Question: Q. 3. When a voltage measuring device is connected to AC mains, the meter shows the steady input voltage of $220 \mathrm{~V}$. this means
(a) Input voltage cannot be AC voltage, but a DC voltage.
(b) Maximum input voltage is $220 \mathrm{~V}$.
(c) The meter reads not $V$ but $\left\langle V^{2}\right\rangle$ and is calibrated to read $\sqrt{\left\langle V^{2}\right\rangle}$.
(d) The pointer of the meter is stuck by some mechanical defect.
[NCERT Exemplar]
Show Answer
Solution:
Ans. Correct option : (c)
Explanation: The voltmeter in AC circuit reads value $\left\langle V^{2}\right\rangle$ and meter is calibrated to rms value $\left\langle V^{2}\right\rangle$ which is multiplied by $\sqrt{2}$ to get $V_{r m s}$. In other words, voltmeter connected to the $\mathrm{AC}$ main read root mean square value of $\mathrm{AC}$ voltage, i.e., $\sqrt{\left\langle V^{2}\right\rangle}$.
