The formula
V = I · R
| Symbol | Quantity | Unit |
|---|---|---|
| V | Voltage | V |
| I | Current | A |
| R | Resistance | Ω |
What it means
Ohm's law says the current through a conductor is proportional to the voltage across it and inversely proportional to its resistance. It is the single most-used relationship in electronics, and it holds for ordinary conductors under ordinary conditions.
How to work it out by hand
- Voltage is current times resistance
- Current is voltage divided by resistance
- Resistance is voltage divided by current
What is worth knowing
Ohm's law is a good description of most materials, not a universal law: diodes, transistors and filament bulbs are all non-ohmic, meaning their resistance changes with the conditions. A filament's resistance rises sharply as it heats, which is why bulbs so often fail at the moment you switch them on — the cold filament briefly draws a large current. The other thing worth internalising is that what harms people is current, not voltage: it takes only around 30 mA through the body to be dangerous, and whether that flows depends on the resistance of the path.