Civil Engineering Chapter 12 Electric Current And Related Variables Engineering What The Current That Flows

147

Chapter 12: Electric Current and Related Variables in Engineering

12.8 What is the current that flows through each of the following light bulbs: 40 W, 60

W, 75 W, 100 W. Each light is connected to a 120-V line.

12.9 If a 1500 W hair dryer is connected to a 120 V line, what is the maximum current

drawn?

SOLUTION

12.10 Referring to Table 12.1, create a table that shows the relative resistance of 1-ft-

long wires having diameters of 1 mil made of the metals given in Table 12.1 to 1-

ft-long copper with a diameter of 1 mil. For example, using the data given in

Table 12.1, the relative resistance of 1-foot-long, 1-mil-diameter aluminum wire

to 1-ft-long, 1 mil copper wire is 1.64 (17.01  /10.37  = 1.64).

148

SOLUTION

Metal Resistance ()

Relative Resistance to

Copper

12.18 Determine the total resistance and the current flow for the circuit shown in the

accompanying figure.

SOLUTION

The light bulbs are connected in a series arrangement; therefore, the total

resistance is given by:

24

total

R

12.19 Determine the total resistance and the current flow in each branch for the circuit

shown in the accompanying figure.

149

SOLUTION

321

12.20 Use Kirchhoff’s current law to determine the missing current in the circuit shown

in the accompanying figure.

150

SOLUTION

12.23 For Example 12.7, we computed the total current drawn by the circuit. Determine

the current in each branch of the circuit given in Example 12.7.

SOLUTION

151

in whole or in part.

Note that as a check

A 3.157.073.0A 3.1 321











III

12.29 According to Sylvania, a light-bulb manufacturer, its 40 W CFL light consumes 9

watts and produces 495 lumens. What is the efficacy for this light?

Energy Consumed by the Lamp (watts) 9 wat

ts watts

12.30 Calculate the efficacy for the following lights: (a) Sylvania accent LED light,

which uses 2 watts and produces 60 lumens, and (b) Sylvania 40 W CFL, which

uses 9 watts and produces 495 lumens.

watts9

(watts) Lampby the ConsumedEnergy

12.31 The Sylvania Super Saver 75 W light uses 20 watts, produces 1280 lumens, and

costs $4.49. As an alternative a generic 75 W incandescent light bulb costs $0.40,

produces 1200 lumens, and has a service life of 750 hours. Compare the

performance of each light by calculating the efficacy for each light and estimate

how much it would cost to run each light for 4 hours per day for 300 days in a

year. Assume electricity costs 9 cents per kWh.

Light Produced (lumens) 495 lumens lumens

152

in whole or in part.

cost=4 hours

day (300 days)(20 W)1 kW

1000 W󰇧$0.09

kW.h󰇨=$2.16

For the generic incandescent 100-W light bulb:

Efficacy=1200

75 =16 lumens/watts

cost=4 hours

day (300 days)(75 W)1 kW

1000 W󰇧$0.09

kW.h󰇨=$8.10

It should be obvious that the Sylvania Super Saver light is more efficient and

economical to operate than the generic incandescent.

12.33 A 20 MW coal-fired power plant has an efficiency of 40%. Estimate the annual

consumption of coal burned in this plant. Assume a heating value of

approximately 7.5 MJ/kg (megajoules per kilogram) for coal.

SOLUTION

153

in whole or in part.

amount of coal required = 210,240,000 kg

year2.2 lbm

1 kg  = 462,528,000 lbm/year

amount of coal required = 462,528,000 lbm

year 1 ton

2,000 lbm = 231,264 ton

year

12.34 A 2,000 W dishwasher is run for approximately one hour for 122 days during a

year. If the electric power company charges 12 cents per kWh, calculate the

annual electric energy consumption of the dishwasher and the associated cost.

12.35 A 500 W clothes washing machine is run for approximately two hours a week for

52 weeks during a year. If the electric power company charges 12 cents per kWh,

calculate the annual electric energy consumption of the clothes washer and the

associated cost.

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12.36 A relatively small house in the United States has an annual electrical energy

consumption of 4000 kWh. Estimate the annual consumption of coal burned in a

power plant to generate the required electrical energy. Assume a heating value of

approximately 7.5 MJ/kg (megajoules per kilogram) for coal, 40% efficiency at

the power plant and 6% loss for the power transmission lines.

SOLUTION

12.37 Rework Problem 12.36 for a larger house with an annual electrical energy

consumption of 10,000 kWh.

155

SOLUTION