Question

An AC source is $120V-60Hz$. The value of voltage after $\frac{1}{720}s$ from start will be

• A. $20.2V$
• B. $42.4V$
• C. $84.8V$
• D. $106.8V$

Answer 1

The correct option is C $84.8V$

From $V=V_{0}sincot$
$V=V_{rms}\sqrt{2}sin\omega t$
After $t=\frac{1}{720}s,$
$V=120\sqrt{2}sin2\pi vt$
$=120\sqrt{2}sin2\pi \times 60\times \frac{1}{720}$
$=120\sqrt{2}sin\frac{\pi }{6}$
$=120\sqrt{2}\times \frac{1}{2}$
$=60\sqrt{2}=84.8V$