element. According to Ohm’s law, they are proportional to each other: V = I R where the coefficient R is called resistance and is expected to be constant (independent of voltage or current). Ohm’s law, formulated by Georg Ohm in 1820s, has been experimentally confirmed for metals and some other materials, of which we build the circuit components called resistors, although their resistance depends on many parameters such as temperature. At the same time, there are many important exceptions to Ohm’s law, such as semiconductor devices, some of which we will discuss later. Accordingly, we use the term ohmic for circuit elements that obey Ohm’s law, and the term non-ohmic for those that lack the linear relationship between current and voltage. For an ohmic circuit element, we expect a linear dependence between voltage across and current through it (also called volt-amp characteristic or I-V curve) as sketched here along with the symbol used for resistors. Electrical engineers dearly love Ohm’s law, because it simplifies circuit equations to linear, algebraic instead of non-linear, differential, etc. It is indeed very convenient: given the voltage and resistance, you can easily find the current through a resistor and the power it absorbs (product of voltage and current, always positive for resistors).
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