Knowledge Organiser: Electrical Power and Energy
Key Terms
- Power (P): Rate of energy transfer (W)
- Watt: 1 joule per second
- kWh: Billing unit (1 kW for 1 hour)
- Energy: Power × time (J or kWh)
Key Facts
- 1 kW = 1000 W
- 1 kWh = 3,600,000 J
- High current = high power loss (I²R)
- Cost = kWh × price per unit
Key Equations
- P = IV
- P = I²R
- P = V²/R
- E = Pt (joules, seconds)
- E = P(kW) × t(h) (kWh)
Exam Tips
- Match equation to given quantities
- kWh needs kW and hours (not seconds)
- Always state units: W, J, or kWh
- Cost = kWh × unit price
Common Mistakes
- Mixing up joules and kilowatt-hours: Use P = E/t (joules, watts, seconds) for physics questions; use kWh = kW × hours for household energy cost questions
- Choosing the wrong power equation: Use P = IV when given current and voltage; use P = I²R when given current and resistance; use P = V²/R when given voltage and resistance
- Not converting time to seconds: E = P × t requires time in seconds — multiply minutes by 60 or hours by 3600
- Confusing power and energy: Power (W) is the rate of energy transfer; energy (J) is the total transferred — a 100 W bulb on for 10 s transfers 1000 J
- Forgetting to square in P = I²R: The current must be squared — students often forget this and just multiply I × R × something
Memory Aid
- Three power equations from one — "PIV is like PIE":
- Start with P = IV (the PIE). To get P = I²R: replace V with IR (from Ohm's Law). To get P = V²/R: replace I with V/R. You only need to remember P = IV and V = IR, then derive the others.
- kWh billing — "kW × Hours = kWh":
- Kilowatts × Hours = Kilowatt-Hours. The unit tells you exactly what to multiply. If given watts, divide by 1000 first. If given time in minutes, divide by 60 first.
- Which equation? — "Got V and I? Use P = VI. Got I and R? Use I²R. Got V and R? Use V²/R":
- Match the letters in the equation to the letters of the quantities you've been given in the question.