Question

find the heat produced if a bulb of rating 20 watt is lite for 2 hours ?

Answer 1

power = energy/ time
time = 2 hours= 2*60*60 sec
20 = energy/(2*60*60)
energy= 20*2*60*60= 144000 joules
= 144 Kjoules

Answer 2

Heat energy = power * time
Heat energy = 20 * 7200 ----> This is 2 hours converted into seconds
 = 144000 Joules

Hope this helps...