Ohm's Law Formula
Learn Ohm's law: V = IR.
The fundamental relationship between voltage, current, and resistance in electrical circuits.
The Formula
Ohm's law states that the voltage across a conductor is directly proportional to the current flowing through it.
The constant of proportionality is the resistance.
This is the most fundamental formula in electrical engineering.
Variables
| Symbol | Meaning |
|---|---|
| V | Voltage (Volts, V) |
| I | Current (Amperes, A) |
| R | Resistance (Ohms, Ω) |
Rearranged 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 1
A 12 V battery drives current through a 4 Ω resistor. What is the current?
I = V / R
I = 12 V / 4 Ω
I = 3 A
Example 2
A circuit carries 0.5 A through a 220 Ω resistor. What is the voltage drop across the resistor?
V = I × R
V = 0.5 A × 220 Ω
V = 110 V
When to Use It
Use Ohm's law when you need to:
- Calculate any one of voltage, current, or resistance when you know the other two
- Analyse simple DC circuits
- Size resistors for LED circuits or voltage dividers
- Troubleshoot electrical faults by measuring and comparing values
Ohm's law applies to resistive (ohmic) components where resistance is constant.
It does not directly apply to non-linear components like diodes or transistors.