Power
Power questions test energy transfer rate in DC circuits and the relationship between voltage, current, and resistance. You must calculate power in different circuit arrangements and distinguish power from energy.
Core rules
- Power formulas: P = VI, P = I²R, and P = V²/R.
- The unit of power is the watt (W); one watt is one joule per second.
- Electrical energy is power × time; practical units include watt-hours and kilowatt-hours.
- In series, current is common, so P = I²R is convenient; in parallel, voltage is common, so P = V²/R is convenient.
- Resistors dissipate power as heat; their wattage rating must exceed the calculated dissipation.
- Efficiency compares useful output power with total input power, usually as a percentage.
Worked example
A 6 Ω resistor carries 2 A. Find its power and the energy used in 5 hours. P = I²R = 2² × 6 = 24 W. Energy = 24 W × 5 h = 120 Wh = 0.12 kWh. Reasoning: power is instantaneous rate; multiplying by time gives energy, and 1 kWh = 1,000 Wh.
Traps
- Treating watts as energy; watts are power, watt-hours are energy.
- Using the wrong formula in a parallel circuit by assuming current is common.
- Forgetting to convert minutes to hours for kWh.
- Choosing a resistor by resistance only and ignoring its power rating.