Ohm's Law and Power Calculations: The Foundation of Electrical Engineering

A practical guide to Ohm's Law (V=IR) and power calculations (P=VI). Learn when and how to apply each formula, with worked examples and common pitfalls.

The Three Forms of Ohm's Law

Where V = voltage (Volts), I = current (Amperes), R = resistance (Ohms). These three equations are really the same relationship viewed from different angles — know any two quantities and you can find the third.

The Power Triangle

Use whichever form has the two quantities you already know. Power is measured in Watts (W).

Worked Example 1: LED Current-Limiting Resistor

You have a 12V supply and a red LED (forward voltage 2V, desired current 20mA).

• Voltage across resistor: V_R = 12V - 2V = 10V
• Current: I = 20mA = 0.02A
• Resistance: R = V/I = 10/0.02 = 500Ω

Use a standard 470Ω or 560Ω resistor. Power dissipated: P = 10 × 0.02 = 0.2W — a 1/4W resistor works.

Worked Example 2: Power Dissipation in a Resistor

A 100Ω resistor has 5V across it:

A standard 1/4W resistor is at its limit! Use a 1/2W resistor for safety margin.

Worked Example 3: Heating Element

A 230V, 2000W heater:

Common Pitfalls

• Forgetting unit conversions: 20mA ≠ 20A. Always convert to base units: 20mA = 0.02A, 4.7kΩ = 4700Ω, 250mW = 0.25W.
• Applying Ohm's Law to non-ohmic components: Diodes, LEDs, and transistors do not follow V=IR. Ohm's Law only applies to resistors and conductors.
• Confusing power in vs power out: Total power in = total power out. If your supply delivers 12W but the load receives 10W, the other 2W is dissipated as heat.

Quick Reference: Which Formula to Use

Use our Power Calculator to compute any electrical quantity instantly.