Current Calculations

The first way to use Ohm’s Law is to determine current in dc circuits. In order to find the current, you must know the voltage and the resistance, or be able to deduce them. Refer to the schematic diagram of following figure. It consists of a dc voltage source, a voltmeter, some wire, an ammeter, and a calibrated, wide-range potentiometer.

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A circuit for working Ohm’s Law problems

Problem 4-1

Suppose that the dc generator in above figure produces 10 V and the potentiometer is set to a value of 10 Ω. What is the current?
This is solved by the formula I = E/R. Plug in the values for E and R; they are both 10, because the units are given in volts and ohms. Then I = 10/10 = 1.0 A.

Problem 4-2

Imagine that dc generator in above figure produces 100 V and the potentiometer is set to 10 kΩ. What is the current?
First, convert the resistance to ohms: 10 kΩ = 10,000 Ω. Then plug the values in: I = 100/10,000 = 0.01 A. You might prefer to express this as 10 mA.

Problem 4-3

Suppose that dc generator in above figure is set to provide 88.5 V, and the potentiometer is set to 477 MΩ. What is the current?
This problem involves numbers that aren’t exactly round, and one of them is huge. But you can use a calculator. First, change the resistance value to ohms, so you get 477,000,000 Ω. Then plug into the Ohm’s Law formula: I = E/R = 88.5 / 477,000,000 = 0.000000186 A. It is more reasonable to express this as 0.186 μA or 186 nA.