Question

Define power. State the difference between 1 watt and 1 watt hour. Establish the relationship between unit of electric energy and SI unit of energy.
An electric heater rated 1000 W/220 V operates for two hours daily. Calculate the cost of energy to
operate for 30 days t 5.00 per kWh.


CBSE Class X Science LA (5 Marks)

Answer 1

Answer:

Power is the amount of work done per unit time. Power is a scalar quantity. One watt means one joule of work done for one second. One watt-hour is the power of one watt for one hour which means 3600 joules of work done in one hour or 3600 seconds.

The unit of electrical energy is joule and the SI unit of energy is joule. The unit of power is watt. So the unit of electrical energy is equal to the SI unit of energy.

The numerical:

The power of electric heater = 1000 W

The time for which the electric heater operates in a day = 2 hr

                                                                                             = 2 X 3600

                                                                                             = 7200 s

For 30 days = 7200 s X 30

                      =  216000 s

In 1 second 1000 joule of work is done

In 216000 seconds 216000 X 1000 joule of work is done

Total Kilowatt-hour = (216000 X 1000) /(3600 X 1000)

                                        = 60 KWh

Total cost = 60 X 5

                    = Rs 300