Currents through parallel resistances

Chapter Currents through parallel resistances

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Currents through parallel resistancesRefer to the schematic diagram of Fig. 5-4. The resistors are called Rn. The total paral-lel resistance in the circuit is R. The battery voltage is E. The current in branch n, con-taining resistance Rn, is measured by ammeter A and is called In.The sum of all the In’s in the circuit is equal to the total current, I, drawn from thesource. That is, the current is divided up in the parallel circuit, similarly to the way thatvoltage is divided up in a series circuit.If you’re astute, you’ll notice that the direction of current flow in Fig. 5-4 is out fromthe positive battery terminal. But don’t electrons flow out of the minus terminal? Yes—but scientists consider current to flow from plus to minus. This is an example of a math-ematical convention or “custom.” Such things often outlast their appropriateness. Backin the early days of electrical experimentation, physicists had to choose a direction forthe flow of current, and plus-to-minus seemed more logical than minus-to-plus. The ex-act nature of electric current flow wasn’t known then. This notation has not beenchanged. It was feared that tampering with it would just cause confusion; some peoplewould acknowledge the change while others would not. This might lead to motors run-ning the wrong way, magnets repelling when they should attract, transistors beingblown out, and other horrors. Just look at the mess caused by the conflict betweenFahrenheit and Celsius temperatures, or between miles and kilometers.Problem 5-3Suppose that the battery in Fig. 5-4 delivers 12 V. Further suppose that there are 12 re-sistors, each with a value of 120 Ω in the parallel circuit. What is the total current, I,drawn from the battery?86 Direct-current circuit analysis5-4Analysis of a current in aparallel circuit. See textfor dicussion-