Ohm's law is the first rule every electronics learner meets. It links three things in a circuit:
- Voltage (V), measured in volts: the "push" that moves electricity.
- Current (I), measured in amps: how much electricity flows.
- Resistance (R), measured in ohms (Ω): how much a part slows the flow.
The formula
V = I × R
Rearrange it to find whichever value you are missing:
| You want | Use |
|---|---|
| Voltage | V = I × R |
| Current | I = V ÷ R |
| Resistance | R = V ÷ I |
Example: a resistor for an LED
A red LED needs about 2 V and 20 mA (0.02 A). Your battery gives 9 V.
- Voltage the resistor must drop: 9 V − 2 V = 7 V
- Resistance: R = 7 ÷ 0.02 = 350 Ω
- Pick the next standard size up: 390 Ω
Too small a resistor lets too much current through and the LED burns out. Too big and it just glows dimly.
Quick tips
- Always check the units: 20 mA is 0.02 A, not 20 A.
- Power in the resistor is P = V × I = 7 × 0.02 = 0.14 W, so a common ¼ W resistor is fine.
- A multimeter lets you measure all three values and check your maths.