The energy needed to keep a 75-watt lightbulb burning for 1.0 h is 270 kJ. Calculate the energy required to keep the lightbulb burning for 4.0 h in joules.

Answer 1

270 kJ/hr x 4.0 hr = 10.8 kJ x 1000 J/kJ = 10800 J = 11,000 J (to 2 significant figures). This simply uses dimensional analysis where you are able to cancel the units and obtain the desired units. Note how hours cancel, and kJ cancel leaving you with J.