Internal Resistance Of A Cell Using Potentiometer — the NEET Physics formula with its derivation, variables, validity constraints and worked solver.
Internal Resistance of a cell using Potentiometer Applicable in physics laboratory experiments for determining the internal resistance of primary cells like Leclanche or Daniel cells using a null-deflection method. When the cell is in open circuit, EMF E is balanced: E = k l1. When the cell is connected to resistance R, terminal potential V is balanced: V = k l2. Using the relation E = V + i r and V = i R, we get E/V = (R + r)/R. Substituting k l1 / k l2 = (R + r)/R leads to l1/l2 = 1 + r/R. Rearranging gives r = R (l1/l2 - 1). The EMF of the driver cell must be greater than the EMF of the test cell. The current in the potentiometer wire must remain constant throughout the measurement. The potentiometer wire must be uniform in cross-section and material. Thinking that l2 can be greater than l1; l1 is always greater because E is greater than V. Forgetting that the driver cell's positive terminal must be connected to the same point as the test cell's positive terminal. Assuming internal resistance is a fixed constant; it actually varies with the current drawn and the age of the cell.