Electricity and numerical problems
Q) In a house , there is daily use of 10 CFL each of 20 W for 5 hours, a rice cooker of 900 W for 1 hour and a LED TV of 50 W for 4 hours. Calculate the monthly bill if the rate of electricity is Rs 10 per unit.
Solution:
Energy
consumed by CFL
Given, power
of a CFL (P) = 20W = 20/1000 = 2/100 kW
Number of CFLs(N) = 10
Time of use(t) = 5 hours
Now, the
electric energy consumed by CFL in a day
= P x N x t
= 2/100 x 10 x 5
= 1 unit(kWh)
Energy
consumed by rice cooker
Given, power
of the rice cooker = 900W = 900/1000 = 9/10 kW
Number of rice cookers(N) = 1
Time of use(t) = 1 hour
Now, the
electric energy consumed by rice cooker in a day
= P x N x t
= 9/10 x 1 x 1 = 0.9 unit(kWh)
Energy
consumed by TV
Given, power
of TV (P) = 50 W = 50/1000 = 5/100 kW
Number of TVs(N) = 1
Time of use(t) = 4 hours
Now, the
electric energy consumed by TV in a day
= P x N x t
= 5/100 x 1 x 4
= 0.2 unit(kWh)
Total
electric energy consumed in a day = 1+0.9+0.2 = 2.1 units
Therefore,
Total electric energy consumed in a month = 30 x 2.1 = 63 units
The bill of
a month = Total units x rate = 63 x 10 = Rs.630
Q) In a building,
10 tube lights of 40 W each are used for 3 hours daily, 4 TVs of 40 W each and
used for 4 hours daily,2 irons of 800W each are used for 2 hours a week and 3
electric bulb of 100 W each are used for 3 hours daily. Calculate the total
cost of electricity in one month. The cost of 1 unit is Rs. 8. If 10 CFL of 10W
each run 3hours a day in place of tube light. Calculate the difference in the
bill.
Solution:
i) For tube
lights
Electrical energy consumed in 1 day = P x N
x t
=
40/1000 x 10 x 3 = 1.2 kWh
ii) For TV
Electrical
energy consumed in 1 day = P x N x t
= 40/1000 x 4 x 4 = 0.64 kWh
iii) For
iron
Electrical
energy consumed in 1 week = P x N x t
= 800/100 x 2 x 2 = 3.2 kWh
= 0.457 kWh
iv) For
electric bulbs
Electrical energy consumed in 1 day = P x N x
t = 100/1000 x 3 x 3 = 0.9 kWh
Now, total
electrical energy consumed in 1 day = 1.2 = 0.64 = 0.457 = 0.9 = 3.197 kWh
Total
electrical energy consumed in 1 month = 3.197 x 30 = 95.91 kWh = 95.91 unit
Total cost
in 1 month = Total unit x Rate = 95.91 x Rs.8 = Rs 767.28
For CFL
bulbs
Electrical
energy consumed in 1 day = P x N x t = 10/1000 x 10 x 3
= 0.3 kWh
Now, total
electrical energy consumed in 1 day including CFL and excluding tube light
= 0.3 + 0.64 + 0.457 + 0.9 += 2.297 kWh
Total
electrical energy consumed in 1month = 2.297 x 30 = 68.91kWh
= 68.91 unit
Total cost
in 1 month = Total unit x Rate
= 68.91
x Rs. 8
= Rs
521.28
The
difference between the bill using tube lights and using CFL
= 767.28 - 521.28 = Rs 264
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