Two identical rectangular rods of metal are welded as shown in figure `(1)` and `20 J` of heat flows through the rods in `1 min`. How long would it take for `20 J` heat to flow through the rods if they are welded as shown in figure `(2)`

Explanation

Let `R` be the thermal resistance of each rod.
`:. ` In first case `(1)/(R_(1))=(1)/(R)+(1)/(R)` or `R_(1)=(R )/(2)`
So the rate of flow of heat in this situation will be
`(DeltaQ)/(Deltat)=(DeltaT)/(R_(1))=(100-0)/(R//2)=(20)/(60)`
`R=600^(@)C//W`
Now for case `(2)`
`R_(2)=R+R=600+600=1200^(@)C//W`
`:. (DeltaQ)/(Deltat)=(DeltaT)/(R_(2))`
`(20)/(t)=(100)/(1200)`
`t=240sec`.