Equation 5 note that the definition of the apparent

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!"## = ࠵? ࠵? [Equation 5] Note that the definition of the apparent resistance is practically the same as the definition of resistance given by Georg Simon Ohm (possibly, he was lucky not to find any samples of non-ohmic conductors). Figure 6 illustrates and compares the apparent and differential resistances for an incandescent lamp. Figure 6. Two mathematical descriptions of a non-ohmic circuit component (here, an incandescent lamp) include the apparent resistance and the differential resistance. The lower panel of Figure 6 emphasizes that the differential resistance is a more sensitive parameter; we discuss more details in the Example at the end of this Unit. Non-ohmic components are widely used in electronics. They belong to several categories, including: Ø Non-ohmic components whose resistance depends on the applied voltage. We have already discussed the example of an incandescent lamp; the example of a semiconductor diode at the end of this Unit presents an opposite dependence of resistance on the applied voltage. Ø Resistive sensors (Section 2-5) whose electric resistance depends on non-electric parameters such as strain/stress, temperature, light intensity, etc. Ø Transistors (trans-resistors) whose electric resistance varies according to the controlling voltage or current . Transistors can serve as electronic switches, amplifiers, etc. (Section 2-2) Ø The voltage-current relationships for capacitors and inductors (Section 3-1) involve time derivatives instead of the algebraic expressions used for resistors . Book Page 36
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EE for the 21 st century Review the basics 1-1-3 Ohm’s law – and beyond © 2015 Alexander Ganago Page 8 of 11 Last printed 2015-07-24 6:18 PM File: 2015 1-1-3 Ohm's.docx Despite its limitations, Electrical Engineers eagerly apply Ohm’s law to the components, for which it formally fails, because the linear equations with constant resistances are much easier to solve than non- linear or differential equations. For example: ü Working with semiconductor diodes, it may be useful to approximate a part of current-voltage characteristic with a straight line (constant differential resistance), which seems natural for the range 1.86 – 1.96 V in Figures 7 and 8 in the Example at the end of this Unit ü For circuits with sinusoidal voltage sources it is possible to apply an extended form of Ohm’s law to capacitors and inductors (Section 3-5). The advantage of doing so is to use algebraic equations for voltages and currents instead of differential equations. E XAMPLE Another example of a circuit component, which does not obey Ohm’s law, is a semiconductor diode whose conductance grows as the voltage applied to the diode increases above a certain limit: see the lab data in Figure 7.
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  • Fall '07
  • Ganago
  • Electric charge, Alexander Ganago

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