Correct use of Ohm’s Law

Chapter 5.6 Correct use of Ohm’s Law

Lessons In Electric Circuits Volume I – DC Book
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Lessons In Electric Circuits Volume I – DC Book

  • 5.6. CORRECT USE OF OHM’S LAW147An interesting rule for total power versus individual power is that it is additive for anyconfiguration of circuit: series, parallel, series/parallel, or otherwise. Power is a measure ofrate of work, and since power dissipated mustequal the total power applied by the source(s)(as per the Law of Conservation of Energy in physics), circuit configuration has no effect on themathematics.• REVIEW:• Power is additive in anyconfiguration of resistive circuit: PTotal = P1 + P2 + . . . Pn5.6Correct use of Ohm’s LawOne of the most common mistakes made by beginning electronics students in their applicationof Ohm’s Laws is mixing the contexts of voltage, current, and resistance. In other words, astudent might mistakenly use a value for I through one resistor and the value for E across aset of interconnected resistors, thinking that they’ll arrive at the resistance of that one resistor.Not so! Remember this important rule: The variables used in Ohm’s Law equations must becommonto the same two points in the circuit under consideration. I cannot overemphasizethis rule. This is especially important in series-parallel combination circuits where nearbycomponents may have different values for both voltage drop andcurrent.When using Ohm’s Law to calculate a variable pertaining to a single component, be surethe voltage you’re referencing is solely across that single component and the current you’rereferencing is solely through that single component and the resistance you’re referencing issolely for that single component. Likewise, when calculating a variable pertaining to a set ofcomponents in a circuit, be sure that the voltage, current, and resistance values are specific tothat complete set of components only! A good way to remember this is to pay close attentionto the two pointsterminating the component or set of components being analyzed, makingsure that the voltage in question is across those two points, that the current in question isthe electron flow from one of those points all the way to the other point, that the resistance inquestion is the equivalent of a single resistor between those two points, and that the power inquestion is the total power dissipated by all components between those two points.The ”table” method presented for both series and parallel circuits in this chapter is a goodway to keep the context of Ohm’s Law correct for any kind of circuit configuration. In a tablelike the one shown below, you are only allowed to apply an Ohm’s Law equation for the valuesof a single verticalcolumn at a time: