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 In a meter bridge experiment resistance are connected as shown in the figure. Initially resistance P=4 and the neutral point N is at 60 cm from A.  Now an unknown resistance R is connected in series to P and the new position of the neutral point is at 80 cm from A.  The value of unknown resistance R is :

  • Option 1)



  • Option 2)


  • Option 3)


  • Option 4)



Answers (1)


As we learnt in 

To find resistance of a given wire using metre bridge and hence determine the specific resistance -

For unknown resistance

X=\frac{\left (100 - l\right )R}{l}

x= Unknown resistance 

R= Given resistance


- wherein


\rho = \frac{X\pi D^{2}}{4l}

= .........ohms

\rho = Resistivity

X = Unknown resistance

D = Diametre

L = length of wire







\frac{4}{Q}=\frac{60}{100-60} \: \: \: \: \cdot \cdot \cdot (i)

\frac{4+R}{Q}=\frac{80}{100-80} \: \: \: \: \cdot \cdot \cdot \cdot (ii)

\\By \: (i) \: and \: (ii) \\ \\ R= \frac{20}{3}\Omega



Option 1)



This option is incorrect

Option 2)


This option is incorrect

Option 3)


This option is incorrect

Option 4)


This option is correct

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