The subscript n denotes any number of

additional

resistance that might be in the equation.

Example: In figure 20 a series circuit consisting of three resistors--one of

10 Ohms, one of 15 Ohms, and one of 30 Ohms--are shown. A voltage source

provides 110 volts. What is the total resistance?

Given:

R1 = 10 Ohms

R2 = 15 Ohms

R3 = 30 Ohms

Solution:

RT = R1 + R2 + R3

RT = 10 Ohms + 15 Ohms + 30 Ohms

RT = 55 Ohms

In some circuit applications, the total resistance is known and the value of

one of the circuit resistors has to be determined. The equation RT = R1 +

R2 + R3 can be transposed to solve for the value of the unknown resistance.

Example: In figure 21 (on the following page) the total resistance of a

circuit containing three resistors is 40 Ohms. Two of the circuit resistors

are 10 Ohms each. Calculate the value of the third resistor (R3).

Given:

RT = 40 Ohms

R1 = 10 Ohms

R2 = 10 Ohms

Solution:

RT - R1 - R2 = R3

Subtract R1 + R2 from both sides of the equation

R3 = RT - R1 - R2

R3 = 40 Ohms - 10 Ohms - 10 Ohms