(The significant figures given in #1 are ridiculous, intended to make your calcu

Question

(The significant figures given in #1 are ridiculous, intended to make your calculated results 'nice'. Please give answers to 2 or 3 significant figures.) A 75 W incandescent light bulb shines in all directions to light a room. These days, we want to replace incandescent bulbs because they are only 5.0666% efficient (i.e 94.9333% of the power is lost as heat). What is the intensity of the light 3.888 meters away? We prefer CFL lights because they are 20% efficient. A store-bought CFL is labeled "75 W" but read the fine print and it really only uses 19 W. What would be the intensity of the light 3.888 meters away?

Explanation / Answer

Given that Power of the ight bulb   is   P = 75 W   distance r = 3.888m       S = P /A                       = 75 W /4 p ( 3.88m ) ^2          0.396 Wb /m^2 b ) S = P /A                       = 19 W /4 p ( 3.88m ) ^2          0.100 Wb /m^2                       = 19 W /4 p ( 3.88m ) ^2          0.100 Wb /m^2

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