26 Electrical Power Systems Technology
POWER IN DC ELECTRICAL CIRCUITS
In terms of voltage and current, power (P) in watts (W) is equal to
voltage (in volts) multiplied by current (in amperes). The formula is P = V
× I. For example, a 120-V electrical outlet with 4 A of current flowing from
it has a power value of
P = V × I = 120 V × 4 A = 480 W.The unit of electrical power is the watt. In the example, 480 W of
power are converted by the load portion of the circuit. Another way to
find power is:
V^2
P = ——
R
This formula is used when voltage and resistance are known, but
current is not known. The formula P = F × R is used when current and
resistance are known. DC circuit formulas are summarized in Figure 2-5.
The quantity in the center of the circle may be found by any of the three
formulas along the outer part of the circle in the same part of the circle.
This circle is handy to use for making electrical calculations for voltage,
current, resistance, or power in DC circuits.
It is easy to find the amount of power converted by each of the resis-
tors in a series circuit, such as the one shown in Figure 2-6. In the circuit
shown, the amount of power converted by each of the resistors, and the
total power, are found as follows:
- Power converted by resistor R 1 :
P 1 = I^2 x R 1 = 2^2 × 20 Ω = 80 W - Power converted by resistor R 2 :
P 2 = I^2 × R 2 = 2^2 × 30 Ω = 120 W - Power converted by resistor R 3 :
P 3 = I^2 × R 3 = 2^2 × 50 Ω = 200 W - Power converted by the circuit:
PT = P 1 + P 2 + P 3 = 80 W + 120 W + 200 W
= 400 W, or
PT = VT × I = 200 V × 2 A = 400 W