Question

Unaware about the fact that analog ammeters and voltmeters can also have zero error, a student recorded following readings while determine resistance by Ohm's law

If the ammeter has no zero error, the zero error is the voltmeter is

Obs, No.	Voltage/V	Current/mA	Obs No.	Voltage/V	Current/mA
1	1.0	40	4	7.0	160
2	3.0	80	5	9.0	200
3	5.0	120

• A. $-1V$
• B. $-1.5V$
• C. $0.5V$
• D. $1V$

Answer 1

The correct option is A $-1V$

$\begin{array}{c}V=IR\\ R=\frac{V}{I}\end{array}$

$\begin{array}{c}\text{Let zero error in}V\text{be}\\ x\text{.}\\ \text{In each}V/I\text{(all will be}\\ \text{equal) we will subtract the}\\ \text{zero error}x\text{.}\\ R=\frac{1-x}{40\times {10}^{-3}}=\frac{3-x}{80\times {10}^{-3}}\end{array}$

$\begin{array}{c}=\frac{5-x}{120\times {10}^{-3}}=\frac{7-x}{160\times {10}^{-3}}\\ =\frac{9-x}{200\times {10}^{-3}}\\ \text{Now,}\\ \frac{1-x}{40}=\frac{3-x}{802}\end{array}$

$\begin{array}{c}2-2x<3-x\\ -1=x\\ x=-1\end{array}$