A student was given the experiment to measure the emf of a unknown cell using potentiometer. He cheated result and wrote the observation arbitrarily and was caught. Which reading helped the teacher to arrive at the conclusion that he cheated?
S. No. Value of rheostat Null point
1. $R_{1}$ 50 cm
2. $R_{2}$ 60 cm
3. $R_{3}$ 70 cm
4. $R_{4}$ 80 cm

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Solution

The correct options are
A 1
B 2
Current at null point will be, $i = \frac{20}{8 + 2 + R} = \frac{20}{10 + R}$ .

Also, at null point, $\frac{8 l}{100} i = 10$

therefore, $i l = 125$

From above equations, $\frac{l}{10 + R} = 6.25$

$R = \frac{l}{6.25} - 10 > 0$

Hence, $l > 62.5$ .

First and second readings voilate the conditions.

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S. No.	Value of rheostat	Null point
1.	$R_{1}$	50 cm
2.	$R_{2}$	60 cm
3.	$R_{3}$	70 cm
4.	$R_{4}$	80 cm

The correct options are A 1 B 2Current at null point will be, i=208+2+R=2010+R.Also, at null point, 8l100i=10therefore, il=125From above equations, l10+R=6.25R=l6.25−10>0Hence, l>62.5.First and second readings voilate the conditions.