Power and Energy in Circuits
Choose the right power formula based on the given quantities; calculate energy and running costs.
Electrical power:
P = IV = I²R = V²/R
Electrical energy:
E = Pt = IVt
Kilowatt-hour (kWh):
- 1 kWh = energy used by 1 kW device for 1 hour
- 1 kWh = 1000 W × 3600 s = 3 600 000 J = 3.6 MJ
Calculating electricity cost:
Energy (kWh) = Power (kW) × Time (h) Cost = Energy (kWh) × Price per kWh
Example: A 2 kW heater runs for 3 hours. Electricity costs $0.20/kWh.
- Energy = 2 × 3 = 6 kWh
- Cost = 6 × 0.20 = $1.20
Energy transfers in components:
| Component | Useful output | Wasted as |
|---|---|---|
| Motor | Kinetic energy | Heat, sound |
| Lamp | Light | Heat (mainly) |
| Resistor | Heat (sometimes useful) | — |
| Battery charger | Chemical energy in battery | Heat |
| Loudspeaker | Sound | Heat |
Comparing energy use:
- A 10 W LED uses same energy in 6 hours as a 60 W incandescent in 1 hour
- Energy label ratings (A+++ to G) indicate efficiency
- P = IV = I²R = V²/R. E = Pt = IVt.
- 1 kWh = 3.6 MJ. Cost = kW × hours × price/kWh.
- Most electrical energy wasted as heat (unless device is a heater).