February 4, 2025

Ohm's Law

Ohm's Law is the foundation of circuit analysis. It describes the relationship between voltage, current, and resistance — three quantities you'll be thinking about constantly in any electronics work. Get comfortable with it early and everything else becomes easier.

The Formula

V = I × R
  • V — voltage in volts (V)
  • I — current in amperes (A)
  • R — resistance in ohms (Ω)

Rearrange it depending on what you're solving for:

V = I × R    (find voltage)
I = V / R    (find current)
R = V / I    (find resistance)

What It's Actually Saying

Voltage is pressure. Current is flow. Resistance is — resistance. Ohm's Law says that if you push harder (more voltage) through the same pipe (same resistance), more current flows. If you narrow the pipe (more resistance) without changing the pressure, less current flows. These relationships are proportional, which is what makes the math clean.

Example Calculations

Find the current: You have a 12V battery and a 4Ω resistor. How much current flows?

I = V / R = 12V / 4Ω = 3A

Find the voltage drop: 2A flows through a 10Ω resistor. What's the voltage across it?

V = I × R = 2A × 10Ω = 20V

Find resistance: You need to limit current to 20mA with a 5V supply. What resistor do you need?

R = V / I = 5V / 0.02A = 250Ω

Use the next standard value up — a 270Ω resistor.

That last example is the one you'll actually use — picking a current-limiting resistor for an LED is probably the first practical application of Ohm's Law most people encounter.

Power

Related to Ohm's Law is the power formula:

P = V × I     (power in watts = voltage × current)
P = I² × R   (useful when you know current and resistance)
P = V² / R   (useful when you know voltage and resistance)

This matters when you're choosing components. A resistor rated for 0.25W will overheat and fail if you're pushing 1W through it. Always calculate the power dissipation and check it against the component rating, with some margin.

Where It Doesn't Apply

Ohm's Law applies to resistive (ohmic) components. Diodes, transistors, LEDs, and capacitors don't follow it — their current-voltage relationship is non-linear. For DC circuit analysis, Ohm's Law is your main tool. For everything involving semiconductors or AC circuits, you need more tools.