# A 100 W bulb is switched on for 10 hrs a day. If the cost of electricity is Rs 10 per unit, then calculate the bill for 1 month of 30 days.

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A 100 W bulb is switched on for 10 hrs a day. If the cost of electricity is Rs 10 per unit, then calculate the bill for 1 month of 30 days.
1. 300
2. 150
3. 200
4. 350

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Correct Answer - Option 1 : 300

Concept:

Power (P): The rate of doing work is called power and it is also known as the rate of consumption of energy.

• The SI unit of power is "watt".
• 1 Watt = 1 Joule/second.
• The commercial unit of electric energy is kilowatt or the Board of Trade (B.O.T) unit.

One kilowatt-hour: It is defined as the electric energy consumed by the appliance of 1 kilowatt in one hour

1 kilowatt-hour =  1 kilowatt × 1 hours = 1000 watt × 3600 s

1 kWh = 3600000 joules = 3.6 × 10J

Calculation:

Given:

P = 100 W, t = 10hr, D = 30 days, Cost per unit (c) = Rs 10

Energy consumed (E) in 30 days = P × t × D

= 100 × 10 × 30

E = 30000 Wh

E = 30 kWh

Bill for electricity = E × c = 30 × 10 = Rs 300