Ohm's Law — Visualised in a Live Circuit Simulator
Ohm's Law is the foundation of all electronics. This guide explains it clearly and lets you prove it yourself with a live simulation running in your browser.
What is Ohm's Law?
Ohm's Law describes the relationship between three fundamental electrical quantities: voltage (V), current (I), and resistance (R). The law states that the voltage across a resistor equals the current through it multiplied by its resistance.
This relationship works in all three forms:
- V = I × R — Find voltage when you know current and resistance
- I = V / R — Find current when you know voltage and resistance
- R = V / I — Find resistance when you know voltage and current
See It Live in the Simulator
Click the button below to load an Ohm's Law demonstration circuit. You'll see a battery, a resistor, and an ammeter (current meter) in a simple loop.
→ Open Ohm's Law CircuitWatch the yellow dots moving around the circuit — those represent electrons (current). The speed and density of the dots show you how much current is flowing.
Experiment: Change the Resistance
In the simulator:
- Double-click the resistor to open its properties
- Change the resistance value from 100Ω to 200Ω
- Click OK and watch what happens to the current
You should see the current drop by half. If voltage stays at 10V and resistance doubles from 100Ω to 200Ω, then current drops from 100mA to 50mA. That's Ohm's Law in action: I = V/R = 10/200 = 0.05A = 50mA.
Experiment: Change the Voltage
- Double-click the voltage source (battery)
- Change the voltage from 10V to 20V
- Watch the current double
With a fixed 100Ω resistor and doubled voltage (20V), current doubles to 200mA. More voltage pushes more current through the same resistance.
Real-World Applications
Ohm's Law is used constantly in electronics design:
- LED current limiting resistors — Calculate the resistor needed to limit current through an LED without burning it out
- Voltage dividers — Use two resistors to create a specific output voltage
- Fuse selection — Calculate maximum current in a circuit to choose the right fuse rating
- Amplifier design — Calculate bias resistors for transistor and op-amp circuits
Try the Voltage Divider
A voltage divider uses two resistors to produce an output voltage between 0 and the supply voltage. Load this example to see it in action:
→ Open Voltage Divider CircuitThe output voltage is: Vout = Vin × R2 / (R1 + R2). Try changing R1 and R2 to see how the output voltage changes.
Circuit Diagrams
Basic Ohm's Law Circuit
The Ohm's Law Triangle
Common Beginner Mistakes
Mixing Up Units
Always convert to base units before calculating: kΩ to Ω (multiply by 1,000), mA to A (divide by 1,000). Mixing kΩ with mA in V = I × R gives a result 1,000,000× off.
Assuming Ohm's Law Applies Everywhere
Ohm's Law only applies to linear resistors. Diodes, transistors, and LEDs are non-linear — their resistance changes with voltage. Using V = I × R on a diode gives wrong answers.
Ignoring Internal Resistance
Real batteries have internal resistance (typically 0.1–2Ω). When current flows, voltage drops across this resistance, reducing terminal voltage below the rated value. This is why a battery "runs down" under load.
Exceeding Power Rating
Ohm's Law gives you current, but you also need to check power: P = V × I = I² × R. A 1/4W resistor with 100mA through it dissipates (0.1)² × 470 = 4.7W — 19× its rating. It will burn up.
Why Ohm's Law Matters in 2026
Ohm's Law is not just a classroom exercise — it is used every day in practical electronics. Before connecting any component, you check whether it will receive the correct voltage and current. Before a component can be damaged, you verify its power dissipation. Every pull-up and pull-down resistor on an I²C bus, every current-limiting resistor on a microcontroller GPIO pin, every voltage divider feeding a sensor to an ADC — all calculated directly from V = I × R and its derivatives.
Experiment: Build and Test Yourself
The best way to truly understand Ohm's Law is to build circuits and change values yourself. Try each of these simulations:
▶ Ohm's Law Circuit ▶ Voltage Divider ▶ Resistor Network