Circuits and waves are two subjects built on the same handful of proportional relationships. Ohm's law (V = IR) fixes how voltage, current and resistance relate in a steady circuit, and electrical power (P = VI) turns that into the watts printed on an appliance label. Waves obey v = fλ — speed equals frequency times wavelength — which is as true for sound in air as it is for light in a vacuum.
Calculate current and power from voltage and resistance with Ohm's law (V = IR, P = V²/R).
I = V / R, P = V² / RCalculate wavelength from wave speed and frequency (λ = v/f) — for light, radio, WiFi and sound.
λ = v / fConvert watts to amps: calculate the current a device draws from its power and supply voltage (I = P / V).
I = P / VUse these to size a cable or fuse, convert an appliance's watts into the amps it will actually draw, or find the wavelength that belongs to a given frequency — the number an antenna length or a speaker crossover really depends on. Circuit results assume direct current and a constant resistance; wave results assume one uniform medium, which is why what happens when light crosses into glass or water is handled by the optics calculators rather than here.