Ohm's Law Calculator

In short Ohm's law is two equations that between them relate four quantities: V = I × R, and P = V × I. Give any two and the other two follow, which is why this calculator takes whichever pair you happen to know. Twelve volts across four ohms draws three amps and dissipates thirty-six watts, and you can arrive at that from any two of those numbers. The two rearrangements worth committing to memory are P = I²R and P = V²/R, because those are the forms that tell you when something is about to get hot — power rises with the SQUARE of current or voltage, so doubling either quadruples the heat. That is the calculation beginners skip. A resistor should dissipate no more than about half its rating in still air, so a part running at 60 mW wants at least an eighth-watt rating and in practice a quarter-watt, which is what everyone stocks. An under-rated resistor discolours, drifts upward in value and eventually fails open, usually after the circuit has been working long enough for you to stop suspecting it. One caveat: Ohm's law assumes a resistance that does not change. That is true enough of a resistor and false of a filament lamp, a motor or any semiconductor, whose resistance moves with temperature and with the current itself.