Power (I²R)
Power lost as heat in a resistor.
Formula
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Example
2² × 5 = 20W
How it works
When electric current flows through a resistor, some electrical energy is converted into thermal energy because the moving charges collide with the atoms in the material. The amount of power turned into heat depends on both how much current is flowing and how strongly the resistor opposes that current. Because electrical power is given by P = VI and Ohm’s law says V = IR, substituting gives P = I^2R.
When to use it
Use this formula when you want to find how much heat a resistor or other resistive component is producing in a circuit. It is especially useful for checking resistor power ratings, wire heating, and energy loss in simple DC circuits.
Worked example, step by step
A resistor has a current of 3 A flowing through it and a resistance of 4 Ω. P = I²R P = 3² × 4 P = 9 × 4 P = 36 W So the resistor dissipates 36 watts of power as heat.
Common mistakes to avoid
- Using the resistance value in ohms but forgetting to square the current
- Mixing up this formula with P = V²/R without first checking which quantities are known
- Leaving out units, especially watts (W) for power and ohms (Ω) for resistance
Did You Know?
The I²R heating law is why thin, overloaded wires get dangerously hot and why fuses exist.
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More in Electronics & Circuits
View allOhm's Law
BasicVoltage equals current times resistance.
Electrical Power (VI)
BasicPower dissipated by a component.
Power (V²/R)
BasicPower from voltage and resistance.
Resistors in Series
BasicSeries resistances add directly.
Resistors in Parallel
IntermediateReciprocals of parallel resistances add.
Capacitors in Parallel
IntermediateParallel capacitances add.