circuit diagram of potentiometer is used to compare e.m. of two cells
Answers
Explanation:
Comparison of emf of two cells: First of all the ends of potentiometer are connected to a battery B1, key K and rheostat Rh such that the positive terminal of battery B1 is connected to end A of the wire. This completes the primary circuit.
Now the positive terminals of the cells C1 and C2 whose emfs are to be compared are connected to A and the negative terminals to the jockey J through a two-way key and a galvanometer. This is the secondary circuit.
Method: (i) By closing key K, a potential difference is established and rheostat is so adjusted that when jockey J is made to touch at ends A and B of wire, the deflection in galvanometer is on both sides. Suppose in this position the potential gradient is K.
(ii) Now plug is inserted between the terminals 1 and 3 so that cell C1 is included in the secondary circuit and jockey J is slided on the wire at P1 (say) to orbit the null point. The distance of P1 from A is measured. Suppose this length is l1i.e. AP1 = l1
therefore The emf of cell C1, begin mathsize 11px style straight epsilon subscript 1 space equals space Kl subscript 1 end style .....(1)
(iii) Now plug is taken off between the terminals 1 and 3 and inserted in between the terminals 2 and 3 to bring cell C2 in the circuit. Jockey is slided on wire and null deflection P2 is noted. Suppose distance of P2 from A is l2 i,e, AP2 = l2
therefore The emf of cell C2, begin mathsize 11px style straight epsilon subscript 2 space end subscript equals space Kl subscript 2 end style .....(2)
Dividing (1) by (2), we get begin mathsize 11px style straight epsilon subscript 1 over straight epsilon subscript 2 equals straight l subscript 1 over straight l subscript 2 end style
Thus emfs of cells may be compared. Out of these cells if one standard cell, then the emf of other cell may be calucated.
Sensitivity: (i) To increase the sensitivity of measurement, the value of potential gradient k is kept least possible. Smaller the value of K, greater is the length (l) of the null deflection; and so greater will be the accuracy of measurement. That is way a very long wire is used in potentiometer.
OR
Determination of internal Resistance of potentiometer:
Circuit: A battery B1, a rheostat (Rh) and a key K is connected across the ends A and B of the potentiometer wire such that positive terminal of battery is connected to points A. This completes the primary ciruuit.
Now the give cell C is connected such that its positive terminal is connected to A and negative terminal to jockey J through a galvanometer. A resistance box (R) and a key K1 are connected across the cell. This completes the secondary circuit.
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