Two metal rods, one made of Invar(alpha = 7 times 10^-7 K^-1) and the other of c

Question

Two metal rods, one made of Invar(alpha = 7 times 10^-7 K^-1) and the other of copper (alpha = 1.7 times 10^-5 K^-1) are observed to have the same length when they are both heated to a temperature of 400 K. When they are cooled to room temperature (300 K): A. the Invar rod is longer than the copper rod. B. it is impossible to know which rod will be longer at 300 K without knowing the common length of the rods at 400 K. C. the copper rod is longer than the Invar rod. D. the rods are still the same length as each other, but both are shorter than they were at 400 K E. the rods are still the same length as each other, but both are longer than they were at 400 K

Explanation / Answer

change n length during expansion or compression

dL = Lo*Alpha*dT

where Alpha = thermal coefficient

now Alpha copper > Alpha invar

during compression

for same dT and same Lo

dL invar > dL copper

=> OPTION (A) invar rod s longer than copper rod.

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