9. Power & Work Done

1. Defining Power

Two Ways to Think About Power

• Rate of Energy Transfer: Power is defined as the rate at which energy is transferred from one store to another.

• Rate of Work Done: Power is also defined as the rate at which work is done.

• Core Concept: In both cases, power tells you how "fast" energy is being used or work is being completed.

2. The Power Equations

The Formulas

Depending on the scenario, you use one of two nearly identical formulas:

1. Power = Energy Transferred / Time

2. Power = Work Done / Time

Units of Measurement

• Power: Measured in Watts (W). 1 Watt is equal to 1 Joule per second.

• Energy / Work Done: Measured in Joules (J).

• Time: Must always be measured in seconds (s).

3. Work Done vs. Energy Transferred

What is "Work Done"?

• Physical Movement: "Work done" is a specific term used when a force is applied to move an object over a distance (e.g., pushing a car).

• General Energy Transfer: If energy is moving without a force causing physical displacement (e.g., electricity charging a phone or heating a lamp), it is simply called "energy transferred."

• Equivalency: Both are measured in Joules and represent the same fundamental concept of energy being moved.

4. Calculation Tips and Unit Conversions

Time Conversions

• Calculations must use seconds. If a question gives time in minutes, multiply by 60 (e.g., 3 minutes = 180 seconds).

Large Energy Values

• Large numbers in Joules are often converted to kilojoules (kJ) by dividing by 1,000 (e.g., 198,000 J = 198 kJ).

Rearranging the Formula

To find energy or work when power and time are known:

• Energy Transferred = Power × Time

• Work Done = Power × Time

5. Summary Table of Units