power (P).

You must understand the relationships which exist between

quantities because they are used throughout your study of electricity. In

the preceding paragraphs, P was expressed in terms of alternate pairs of the

other three basic quantities E, I, and R. In practice, you should be able

to express any one of these quantities in terms of any two of the others.

Figure 18 is a summary of 12 basic formulas you should know.

The four

quantities E, I, R, and P are at the center of the figure. Adjacent to each

quantity are three segments. Note that in each segment, the basic quantity

is expressed in terms of two other basic quantities, and no two segments are

alike.

FIGURE 18.

SUMMARY OF BASIC FORMULAS.

For example, the formula wheel in figure 18 could be used to find the

formula to solve the following problem:

A circuit has a voltage source that delivers 6 volts and the circuit uses 3

watts of power. What is the resistance of the load?

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