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Explain the determination of the internal resistance of a cell using voltmeter. |
| Answer» <html><body><p></p>Solution :Determination of internal <a href="https://interviewquestions.tuteehub.com/tag/resistance-1186351" style="font-weight:bold;" target="_blank" title="Click to know more about RESISTANCE">RESISTANCE</a>: The emf of cell `xi` is measured by connecting a high resistance voltmeter across it without connecting the external resistance R. Since the voltmeter draws verylittle current for <a href="https://interviewquestions.tuteehub.com/tag/deflection-17189" style="font-weight:bold;" target="_blank" title="Click to know more about DEFLECTION">DEFLECTION</a>, the circuit may be considered as open. Hence, the voltmeter <a href="https://interviewquestions.tuteehub.com/tag/reading-19542" style="font-weight:bold;" target="_blank" title="Click to know more about READING">READING</a> gives the emf of the cell. Then external resistance R is included in the circuit and current I is extablished in the circuit. The potential differenceacross R is equal to the potential difference across the cell (V). <br/> <img src="https://d10lpgp6xz60nq.cloudfront.net/physics_images/FM_PHY_XII_V01_C02_E01_040_S01.png" width="80%"/> <br/> <img src="https://d10lpgp6xz60nq.cloudfront.net/physics_images/FM_PHY_XII_V01_C02_E01_040_S02.png" width="80%"/> <br/> The potential drop across the resistor R is <br/> `V=IR "" ...(1)` <br/> Due to internal resistance r of the cell, the voltmeter reads a value V, which is less than the emf of cell `xi`. It is because, <a href="https://interviewquestions.tuteehub.com/tag/certain-407894" style="font-weight:bold;" target="_blank" title="Click to know more about CERTAIN">CERTAIN</a> amount of voltage (Ir) has dropped across the internal resistance r. <br/> Then`V=xi-Ir` <br/> `Ir=xi-V "" ...(2)` <br/> <a href="https://interviewquestions.tuteehub.com/tag/dividing-957391" style="font-weight:bold;" target="_blank" title="Click to know more about DIVIDING">DIVIDING</a> equation (2) by equation (1), we get <br/> `(Ir)/(IR)=(xi-V)/(V)` <br/> `r=|(xi-V)/(V)|R"" ...(3)` <br/> Since `xi, V and R` are known, internal resistance r can be determined.</body></html> | |