Question

A 75 watt incandescent light bulb and an 18 watt compact fluorescent bulb are equally bright. How much warmer will an insulated closed 12 by 11 by 8.0 foot room get in 1.0 hour if it is lighted by an incandescent bulb than if it is lighted by a fluorescent bulb? At room temperature, the specific heat of air is 1.007 J/gC and its density is 1.2 g/L. The watt is the SI unit of power; 1 watt = 1 J/s.

Answer 1

1ft = 3.048 dm
volume = 12*11*8 ft^3 = 12*11*8*(3.048) dm^3
                                              =29902.6 dm^3
                                              =29902.6 L

density of air = 1.2 g/L

mass of air = density * volume
                        =1.2 * 29902.6
                        = 35883.11 g

for incandescent bulb:
Heat given in 1 hor = P*t
       = 75 W*3600 s
       = 270,000 J

use:
heat = m*C*delta Ti
270000 = 35883.11*1.007*delta Ti
delta Ti = 7.5 oC

for fluorescent bulb:
Heat given in 1 hor = P*t
       = 18 W*3600 s
       = 64800 J

use:
heat = m*C*delta TF
64800 = 35883.11*1.007*delta TF
delta TF= 1.58 oC

Ratio = 7.5/1.58
           = 4.75

Answer:4.75