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Q.

Two rods of equal cross-sections, one of copper and the other of steel are joined to form a composite rod of length 2.0 m at 20°C, the length of the copper rod is 0.5 m. When the temperature is raised to 120°C, the length of composite rod increases to 2.002 m. If the composite rod is fixed between two rigid walls and thus not allowed to expand, it is foundthat the length of the component rods also do not change with increase in temperature. calculate the Young's modulus of steel. Given Young's modulus of copper =1.3×1011 N/m2, the coefficient of linear expansion of copper αc=1.6×10-5/c  .

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a

2.6×1011 Pa

b

1.6×1010 Pa

c

1.3×1010 Pa

d

0.9×1010 Pa

answer is A.

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Detailed Solution

The change in length of copper rod due to change in temprature from 20°C to 120°C.

Δl1=lcαcΔT;0.5αc(120-20)=50αc

For steel, Δl2=lsαsΔT=1.5αs(120-20)=150αs

Total change in length, Δl=Δl1+Δl2=50αc+150αs

It is given that Δl=0.002 m

50αc+150αs=0.002 or, αs=0.002-50αc150 =0.002-50×1.6×10-5150=0.8×10-5/c   F=YAαΔT  ;  Ysαs=Ycαc Ys=2.6×1011 N/m2

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