Overview
This lecture explains what power is in physics, covers its two main definitions, demonstrates how to calculate it using examples, and highlights related units and conversions.
Definitions and Equations for Power
- Power is the rate at which energy is transferred or work is done.
- Power can be calculated by: Power = Energy Transferred ÷ Time.
- Alternatively, Power = Work Done ÷ Time.
- Work done refers to energy transferred when a force moves an object over a distance.
- Power is measured in watts (W), energy/work in joules (J), and time in seconds (s).
Calculating Power: Examples
- For devices like lamps, use Power = Energy Transferred ÷ Time since no force movement is involved.
- Example: A lamp transferring 1,200 J in 20 s has 1,200 ÷ 20 = 60 W power.
- Another lamp transferring 1,500 J in 30 s has 1,500 ÷ 30 = 50 W, so the first lamp is more powerful.
Rearranging Formulas and Unit Conversions
- To find energy transferred, rearrange: Energy Transferred = Power × Time.
- Always convert time to seconds (e.g., 3 min × 60 = 180 s).
- For a microwave of 1,100 W running for 180 s: 1,100 × 180 = 198,000 J.
- To convert joules to kilojoules, divide by 1,000; 198,000 J = 198 kJ.
Work Done Example
- When a force moves an object (e.g., pushing a car), use Power = Work Done ÷ Time.
- Example: 9 kJ (9,000 J) of work over 20 s: 9,000 ÷ 20 = 450 W.
Key Terms & Definitions
- Power — Rate at which energy is transferred or work is done (measured in watts, W).
- Energy Transferred — Amount of energy moved from one store to another (measured in joules, J).
- Work Done — Energy transferred by a force moving an object over a distance (measured in joules, J).
- Watt (W) — Unit of power, equivalent to one joule per second.
Action Items / Next Steps
- Practice using both definitions and formulas for power with different examples.
- Convert time and energy units as needed (minutes to seconds, joules to kilojoules).
- Review related questions, flashcards, and exam-style problems for further study.