Question

Calculate the amount of energy that is saved by replacing a 75-watt incandescent light bulb with an 18-watt fluorescent bulb during the 1500-hour life of the incandescent bulb. one watt is the equivalent of one joule of energy used per second.

Answer 1

First of all, let's convert the life duration of the bulb from hours into seconds:
$t=1500 h = 5.4 \cdot 10^6 s$

So now we can calculate the total energy consumed by the first light bulb:
$E_1=Pt = (75 W)(5.4 \cdot 10^6 s)=4.05 \cdot 10^8 J$

While the total energy consumed by the second light bulb is
$E_2=Pt = (18 W)(5.4 \cdot 10^6 s)=9.7 \cdot 10^7 J$

So the total energy saved by replacing the light bulb is
$\Delta E=E_1 - E_2 = 4.05 \cdot 10^8 J-9.7 \cdot 10^7 J=3.08 \cdot 10^8 J$