SubjectsPhysicsGeneral Physics

General Physics

Power

1 real objectives extracted from your Physics notes.

Objective 1

Meaning of Power

Power is defined as:

The rate at which work is done, or the rate at which energy is transferred.

Power tells us how fast work is done, not how much work is done.

Formula for Power

P=WtP = \frac{W}{t}

Where:

  • P = power (watts, W)
  • W = work done or energy transferred (joules, J)
  • t = time taken (seconds, s)

Unit definition:

1 watt=1 joule per second1\ \text{watt} = 1\ \text{joule per second}

Understanding the Formula Conceptually

  • If more work is done in the same time, power is greater.
  • If the same work is done in less time, power is greater.
  • Doing work slowly means low power, even if the total work is large.

Power in Everyday Life

Examples:

  • A fast elevator has higher power than a slow one lifting the same load.
  • An electric kettle with higher power boils water faster.
  • Two people may do the same work, but the one who finishes first is more powerful.

Worked Examples (Teaching Core)

Example 1: Calculating Power

A machine does 600 J of work in 10 s.

Calculate the power.

Solution

P=Wt=60010=60 WP = \frac{W}{t} = \frac{600}{10} = 60\ \text{W}

Example 2: Calculating Work Done

A motor operates at a power of 200 W for 15 s.

Calculate the work done.

Rearranging:

W=P×tW = P \times t
W=200×15=3000 JW = 200 \times 15 = 3000\ \text{J}

Example 3: Calculating Time

An appliance uses 1200 J of energy at a power of 300 W.

Calculate the time taken.

Rearranging:

t=WPt = \frac{W}{P}
t=1200300=4 st = \frac{1200}{300} = 4\ \text{s}

Visual Aid: Formula Triangle

This helps learners remember how to rearrange the equation.

Key Examination Tips (High-Value)

  • Always use joules (J) and seconds (s).
  • Write the formula first.
  • Check that time is not in minutes unless converted.
  • Include units in final answers.
  • Power answers should be realistic (not extremely large or negative).

Common Examination Errors (Examiner Insight)

Students often:

  • confuse power with energy,
  • divide the wrong way round,
  • forget to convert minutes to seconds,
  • omit units.

Remember: power is about speed of doing work.

Exam-Style Questions (Original)

Question 1

Define power.

Question 2

A force does 900 J of work in 15 s.

Calculate the power developed.

Question 3

A machine operates at 500 W for 20 s.

Calculate the work done.

Question 4

Two students lift the same load to the same height.

Student A takes 10 s, Student B takes 5 s.

Who has greater power? Explain your answer.

Worked Solutions (Beyond Excellent)

Solution 1

Power is the rate at which work is done or energy is transferred.

Solution 2

P=90015=60 WP = \frac{900}{15} = 60\ \text{W}

Solution 3

W=500×20=10 000 JW = 500 \times 20 = 10\,000\ \text{J}

Solution 4

Student B has greater power because the same work is done in a shorter time.

End-of-Objective

A learner who has mastered this objective can:

  • define power correctly,
  • relate power to work and time,
  • use P=WtP = \frac{W}{t} confidently,
  • solve simple numerical problems accurately.
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