The one triangle that runs everything electrical
Ohm's law ties the three fundamentals together — voltage pushes, current flows, resistance resists — and the power law converts that into watts, which is what you get billed for and what makes things hot.
V = I × R I = V ÷ R R = V ÷ I P = V × I
Worked example
A 1,500 W space heater on a 120 V circuit: I = P ÷ V = 12.5 A. That's why two of them on the same 15 A circuit (25 A total) trips the breaker every time — and why the heater's cord gets warm, since its resistance works out to just 9.6 Ω doing serious work.
Where the trades actually use this
Converting a nameplate wattage to amps to size a circuit, figuring what a generator can carry, spotting a bad element (resistance way off spec), or working out why a long extension cord murders a compressor. It's four tiny formulas that answer half the electrical questions on any job.