NEET Questions Solved

A wire of length L and 3 identical cells of negligible internal resistances are connected in series. Due to current, the temperature of the wire is raised by ΔT in a time t. A number N of similar cells is now connected in series with a wire of the same material and cross–section but of length 2 L. The temperature of the wire is raised by the same amount ΔT in the same time t. the value of N is 

(1) 4

(2) 6

(3) 8

(4) 9

(2) Let R and m be the resistance and mass of the first wire, then the second wire has resistance 2R and mass 2m. Let E = emf of each cell, S = specific heat capacity of the material of the wire. For the first wire, current i1=3ER and i12Rt=mSΔT

For the second wire, i2=NE2R and i22(2R)t=2mSΔT. Thus, i1=i2 or N=6.  

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