Power is
Rate of doing work
Power = Work/Time Taken
P = W/T
Example
Suppose a person does 30 Joules work in 5 Seconds
Calculate Power
Ans
Work = 30 Joules
Time Taken = 5 seconds
Power = Work/Time
P = W/T
P = 30 Joules/5 second
= 6 Joules/second
What is average power?
Average power is defined as the total work done/total energy consumed divided by the total time taken.
Average power = Total Work done / Total Time
What is SI Unit of Power?
SI unit of Power is Watt
it is named after famous scientist James Watt
Note
1 Watt = 1 Joule/Second
What are Other Units of Power
SI unit of Power is Watt but it is also measured in KW,MW,bhp etc
Different Units of Power are
| Name | Symbol | FORUMULA |
| Watt | W | 1 Watt = 1 Joule/second |
| Kilowatt | KW | 1 KW = 1000 W |
| Megawatt | MW | 1 MW = 10,00,000W |
| Horsepower | hp | 1 HP = 746W = 0.75KW |
Questions
Q 1 Page 156 - What is power?
Q 2 Page 156 - Define 1 watt of power
Q 3 Page 156 - A lamp consumes 1000 J of electrical energy in 10 s. What is its power?
Q 4 Page 156 - Define average power
Example 11.7 - Two girls, each of weight 400 N climb up a rope through a height of 8 m. We name one of the girls A and the other B. Girl A takes 20 s while B takes 50 s to accomplish this task. What is the power expended by each girl?
Example 11.8 - A boy of mass 50 kg runs up a staircase of 45 steps in 9 s. If the height of each step is 15 cm, find his power. Take g = 10 m s –2 .
Example 11.9 - An electric bulb of 60 W is used for 6 h per day. Calculate the ‘units’ of energy consumed in one day by the bulb.
NCERT Question 9 - A certain household has consumed 250 units of energy during a month. How much energy is this in joules?
NCERT Question 14 - An electric heater is rated 1500 W. How much energy does it use in 10 hours?
NCERT Question 20 - Find the energy in kWh consumed in 10 hours by four devices of power 500 W each.
