The interdependence among current, voltage, and resistance in dc circuits is called Ohm’s Law, named after the scientist who supposedly first quantified it. Three formulas denote this law:
E = IR
I = E/R
R = E/I
You need only remember the first of these formulas in order to derive the others. The easiest way to remember it is to learn the abbreviations E for voltage, I for current, and R for resistance, and then remember that they appear in alphabetical order with the equal sign after the E. Sometimes the three symbols are arranged in the so-called Ohm’s Law triangle, shown in following figure. To find the value of a quantity, cover it up and read the positions of the others.
The Ohm’s Law triangle. The voltage is E, the current is I, and the resistance is R. These quantities are expressed in volts, amperes, and ohms, respectively.
Remember that you must use units of volts, amperes, and ohms for the Ohm’s Law formulas to yield a meaningful result! If you use, say, volts and microamperes to calculate a resistance, you cannot be sure of the units you’ll end up with when you derive the final result. If the initial quantities are given in units other than volts, amperes, and ohms, convert to these units, and then calculate. After that, you can convert the calculated current, voltage, or resistance value to whatever size unit you want. For example, if you get 13,500,000 Ω as a calculated resistance, you might prefer to say that it’s 13.5 MΩ.