electric power
Summary
Electric power is the rate at which a circuit element transfers or converts electrical energy, measured in watts (J/s). The key relation is P = IV = I²R = V²/R; which form to use depends on whether current or voltage is the same across the elements.
Detail
Meaning: Electric power is the rate at which electrical energy is transferred or converted in a circuit element. A battery supplies power to the circuit; a resistor dissipates it as thermal energy. Power depends on both the current through an element and the potential difference across it.
Equation: P = IV = I²R = V²/R, where P is power in watts (W = J/s), I is current in amperes (A), V is potential difference in volts (V), and R is resistance in ohms (Ω). P = IV works for any element; the other two forms apply to resistors.
On the MCAT: Passages ask for the power of a heater, bulb, or wire. Two bulbs in series share the same current, so the larger resistance dissipates more power (I²R); in parallel they share the same voltage, so the smaller resistance dissipates more (V²/R). Transmission lines use high voltage so current is low for a given power, cutting I²R loss.
Common confusion: Higher resistance does not always mean more power. That holds at fixed current, but at fixed voltage power falls as R rises. Also, a kilowatt-hour is a unit of energy (3.6 × 10⁶ J), not power.
Sources
- Physics (Giancoli)
- University Physics (Young and Freedman)
- Fundamentals of Physics (Halliday, Resnick, and Walker)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.