How Ohm's law works
Ohm's law ties the three electrical basics together:
V = I × R
Voltage in volts, current in amperes, resistance in ohms. Power joins via P = V × I, which combines with Ohm's law into the two alternates every technician memorises:
P = I² × R P = V² ÷ R
Those alternates answer the two common real questions: how hot does this resistor get (I²R — dissipation at a given current) and what does this heater draw (V²/R — dissipation at a fixed supply voltage). The four-form wheel lets you solve any third or fourth quantity from any known pair; the calculator implements all four combinations.
Worked example
A 12 V supply feeding a 0.5 A LED driver: R = 12 ÷ 0.5 = 24 Ω and P = 12 × 0.5 = 6 W. Halve the resistance to 12 Ω and current doubles to 1 A while power jumps fourfold to 12 W — current scales as 1/R but power as V²/R, which is why undervoltage heaters and undersized wiring behave so differently from intuition.
Where it stops being exact
Ohm's law is exact for ohmic conductors at constant temperature. Filament lamps, diodes and thermistors have resistance that shifts with temperature or voltage, so treat their results as operating-point values, not constants.