What Is Ohm's Law?

Ohm's Law is the relationship between three fundamental electrical quantities: voltage (V), current (I), and resistance (R). Discovered by German physicist Georg Simon Ohm in 1827, it can be expressed in one simple equation:

V = I × R

In plain English: the voltage across a conductor equals the current flowing through it multiplied by its resistance. This single formula unlocks the ability to analyze almost any basic electrical circuit.

Understanding the Three Variables

Before you can apply Ohm's Law, you need to understand what each variable actually represents:

  • Voltage (V) — measured in volts (V). Think of it as electrical pressure — the force that pushes electrons through a circuit. A 9V battery has more "push" than a 1.5V AA cell.
  • Current (I) — measured in amperes (A). This is the rate at which electrons actually flow. More current means more electrons moving per second.
  • Resistance (R) — measured in ohms (Ω). Resistance is opposition to current flow. Higher resistance means less current for the same voltage.

The Ohm's Law Triangle

A handy memory trick is the Ohm's Law Triangle. Write V at the top, I on the bottom left, and R on the bottom right. To find any one value, cover it up and the remaining symbols show the formula:

  • Cover V → V = I × R
  • Cover I → I = V ÷ R
  • Cover R → R = V ÷ I

Practical Examples

Example 1: Finding Current

You have a 12V battery connected to a 100Ω resistor. How much current flows?

I = V ÷ R = 12 ÷ 100 = 0.12 A (120 mA)

Example 2: Finding Resistance

A 5V circuit needs to limit current to 20 mA (0.02 A). What resistor do you need?

R = V ÷ I = 5 ÷ 0.02 = 250 Ω

You'd choose the nearest standard value — a 270Ω resistor would work well here.

Example 3: Finding Voltage Drop

A 470Ω resistor has 50 mA flowing through it. What is the voltage across it?

V = I × R = 0.05 × 470 = 23.5 V

Where Ohm's Law Applies — and Where It Doesn't

Ohm's Law applies to ohmic conductors — materials like copper wire and carbon resistors where resistance stays constant regardless of voltage or current. It does not perfectly apply to:

  • Diodes — resistance changes dramatically with voltage direction
  • Transistors — non-linear devices that amplify or switch
  • Light bulbs — resistance increases as the filament heats up

For those components, Ohm's Law still gives useful approximations, but more advanced models are needed for precision.

Why Ohm's Law Matters

Every time you design a circuit, choose a resistor, or troubleshoot a problem, Ohm's Law is your starting point. It lets you predict how a circuit will behave before you ever connect a wire. Master this formula first, and everything else in electronics becomes much easier to understand.