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Q. 11.10 A brass rod of length 50\; cm and diameter 3.0\; mm is joined to a steel rod of the same length and diameter. What is the change in length of the combined rod at 250^{\circ}C,  if the original lengths are at 40.0^{\circ}C\; ? Is there a ‘thermal stress’ developed at the junction ? The ends of the rod are free to expand (Co-efficient of linear  expansion\; of\; brass=2.0\times 10^{-5}K^{-1},\; steel=1.2\times 10^{-5}K^{-1}).

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Length of the rods l = 50 cm

Co-efficient of linear expansion of brass, \\\alpha _{b}=2\times 10^{-5}K^{-1}

Co-efficient of linear expansion of steel, \\\alpha _{s}=1.2\times 10^{-5}K^{-1}

Initial Temperature T1 = 40.0 oC

Final Temperature T2 = 250 oC

Change in length of brass rod is

\\\Delta l_{t}=l_{t}\alpha_{b} \Delta T\\ \Delta l_{t}=50\times 2.0\times 10^{-5}\times (250-40)\\ \Delta l_{t}=0.21cm

Change in length of the steel rod is

\\\Delta l_{s}=l_{s}\alpha_{s} \Delta T\\ \Delta l_{s}=50\times 1.2\times 10^{-5}\times (250-40)\\ \Delta l_{s}=0.126cm

Change in length of the combined rod is

\\\Delta l=\Delta l_{s}+\Delta l_{b}\\ \Delta l=0.126+0.21\\ \Delta l=0.336cm

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