 
TM 9254
72.
Direct Current  Continued
d.
DC Circuit Evaluation. The four quantities (voltage, current, resistance, and power) are the basic quantities
used in evaluating a DC circuit. Table 71 provides a useful tool for determining the unknown values if any two of these
quantities are known. Enter the table in the row indicating the known values. Find the appropriate formula(s) for the
desired unknown(s) under the proper column heading.
Table 71. DC Relationships
Known
Formulas for Determining Unknown Values of
Values
I
R
E
P
I & R


E = IR
P = I^{2}R
I & E

R =
I
E

P = IE
I & P

R =
I
P
2
E =
I
P

R & E
I =
R
E


P =
R
E^{2}
R & P
I =
R
P

E = PR

E & P
I =
E
P
R =
P
E^{2}


e.
Resistance. Resistance is the opposition to the flow of current. The total or equivalent resistance of a circuit
is dependent on how the resistors are connected. Resistors can be connected in series as shown in figure 72 or in
parallel as shown in figure 73. The formulas for calculating series and parallel resistance are given below.
(1)
The equivalent resistance of a series circuit is the algebraic sum of all the resistors in the circuit. This is
stated mathematically as follows:
Figure 72. Series Circuit
72
