Understanding Ohm's Law: The Foundation of Electronics
Published Apr 14, 2026 · 5 min read
Ohm's Law is the E = mc² of electronics. Three variables — voltage (V), current (I), and resistance (R) — and one relationship: V = I × R. Master this and you can design any basic circuit.
The Water Analogy
| Electrical | Water | Unit |
|---|---|---|
| Voltage (V) | Water pressure | Volts (V) |
| Current (I) | Flow rate | Amps (A) |
| Resistance (R) | Pipe narrowing | Ohms (Ω) |
Higher pressure (voltage) pushes more water (current) through the pipe. A narrower pipe (resistance) reduces flow.
The 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
Example: LED needs 20mA (0.02A) with 2V forward voltage on a 12V supply. Resistor needed: R = (12 − 2) / 0.02 = 500Ω (use 470Ω or 510Ω standard value).
The Power Equation
P = V × I (power in watts). Combined with Ohm's Law:
- P = I² × R (useful when you know current and resistance)
- P = V² / R (useful when you know voltage and resistance)
A 60W light bulb on 120V draws: I = 60/120 = 0.5A. Its resistance: R = 120/0.5 = 240Ω.
Practical Applications
- LED circuits: Calculate the correct current-limiting resistor
- Wire sizing: Higher current needs thicker wire (lower resistance)
- Fuse selection: Know the max current to choose the right fuse
- Battery life: A 2000mAh battery powering a 200mA device lasts ~10 hours
- Heat management: P = I²R tells you where heat is generated in a circuit
Try it: Use our Ohm's Law Calculator to solve for any variable — enter two values, get the rest.