Homework Solution

TP 3-8

Given:

    A 100 W light bulb is left on for one month (30 days)

    e_gen e_trans = 0.30

    Fuel consumption for a kerosene wick stove is 6 MJ/meal

Find:

    a) chemical energy (in joules) wasted at the power plant

    b) number of equivalent meals from wasted energy

Science:

    e = useful energy out
               energy in

    unit conversions 

Solve:

    a)

    Energy used by the light bulb:

        E_bulb Dt = (100 W) Dt = (100 J/s) Dt = (100 J/s)(30 d)(24 h/d)(3600 s/h) = 2.592x10⁸ J  

        e_gen e_trans =              energy delivered to light bulb
                        energy used to generate and transmit to the light bulb  

    Energy used to generate and transmit electrical energy to the light bulb:

       E_gen,trans = E_bulb/e_gen e_trans = (2.592 x10⁸ J)/(0.30) = 8.64x10⁸ J

    b)

    Number of equivalent meals:

        N = E_gen,trans/(6x10⁶ J/meal) = (8.64x10⁸ J)/(6x10⁶ J/meal) =144 meals