Page 89 - Teach Yourself Electricity and Electronics

Current calculations 69

Ohm’s Law

The interdependence between current, voltage, and resistance is one of the most fun-
damental rules, or laws, in electrical circuits. It is called Ohm’s Law, named after the sci-
entist who supposedly first expressed it. Three formulas denote this law:
E IR
I E/R
R E/I

You need only to remember the first one in order to derive the others. The easiest
way to remember it is to learn the abbreviations E for EMF or voltage, I for current and
R for resistance, and then remember that they appear in alphabetical order with the
equals sign after the E.
Sometimes the three symbols are written in a triangle, as in Fig. 4-6. To find the
value of one, you cover it up and read the positions of the others.

4-6 Ohm’s Law triangle.

It’s important to remember that you must use units of volts, amperes, and ohms in
order for Ohm’s Law to work right. If you use volts, milliamperes, and ohms or kilovolts,
microamperes, and megohms you cannot expect to get the right answers.
If the initial quantities are given in units other than volts, amperes, and ohms, you
must convert to these units, then calculate. After that, you can convert the units back
again to whatever you like. For example, if you get 13,500,000 ohms as a calculated re-
sistance, you might prefer to say that it’s 13.5 megohms.

Current calculations

The first way to use Ohm’s Law is to find current values 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 Fig. 4-7. It consists of a variable dc generator, a
voltmeter, some wire, an ammeter, and a calibrated, wide-range potentiometer. Com-
ponent values have been left out of this diagram, so it’s not a wiring diagram. But