Ncert Exercises & Solutions

When a voltage measuring device is connected to AC mains, the meter shows the steady input voltage of \(220~\text{V}\). This means:

1.	input voltage cannot be AC voltage, but a DC voltage
2.	maximum input voltage is \(220~\text{V}\)
3.	the meter reads not \(v\) but \(<v^2>\) and is calibrated to read \(\sqrt{<v^2>}\)
4.	the pointer of the meter is stuck by some mechanical defect

Hint: The voltmeter is calibrated to read the root mean square value.

Explanation: In AC voltage, the steady input voltage is the RMS value of the voltage. The voltmeter connected to the AC mains measures the mean value \((<v^2>)\) and is calibrated to display the RMS value. It does this by multiplying the measured value \(< v^2>\) by the form factor to obtain the RMS voltage. Therefore, the meter reads not \(v\) but \(<v^2>\) and is calibrated to read \(\sqrt{<v^2>}.\)
Hence, option (3) is the correct answer.

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