Section 3.8

Power

Power, work and energy (kinetic and potential); dissipation of power by a resistor; the power formula and the maximum power transfer theorem.

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Main ideas

Power is the rate of doing electrical work, measured in watts. P = V × I.
Substituting Ohm's law gives two more forms: P = I²R and P = V²/R. Choose the one using the two quantities you are given.
Energy is power multiplied by time: W = P × t, in joules or kilowatt-hours.
Maximum power is transferred to a load when the load resistance equals the source's internal resistance.
⚠ Exam trap: pick the power formula that matches the given quantities. Converting unnecessarily through Ohm's law first invites arithmetic errors.

Key formulas

Solved examples

A 28 V landing lamp draws 5 A. What power does it dissipate?
P = V × I = 28 × 5 = 140 W.
A 4 Ω resistor carries 3 A. Find the power dissipated.
Current and resistance are given, so use P = I²R = 3² × 4 = 9 × 4 = 36 W.

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