Power
Power is the rate of doing work or the rate of energy transfer.
In one line: Power is work speed.
Practice This ConceptUnderstand it
Power tells how quickly work is done. Two people may do the same work, but the one who completes it in less time has greater power. Its SI unit is watt, where 1 watt means 1 joule of work done per second.
Example
Rule to remember
P = W/t = E/t, where P is power in watt, W or E is work or energy in joule, and t is time in second.
See it worked out
Follow the example line by line, then use the same method on new questions.
Worked example
- 1A pump does 1200 J of work in 20 s.
- 2Power = 1200/20 = 60 W.
Method you can reuse
Visual tip
A simple comparison table of work, time, and power helps in ratio-based questions.
Check yourself
Answer the question in your head before opening the answer.
Try it yourself
A machine does 500 J of work in 10 s. What is its power?
Think first, then show answerHide answer
Common wrong answer
Multiplying work and time.
Why it loses marks: Power is work done per unit time.
Don't mix this up
Exam tip
Write it in the exam
Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.
How CBSE asks it
- 1
Enough for 1 mark
Power is the rate of doing work or the rate of energy transfer.
- 2
Add for 2 marks
Example: If two motors lift the same load to the same height but one takes less time, that motor has greater power.
- 3
Add for 3 marks
Power tells how quickly work is done.
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