Which equation correctly relates power, voltage, and current in a circuit?

Which equation correctly relates power, voltage, and current in a circuit?

• A. Power = Voltage × Resistance
• B. Power = Current × Resistance
• C. Power = Voltage × Current
• D. Power = Voltage ÷ Current

Answer: (C)

Power in a circuit is the rate at which energy is transferred, and it is found by multiplying the voltage across an element by the current through it. This product, voltage times current, gives power in watts because a volt times an ampere equals a watt. If you know the resistance and use Ohm’s law (V = IR), you can also express power as P = I^2R or P = V^2/R, but the fundamental relationship linking all three quantities is P = VI. Why the other expressions don’t fit: multiplying voltage by resistance would give V·R, which isn’t watts in general. Multiplying current by resistance gives I·R, which equals voltage, not power. Dividing voltage by current gives V/I, which equals resistance, not power.

Power in a circuit is the rate at which energy is transferred, and it is found by multiplying the voltage across an element by the current through it. This product, voltage times current, gives power in watts because a volt times an ampere equals a watt. If you know the resistance and use Ohm’s law (V = IR), you can also express power as P = I^2R or P = V^2/R, but the fundamental relationship linking all three quantities is P = VI.

Why the other expressions don’t fit: multiplying voltage by resistance would give V·R, which isn’t watts in general. Multiplying current by resistance gives I·R, which equals voltage, not power. Dividing voltage by current gives V/I, which equals resistance, not power.