Question

It turns out that most of the electricity we use in Ohio comes from burning coal. Coal yields about 30 Gigajoules (GJ = 109 J) of electricity per metric ton (1000 kg). Assuming that the coal plant is 30% efficient (i.e. the plant captures 30% of the energy converted by burning coal), how much total coal has to be burned to keep that 100 W light bulb lighted for the week?

Answer 1

Answer:

0.6048 kg of coal

Explanation:

Mass energy input = mass energy output/efficiency

Mass energy output = 30 GJ/ton= 3×10^10 J/1000 kg = 3×10^7 J/kg

Efficiency = 30% = 0.3

Mass energy input = 3×10^7/0.3 = 1×10^8 J

Mass of coal to be burned = power×time/mass energy input

Power = 100 W

Time = 1 week = 604,800 s

Mass of coal to be burned = 100×604,800/1×10^8 = 0.6048 kg