28 Electrical Power Systems Technology
- Power converted by resistor R 2 :
V^2 302 900
P 2 = — = —— = —— = 90 W
R 2 10 10
- Power converted by resistor R 3 :
V^2 302 900
P 3 = — = —— = —— = 45 W
R 3 20 20
- Total power converted by the circuit:
PT = P 1 = P 2 + P 3 = 180 W + 90 W + 45 W = 315 W
Figure 2-7. Finding power values in a parallel circuit.The watt is the basic unit of electrical power. To determine an actu-
al quantity of electrical energy, one must use a factor that indicates how
long a given power value continued. Such a unit of electrical energy is
called a watt-second. It is the product of watts (W) and time (in seconds).
The watt-second is a very small quantity of energy. It is more common to
measure electrical energy in kilowatt-hours (kWh). It is the kWh quan-
tity of electrical energy that is used to determine the amount of electrical
utility bills. A kilowatt-hour is 1000 W in 1 h of time, or 3,600,000 W per
second.
As an example, if an electrical heater operates on 120 V, and has a
resistance of 200, what is the cost to use the heater for 200 h at a cost of 5
cents per kWh?
- V^2 1202 14,400
P = — = —— = ——— = 720 W = 0.72 kW.
R 20Ω 20 Ω