Ohm's Law Explained Like You're Five: Voltage, Current, and Resistance

Updated on February 12, 2026

My cousin called me last month, completely baffled. He'd just bought his first Arduino kit, soldered two wires together, plugged it in — and the LED immediately died. "I don't get it," he said. "I did exactly what the video showed." Sound familiar? That one dead LED is how most people's electronics journey begins. And 90% of the time, the culprit is not understanding one single equation: Ohm's Law.

Here's the wild part — once it clicks, it really clicks. You'll look at a circuit schematic and just know what's happening, the same way you know water will flow downhill. Let's get there.

First, Forget Everything "Electric" and Think About Water

Seriously. Close your eyes and picture a garden hose connected to a tap.

When you turn the tap on, water pushes through the hose. The water pressure building up at the tap? That's voltage. The actual flow of water moving through the hose? That's current. And the width (or narrowness) of the hose — how much it resists the water pushing through it? That's resistance.

A fat, wide hose lets a lot of water gush through easily — low resistance. A narrow garden hose? The water squeezes through slowly — high resistance. Same tap pressure, completely different flow.

Electricity works identically. Replace "water pressure" with voltage (measured in Volts, V), replace "water flow" with current (measured in Amperes, A), and replace "hose narrowness" with resistance (measured in Ohms, Ω).

The Equation That Runs the World

Georg Simon Ohm — a German physicist working in the 1820s — figured out that these three things always have a fixed relationship. He wrote it as:

V = I × R
(Voltage = Current × Resistance)

Three letters. That's it. This equation is used every single day by everyone from hobbyists prototyping in their garage to engineers designing the chips inside your phone.

The beautiful thing about algebra is you can rearrange this into three forms depending on what you're trying to find:

  • V = I × R → "What's the voltage across this component?"
  • I = V ÷ R → "How much current will flow through this wire?"
  • R = V ÷ I → "What resistance do I need here?"

A classic trick people teach: draw a triangle with V at the top, and I and R side by side at the bottom. Cover the one you want to find, and the remaining two show you what to do. Cover V → multiply I and R. Cover I → divide V by R. Cover R → divide V by I. Simple.

Back to That Dead LED

Let's use Ohm's Law to solve my cousin's problem — and understand why it fried.

A standard red LED needs about 2 volts to operate, and it can safely handle 20 milliamps (0.02 A) of current. That's it. Give it more current, and you've got a tiny, expensive piece of fried silicon.

My cousin was using a 5V Arduino pin. He connected the LED directly — no resistor in between. So let's see what happened using Ohm's Law.

An LED has very little internal resistance on its own — maybe 10–20 ohms when forward-biased. Let's say 15 Ω. The voltage available was 5V. So:

I = V ÷ R
I = 5 ÷ 15
I = 0.333 Amperes = 333 milliamps

He sent 333 milliamps through something rated for 20. The LED didn't stand a chance.

What he needed was a current-limiting resistor. Here's how to calculate the right one:

The LED drops 2V, so the resistor needs to handle the remaining 5V − 2V = 3V. We want the current to stay at 20mA (0.02A). So:

R = V ÷ I
R = 3 ÷ 0.02
R = 150 Ω

A 150-ohm resistor placed in series with that LED would've saved it. A 150Ω resistor costs less than half a cent. That's Ohm's Law doing its job.

Voltage: The "Pressure" That Makes Electrons Move

Voltage is often called "potential difference" — a fancier way of saying it's the difference in electrical pressure between two points. Your USB port provides 5V. A AA battery gives 1.5V. A car battery pushes 12V.

Higher voltage means more "push." More push through the same resistance means more current flows. Makes sense, right? Turn up the tap pressure with the same-width hose, and more water gushes through.

One thing beginners often mix up: voltage doesn't flow. Only current flows. Voltage is more like the difference in height between a hilltop and a valley — it describes the potential for things to move, not the movement itself.

Current: The Actual Flow of Electrons

Current is electrons physically moving through a conductor. In metals like copper wire, electrons are loosely attached to their atoms, so they're free to drift when voltage pushes them.

One ampere equals roughly 6.24 × 10¹⁸ electrons passing a point every second. That number is absurd, but the unit itself is practical. Your phone charger might draw 1–2A. A single LED needs 0.02A. A clothes dryer pulls 15–20A.

A key thing the water analogy nails perfectly: current is the same everywhere in a simple series circuit. If water flows through a single pipe with no branches, the same amount of water passes every point along the pipe — whether it's the start, middle, or end. Same with current through a series circuit. What changes from point to point is the voltage, which drops across each component.

Resistance: The Gatekeeper

Resistance is the opposition to current flow. Every material has some resistance — copper wire has very little (which is why we use it), while nichrome wire has a lot (which is why it's used in toasters and heaters).

Resistors — those tiny striped cylinders you see on every circuit board — are deliberately designed to have specific resistance values. They protect sensitive components, divide voltages, set bias points, and do a hundred other jobs.

Temperature affects resistance too. Most metals get more resistive as they heat up. This is actually why incandescent lightbulbs work the way they do — the tungsten filament gets so hot it glows, and its resistance spikes as it heats, limiting current automatically.

A Real Example: Designing a Simple Battery-LED Circuit

Say you have a 9V battery and a white LED (forward voltage: ~3.2V, max current: 20mA). What resistor do you need?

Step 1 — Find the voltage across the resistor:

V_resistor = 9V − 3.2V = 5.8V

Step 2 — Apply Ohm's Law to find resistance:

R = V ÷ I = 5.8 ÷ 0.02 = 290 Ω

Standard resistor values don't include 290Ω, so you'd pick the nearest common value: 300Ω. That means your current will actually be:

I = 5.8 ÷ 300 = 0.0193A ≈ 19.3mA

Perfectly safe. Your LED lives.

Using an Online Calculator to Speed This Up

Once you understand the concepts, engineering calculators become genuinely powerful tools rather than black boxes you blindly trust. An Ohm's Law calculator lets you punch in any two of the three values and instantly get the third. They're especially handy when you're iterating quickly — trying different resistor values to see how current changes, or working out whether a particular voltage source is appropriate for a component.

The best ones also handle power calculations alongside Ohm's Law. Power (measured in Watts) is calculated as P = V × I, or equivalently P = I² × R. This matters because resistors have power ratings — a 1/4-watt resistor doing the job of a component that's dissipating 1 watt will get very hot, very fast, and potentially fail. Knowing the power involved is just as important as knowing the voltage, current, and resistance.

The One Thing That Makes This All Stick

Don't just read about it. Buy a $10 multimeter and a bag of mixed resistors. Set the multimeter to measure resistance, clip it across a resistor, and verify the value. Then build the simple LED circuit above, measure the actual voltage across each component, and watch the numbers match what Ohm's Law predicts.

That moment — when your calculated number shows up on the display — is the moment electronics stops being magic and starts being a language you speak. V = I × R isn't just an equation. It's a lens that makes the invisible visible.

Everything else in electronics — transistors, op-amps, microcontrollers, RF circuits — builds on top of this one relationship. Get this down, and the rest becomes a matter of adding vocabulary to a grammar you already understand.

Now go rescue that LED.