How do you calculate power dissipated by a resistor from current and resistance?

• A. P = VI.
• B. P = I^2 × R.
• C. P = I × R.
• D. P = V^2 / R.

Answer: (B)

Power in a resistor is the rate at which it converts electrical energy to heat, and it equals the product of voltage and current: P = VI. If you know the current through the resistor and its resistance, use Ohm’s law V = IR to replace the voltage with IR in P = VI. This gives P = I × (IR) = I^2R. So, the power dissipated is the current squared times the resistance.

If you look at the other formulas, they require voltage or are not expressed as power: P = VI needs voltage, P = V^2/R needs voltage, and P = IR would give a voltage, not power, so it’s not applicable when you only know current and resistance.