# COMMERCIAL UNIT OF ELECTRICAL ENERGY AND POWER

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### Domestic Electric Circuit and Connections

#### Disadvantages of Series Circuits for Domestic Wiring

•  If one bulb gets fused (or blows off), then the circuit breaks and all the bulbs are turned off
• In series circuit, the appliances do not get the same voltage (220 V) as that of the power supply line
• In series circuit, the appliances do not get the same voltage (220 V) as that of the power supply line

#### Advantages of Parallel Circuits in Domestic Wiring

• if one electrical appliance stops working due to some defect, then all other appliances keep working normally
• each electrical appliance gets the same voltage (220 V) as that of the power supply line.
• the overall resistance of the household circuit is reduced due to which the current from the power supply is high.

## ELECTRIC POWER

• The rate of doing work is called power
• Electric power is the electrical work done per unit time
• Power = Work done /Time taken
• The power of 1 watt is a rate of working of 1 joule per second.
• A bigger unit of electric power called kilowatt is used for commercial purposes.
• 1 kilowatt = 1000 watts or 1 kW = 1000 W
• When an electrical appliance consumes electrical energy at the rate of 1 joule per second, its power is said to be 1 watt.
• The rate at which electrical work is done or the rate at which electrical energy is consumed, is called electric power

Power = Work done / Time taken

the work done W by current I when it flows for time t under a potential difference V is given by :

W =V × I × t  joules

Sample Problem :

An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed will be : (a) 100 W (b) 75 W (c) 50 W (d) 25 W

### COMMERCIAL UNIT OF ELECTRICAL ENERGY : KILOWATT-HOUR

• 1 kilowatt-hour = 36,00,000 joules (or 3.6 × 106 J)

#### Sample Problems

1. A refrigerator having a power rating of 450 W operates for 15 hours a day. Calculate the cost of electrical energy to operate it for a month of 30 days. The rate of electrical energy is Rs. 3 per kWh.
2. An electric heater draws a current of 15 A from a 220 V supply. What is the cost of using the heater for 5 hours everyday for 30 days if the cost of 1 unit (1 kWh) is Rs. 10
3. What will be the current drawn by an electric bulb of 40 W when it is connected to a source of 220 V ?
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