Ohm's Law Explained
Ohm's Law, formulated by German physicist Georg Simon Ohm in 1827, is one of the most fundamental relationships in electrical engineering: voltage equals current times resistance. It holds for most conductors (called ohmic conductors) under constant temperature conditions.
Ohm's Law — Four RelationshipsV = I × R (Voltage)
I = V ÷ R (Current)
R = V ÷ I (Resistance)
P = V × I (Power)
I = V ÷ R (Current)
R = V ÷ I (Resistance)
P = V × I (Power)
SI Units Reference
| Quantity | Symbol | Unit | Abbreviation |
|---|---|---|---|
| Voltage | V | Volt | V |
| Current | I | Ampere | A |
| Resistance | R | Ohm | Ω |
| Power | P | Watt | W |
💡 Memory trick: Use the Ohm's Law triangle. Cover the variable you want to find and the remaining two show the operation: V on top, I and R at the bottom (V = I×R).
Practical Examples
- LED circuit: 5V supply, LED needs 20mA (0.02A) → R = 5÷0.02 = 250Ω resistor needed
- Household appliance: 100W light bulb at 230V → I = P÷V = 100÷230 = 0.43A
- Car battery: 12V car battery, starter motor draws 200A → R = 12÷200 = 0.06Ω
⚡ Calculate any Ohm's Law variable — enter any two known values
Open Ohm's Law Calculator →ohms law calculatorvoltage calculatorcurrent calculatorresistance calculatorelectrical power calculatorV=IR calculatorP=VIohms law formula