Question

A wire of length L and 3 identical cells of negligible internal resistance are connected in series. Due to the current, the temperature of the wire is raised by `DeltaT` 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 2L. The temperature of the wire is raised by the same amount `DeltaT` in the same time t. the value of N is
A. 4
B. 6
C. 8
D. 9

Answer 1

Correct Answer - B

`H = (3E)^(2))/(R ) t = ms Delta T` ..(i)

`H = ((NE)^(2))/(2R)t = 2 mss Delta T` ..(ii)
[ ddue to doubling of length , resistace and mass of wire is doubled]
`(ii)//(i)`
`(N^(2))/(2xx9)=2 rArr N^(2)=36 rArr N=6`