What is Power
We have studied in last class that
Rate of Doing Work is called Power
Power = Work Done/Time Taken
It is measured in Joule per second
Suppose 20 Joules of work is done in 4 seconds
Power =Work Done/Time Taken
=20 Joules/4 Seconds
= 5 Joules/Second
What is Electric Power
Rate at which electric energy is consumed in a electrical circuit
Power = VI
How is Power Formula Derived
What is SI unit of Power
It is Watt
1 Watt = 1 Volt × 1 Ampere
1 watt is power generated by Electrical Device when
When an electrical appliance is operated at Potential Difference Of 1 Volt and a current of 1A flows through it.
Different formulas for Calculating Power
What is Commercial Unit of Electric Power
It is measured in Kilowatt Hour (KWh)
1 Kilowatt hour = 1000 Watt × 3600 Second
1 Kilowatt hour = 3600000 Watt Second
1 Kilowatt hour = 3.6 × 10 6 watt seconds
1 Kilowatt hour = 3.6 × 10 6 Joule
NCERT Question 2 -
Which of the following terms does not represent electrical power in a circuit?
(a) I 2 R (b) IR 2 (c) VI (d) V 2 /R
NCERT Question 3 -
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
NCERT Question 12 - Several electric bulbs designed to be used on a 220 V electric supply line, are rated 10 W. How many lamps can be connected in parallel with each other across the two wires of 220 V line if the maximum allowable current is 5 A?
NCERT Question 14 -
Compare the power used in the 2 Ω resistor in each of the following circuits:
(i) a 6 V battery in series with 1 Ω and 2 Ω resistors, and (ii) a 4 V battery in parallel with 12 Ω and 2 Ω resistors.
NCERT Question 15 - Two lamps, one rated 100 W at 220 V, and the other 60 W at 220 V, are connected in parallel to electric mains supply. What current is drawn from the line if the supply voltage is 220 V?
NCERT Question 16 - Which uses more energy, a 250 W TV set in 1 hr, or a 1200 W toaster in 10 minutes?
NCERT Question 17 - An electric heater of resistance 8 Ω draws 15 A from the service mains 2 hours. Calculate the rate at which heat is developed in the heater.
Q1 Page 220 - What determines the rate at which energy is delivered by a current?
Q2 Page 220 - An electric motor takes 5 A from a 220 V line. Determine the power of the motor and the energy consumed in 2 h
Example 12.12 - An electric bulb is connected to a 220 V generator. The current is 0.50 A. What is the power of the bulb?
Example 12.13 - An electric refrigerator rated 400 W operates 8 hour/day. What isthe cost of the energy to operate it for 30 days at Rs 3.00 per kW h?
Example 12.10 - An electric iron consumes energy at a rate of 840 W when heating is at the maximum rate and 360 W when the heating is at minimum. The voltage is 220 V. What are the current and the resistance in each case?
Question 13 -
Find out the following in the electric circuit given in Figure 12.9
(a) Effective resistance of two 8 W resistors in the combination
(b) Current flowing through 4 W resistor
(c) Potential difference across 4 W resistance
(d) Power dissipated in 4 W resistor
(e) Difference in ammeter readings, if any.