Paper 3 Alternating Current Answers

Q1 RMS from a Sinusoid

1. $I_{rms}=I_0/\sqrt{2}=5.0/\sqrt{2}=3.54\mathrm{A}$. [2]
2. $P=I_{rms}^{2}R=(3.54{)}^2(6.0)=75\mathrm{W}$. [2]
3. The arithmetic mean over a full cycle is zero, but heating depends on $I^{2}R$, which is always positive; rms accounts for mean square current. [3]

Q2 AC Waveform Quantities

1. $V_0=40/2=20\mathrm{V}$. [1]
2. $V_{rms}=20/\sqrt{2}=14.1\mathrm{V}$. [2]
3. $f=1/T=1/(8.0\times 10^{-3})=125\mathrm{Hz}$. [2]