Which expression correctly represents electrical power in terms of voltage and current?

• A. P = V x I
• B. P = I x R
• C. P = V / I
• D. P = V + I

Answer: (A)

Power is the rate at which electrical energy is transferred. When you know the voltage across a component and the current flowing through it, the energy per unit time delivered to the component is the product of those two quantities. This works because voltage is energy per unit charge (joules per coulomb) and current is charge per unit time (coulombs per second). Multiply them and you get energy per unit time (joules per second), which is watts. So the expression for power in terms of voltage and current is P = V × I.

For example, a device with 120 V across it drawing 0.5 A uses 60 W of power (120 × 0.5 = 60). The other forms don’t represent power in terms of voltage and current: I × R equals voltage, V ÷ I equals resistance, and V + I combines unlike quantities without giving power.