Question

Kabir bought 5 new light bulbs of 50 W each in addition to the 7 bulbs he already had in his house which were also 50 W each. Calculate his electricity bill, if he keeps the new bulbs on for 5 hours and the older bulbs on only for 3 hours, and the cost of one unit of electricity is Rs. 60.

(a) Rs.136

(b) Rs.137

(c) Rs.138

(d) Rs.139

The question was asked in final exam.

My question is based upon Current Electricity topic in portion Current Electricity of Physics – Class 12

Answer 1

Correct option is (c) Rs.138

To explain: New bulbs = 5; Old bulbs = 7; Total wattage of new bulbs = 5 × 50;

Total wattage of old bulbs = 7 × 50; Time the new bulbs are kept on = 5 hours;

Time the old bulbs are kept on = 3 hours

The number of units of electricity consumed by new bulbs (n1) = \(\frac {(total \, wattage \, \times \, time \, in \, hours)}{1000}\)

= \(\frac {50 \times 5 \times 5}{1000}\) = 1.25

The number of units of electricity consumed by old bulbs (n2) = \(\frac {50 \times 7 \times 3}{1000}\) = 1.05                                                                          Total bill of electricity = number of units of electricity consumed × amount for one unit of electricity

= (n1 + n2) × 60

= (1.25 + 1.05) × 60

= 138

Therefore, his electricity bill will be Rs. 138.