Finally, if we divide equation 2 by I, we solve for resistance, This relationship is true only if the resistance in the circuit remains constant. If the resistance changes, current also changes. A graph of this relationship is shown in

relationship between voltage and current in this example shows voltage plotted horizontally along the X axis in values from 0 to 120 volts. The corresponding values of current are plotted vertically in values from 0 to 6.0 amperes along the Y axis. A straight line drawn through all the points where the

and is called a linear relationship.

All three formulas presented in this section are equivalent to each other and are simply different ways of expressing Ohm’s Law.

The various equations, which may be derived by transposing the basic law, can be easily obtained by using the triangles in Figure 12-40.

The triangles containing E, R, and I are divided into two parts, with E above the line and I × R below it. To determine an unknown circuit quantity when the other two are known, cover the unknown quantity with a thumb. The location of the remaining uncovered letters in the triangle indicate the mathematical operation to be performed. For example, to find

thumb. The result indicates that E is to be divided by I, or R

I × R. This chart is useful when learning to use Ohm’s Law. It should be used to supplement the beginner’s knowledge of the algebraic method.

While wire of any size or resistance value may be used, the word “conductor” usually refers to materials that offer low resistance to current flow, and the word “insulator” describes materials that offer high resistance to current. There is no distinct dividing line between conductors and insulators; under the proper conditions, all types of material conduct some current. Materials offering a resistance to current flow midway between the best conductors and the poorest conductors (insulators) are sometimes referred to as “semiconductors,” and find their greatest application in the field of transistors.

The best conductors are materials, chiefly metals, which possess a large number of free electrons; conversely, insulators are materials having few free electrons. The best conductors are silver, copper, gold, and aluminum; but some nonmetals, such as carbon and water, can be used as conductors. Materials such as rubber, glass, ceramics, and plastics are such poor conductors that they are usually used as insulators. The current flow in some of these materials is so low that it is usually considered zero. The unit used to measure resistance is called the ohm. The symbol for the

the capital letter “R” refers to resistance. The resistance of a conductor and the voltage applied to it determine the number of amperes of current flowing through the conductor. Thus, 1 ohm of resistance limits the current flow to 1 ampere in a conductor to which a voltage of 1 volt is applied.