Ohm's law
V = I × R: voltage (volts) equals current (amps) times resistance (ohms). Rearranged, I = V ÷ R and R = V ÷ I. Power is P = V × I (also I²R or V²/R). With 12 V across 6 Ω, the current is 2 A and the power is 24 W.
LED resistor example
To run a red LED (2 V, 20 mA) from a 5 V USB supply, the resistor drops 3 V at 0.02 A, so R = 3 ÷ 0.02 = 150 Ω, and it dissipates 0.06 W, so a standard ¼ W resistor is fine. UK mains is 230 V, so a 3 kW kettle draws about 13 A, which is why plugs have 13 A fuses. Don't work on mains wiring unless you're qualified.
Frequently asked questions
What is the formula for Ohm's law?
V = I × R.
How do I calculate power?
P = V × I.
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