Ohm's Law
The single most useful equation in electronics: how voltage, current and resistance bind each other.
Builds on: 0.2 Voltage: Energy per Charge0.3 Current: Charge in Motion1.1 Your First Circuit
Resistance: friction for charge
As electrons drift through a material they constantly collide with its atoms, giving up energy as heat. How much a component impedes flow is its resistance, measured in ohms (Ω). Thin wires resist more than thick ones, long more than short, and some materials vastly more than others — that’s why we build heaters out of nichrome and wires out of copper.
The law
In 1827 Georg Ohm found that for most conductors, the current is simply proportional to the voltage pushing it. Double the push, double the flow. The constant between them is the resistance:
This one line answers three everyday questions. How much current will flow? I = V/R. What resistor do I need to get a wanted current? R = V/I. What voltage appears across this part? V = I·R. You will use it in literally every lesson from here on.
Worked examples
- A 9 V battery across a 450 Ω resistor: I = 9 / 450 = 0.02 A = 20 mA.
- You want 15 mA to flow from a 5 V supply: R = 5 / 0.015 ≈ 333 Ω (you’d grab the standard 330 Ω part).
- 2 A flows through a car headlight on a 12 V battery: the filament’s hot resistance is R = 12 / 2 = 6 Ω.
Voltage is the push, resistance is the squeeze, current is the result. More push → more flow. More squeeze → less flow. If you remember nothing else from this course, remember V = I·R.
Ohmic and non-ohmic
Components that obey V = I·R with a constant R are called ohmic — resistors and wires behave this way. Plenty of interesting parts do not: a lamp filament’s resistance rises as it heats, and diodes and LEDs (Unit 3) barely conduct at all until the voltage crosses a threshold, then conduct furiously. Ohm's law still applies to every resistor in those circuits — it just doesn’t describe the exotic parts themselves.
⚡ Lab — The V–I–R Machine
One loop, two knobs, one law.
- Hold R constant and sweep V: current tracks it in perfect proportion.
- Hold V at 9 V and sweep R across three decades — watch the current dive.
- Find settings that give exactly 20 mA. There is more than one way!