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Power

Power is the rate of doing work or the rate of energy transfer.

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Student-friendly explanation

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.

How to write this in exams

  1. 1

    Start with the exact idea

    Power is the rate of doing work or the rate of energy transfer.

  2. 2

    Then show how to use it

    Identify work or energy, identify time in seconds, divide work by time, and write watt as unit.

  3. 3

    Add one concrete example

    If two motors lift the same load to the same height but one takes less time, that motor has greater power.

  4. 4

    Avoid this incomplete answer

    Multiplying work and time is wrong because power is work done per unit time.

Definition

Power is the rate of doing work or the rate of energy transfer.

Example

If two motors lift the same load to the same height but one takes less time, that motor has greater power.

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.

Memory hook

Power is work speed.

Examples and method

Worked example

A pump does 1200 J of work in 20 s. Power = 1200/20 = 60 W.

Method to apply

Identify work or energy, identify time in seconds, divide work by time, and write watt as unit.

Diagram support

A simple comparison table of work, time, and power helps in ratio-based questions.

How CBSE asks it

Exams ask numerical power, unit conversion, or comparison of two machines doing the same work in different times.

Avoid common mistakes

Common confusion

Students often think more power always means more total work, but power also depends on time taken.

Common wrong answer

Multiplying work and time is wrong because power is work done per unit time.

Exam tip

For comparison questions, first check whether work done is same or different, then compare time.

Quick check

A machine does 500 J of work in 10 s. What is its power?

Its power is 50 W because power = work/time = 500/10 = 50 joules per second.

Answer writing and exam use

1-mark answer

Power is the rate of doing work or the rate of energy transfer.

2-mark answer

Power is the rate of doing work or the rate of energy transfer. 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. If two motors lift the same load to the same height but one takes less time, that motor has greater power.

3-mark answer

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. 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. A pump does 1200 J of work in 20 s. Power = 1200/20 = 60 W. Exams ask numerical power, unit conversion, or comparison of two machines doing the same work in different times. Multiplying work and time is wrong because power is work done per unit time.
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