Knowledge Organiser: Power
Key Equations
- P = E/t — power from energy and time
- P = W/t — power from work done and time
- P = IV — electrical power
- P = I²R — electrical power (current + resistance)
- P = V²/R — electrical power (voltage + resistance)
- Energy (kWh) = Power (kW) × Time (h)
Key Terms
- Power — rate of energy transfer
- Watt (W) — 1 J/s
- Kilowatt (kW) — 1,000 W
- Kilowatt-hour (kWh) — unit of energy (not power)
Unit Conversions
- 1 kW = 1,000 W
- 1 MW = 1,000,000 W
- 1 kWh = 3,600,000 J
- Minutes → seconds: × 60
- Hours → seconds: × 3,600
Common Pitfalls
- kWh is energy, not power
- Always convert time to seconds for P = E/t
- Higher power ≠ more total energy used
- Power is a scalar (no direction)
Memory Aid
- Cover P → P = E/t
- Cover E → E = P × t
- Cover t → t = E/P
Common Mistakes
Misconception 1: "Higher power means more total energy" Not necessarily. Power tells you the rate of energy transfer, not the total. A 60 W bulb left on for 10 hours transfers 2,160,000 J. A 600 W device on for 1 hour also transfers 2,160,000 J. Same energy, very different power ratings. To find total energy, you multiply power by time. Misconception 2: "Power and energy are the same thing" They are closely related but different quantities. Energy is measured in joules and represents the amount transferred. Power is measured in watts and represents how quickly it is transferred. The relationship is P = E/t. Misconception 3: "kWh is a unit of power" The kilowatt-hour (kWh) is a unit of energy, not power. It combines the unit of power (kilowatt) with a unit of time (hour) to give energy. 1 kWh = 1,000 W × 3,600 s = 3,600,000 J.