Why it matters
Amplifier power ratings, speaker impedance, wiring choices, and fuse sizing all come straight out of these few equations. Get the basics and the rest of the electronics reference stops being magic.
How it works
Three quantities, one law:
- Voltage (V, in volts) is electrical pressure — the push.
- Current (I, in amps) is the flow of charge that push causes.
- Resistance (R, in ohms, Ω) is how much the path opposes that flow.
Ohm’s law links them: V = I × R. Rearranged, current is I = V / R. So for a fixed voltage, lower resistance means more current.
Power is the rate of energy delivery: P = V × I (watts). Substituting Ohm’s law gives the two forms that matter for audio: P = V² / R and P = I² × R. Both say the same thing — at a fixed voltage, a lower-ohm load draws more power and more current.
In speakers, the “resistance” is really impedance, because it changes with frequency. But the relationships above still drive everything: that’s why a 4-ohm speaker pulls roughly twice the current of an 8-ohm speaker from the same amplifier, and why low loads stress amps.
Common mistakes
Simple example
Related concepts
Related concepts
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