Question

How much energy does a 60.0 w light bulb use in half an hour? is the light bulb is 12% efficient, how much thermal energy does it generate?

Answer 1

(a) The power of the bulb is P=60.0 W. Half an hour corresponds to a time of
$t=0.5 h=30 min = 1800 s$
so the energy used by the bulb during this time is
$E=Pt = (60.0 W)(1800 s)=1.08 \cdot 10^5 J$

(b) The efficiency of the bulb is 12%, it means that it converts only 12% of the input energy into thermal energy, so the thermal energy is
$E_{out}= E_{in} \cdot \frac{12}{100}=1.3 \cdot 10^4 J$