Environmental Engineering Reference
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R 1
12 ohms
R T
50 ohms
R 2
12 ohms
R 3
? ohms
FIGURE 10.12
Calculating the value of one resistance in a series circuit in Example 10.13.
R T = R 1 + R 2 + R 3
R T = 10 + 12 + 25 = 47 W
Equation 10.8 can be transposed to solve for the value of an unknown resistance;
for example, transposition can be used in some circuit applications where the total
resistance is known but the value of a circuit resistor has to be determined.
Example 10.13
Problem: The total resistance of a circuit containing three resistors is 50 ohms (see
Figure 10.12). Two of the circuit resistors are 12 ohms each. Calculate the value of
the third resistor ( R 3 ).
Solution:
Given:
R T = 50 ohms
R 1 = 12 ohms
R 2 = 12 ohms
R T = R 1 + R 2 + R 3
R 3 = R T - R 1 - R 2
R 3 = 50 - 12 - 12 = 26 W
Note: When resistances are connected in series, the total resistance in the circuit
is equal to the sum of the resistances of all the parts of the circuit.
s eries C irCuit C urrent
Because there is but one path for current in a series circuit, the same current ( I ) must
flow through each part of the circuit. Thus, to determine the current throughout a
series circuit, only the current through one of the parts must be known. The fact that
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