Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

356 Chapter 12 Electric Current and Related Parameters

types, maximum operating temperatures, insulating materials, outer cover sheaths, the type

usage, and the specific location where a wire should be used.

Electric Power

The electric power consumption of various electrical components can be determined using the

following power formula:

(12.4)

wherePis power in watts,Vis the voltage, andIis the current in amps. The kilowatt hour units

are used in measuring the rate of consumption of electricity by homes and business. One kilo-

watt hour represents the amount of power consumed during 1 hour by a device that uses one

kilowatt (kW), or 1000 joules per second.

Example 12.2 The electric resistance of a light bulb is 145 . Determine the value of current flowing through

the lamp when it is connected to a 120-volt source.

Using Ohm’s law, Equation (12.3), we have

Example 12.3 Assuming that your electric power company is charging you 10 cents for each kWh usage,

estimate the cost of leaving five 100-W light bulbs on from 6 p.m. until 11 p.m.every night for

30 nights.

Series Circuit

Electrical components can be connected in either a series or a parallel arrangement. As we men-

tioned earlier in this chapter, there are many different types of electrical components. Here in

this section we will only look at resistors and capacitors.

Aresistoris an electrical component that resists the flow of either direct or alternating cur-

rent. Resistors are commonly used to protect sensitive components or to control the flow of cur-

rent in a circuit. Moreover, a resistor can be used to divide or control voltages in a circuit. There

are two broad groups of resistors: fixed-value resistors and variable resistors. As the name implies,

the fixed-value resistor has a fixed value. On the other hand, the variable resistors, sometimes

called potentiometers, can be adjusted to a desired value. The variable resistors

are used in various circuits to adjust the current in the circuit. For example, variable resistors

are used in light switches that allow you to dim the light. They are also used to adjust the

$7.50

750 cents

1 5 light bulbs2a 100

W

light bulb

ba

1 kW

1000 W

ba 5

h

nights

b130 nights2a 10

cents

kWh

b

I

V

R



120

145

0.83 A

VRI

PVI

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