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Question: Answered & Verified by Expert
The formula for internal resistance of a cell will be, (Here \( l_{1} \& l_{2} \) are the balancing lengths of cell in open and closed circuit respectively.)
PhysicsCurrent ElectricityJEE Main
Options:
  • A \( \mathrm{r}=\left(\frac{\mathrm{l}_{1}-\mathrm{l}_{2}}{\mathrm{l}_{2}}\right) \mathrm{R} \)
  • B \( \mathrm{r}=\left(\frac{\mathrm{l}_{2}-\mathrm{l}_{1}}{\mathrm{l}_{2}}\right) \mathrm{R} \)
  • C \( \mathrm{r}=\left(\frac{\mathrm{l}_{1}-\mathrm{l}_{2}}{\mathrm{l}_{1}}\right) \mathrm{R} \)
  • D \( \mathrm{r}=\left(\frac{\mathrm{l}_{2}-\mathrm{l}_{1}}{\mathrm{l}_{1}}\right) \mathrm{R} \)
Solution:
1111 Upvotes Verified Answer
The correct answer is: \( \mathrm{r}=\left(\frac{\mathrm{l}_{1}-\mathrm{l}_{2}}{\mathrm{l}_{2}}\right) \mathrm{R} \)

Given that balancing lengths of potentiometer in open and closed circuit as l1,l2 and If R is the resistance of resistance box and x is the potential gradient then 

Internal resistance of the cell will be,r=EVVR=l1xl2xl2xR=l1l2l2R

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