12 cells each having same emf are connected in series with some cells wrongly connected. The arrangement is connected in series with an ammeter and two cells which are in series. Current is 3 A when cells and battery aid each other and is 2 A when cells and battery oppose each other. The number of cells wrongly connected is
Text Solution
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Let n be the number of wrongly connected cells.
Number of cells helping one another 
Total e.m.f. of such cells 
Total e.m.f. of cells opposing = nE
Resultant e.m.f. of battery 
Total resistance of cells = 12r
( Q resistance remains same irrespective of connections of cells)
With additional cells
Total e.m.f. of cells when additional cells help battery = (12 – 2n) E + 2E
Total resistance = 12r + 2r = 14r
∴ ∴ 
......(i)
Similarly when additional cells oppose the battery
......(ii)
Solving (i) and (ii), n = 1
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