Alternating Current & RMS — A/L Physics

What must I remember about Alternating Current & RMS?

• Sinusoidal AC Waveform: Instantaneous voltage v(t) = V0 sin(ωt) and current i(t) = I0 sin(ωt ± φ), where V0, I0 are peak amplitudes, ω = 2π f is angular frequency, and φ is phase angle.
• Root-Mean-Square (RMS) Values: Effective DC equivalent heating value of an alternating waveform: V_rms = V0 / √2 ≈ 0.707 V0; I_rms = I0 / √2 ≈ 0.707 I0; Standard domestic mains in Sri Lanka is 230 V RMS at 50 Hz.
• Mean (Average) Value of AC: Over a full cycle, mean value is strictly ZERO; Average value over half cycle: V_avg = (2 / π) V0 ≈ 0.637 V0.
• AC in Purely Resistive Circuit: Current and voltage are strictly IN PHASE (φ = 0); instantaneous power fluctuates between 0 and V0 I0; Average Power Dissipation P_avg = V_rms * I_rms = I_rms² R.

Test yourself (1 minute)

1. Hot-Wire / Moving-Iron Meters

   Conventional moving-coil DC meters read ZERO on AC because average torque is zero; AC measuring instruments (hot-wire or moving-iron) utilize I² heating or magnetic repulsion to read TRUE RMS values directly.

2. Form Factor and Peak Factor

   Form Factor = V_rms / V_avg = (V0/√2) / (2V0/π) = π / (2√2) ≈ 1.11 (for pure sine wave); Peak Factor = V0 / V_rms = √2 ≈ 1.414.

3. Peak insulation stress

   A 230 V RMS AC line has a maximum peak voltage of V0 = 230 * √2 ≈ 325 V; electrical insulation must withstand peak voltage, not RMS.

Practise Alternating Current & RMS questions today

Source: NIE G.C.E. (A/L) Physics Resource Book - Unit 8: AC Theory

What should I study first?

What comes next?

Next in the syllabus

Where does Alternating Current & RMS fit in the syllabus?

Unit 8.4 · Year 2 · Term 1