The physics
An alternating current (AC) keeps changing. It grows, falls to zero, then flows the other way. We write a sine AC as i = I₀ sin ωt. I₀ is its peak.
i = I₀ sin ωt ω = 2πf = 2π / T
A heater's power is i²R. The square is never negative, so AC heats in both directions. Its average over a cycle is what counts.
The rms value is the steady DC that heats a resistor just as fast. It is the square root of the average of i².
Irms = √(average of i²) for a sine: Irms = I₀ / √2
A plain “220 V” on the mains is an rms value. The peak of that sine is 220√2, about 311 V.
For a steady part plus a sine, add the squares: Irms² = Idc² + I₀²/2. For a straight ramp from 0 to I₀, Irms = I₀ / √3.