物理熱力學問題...尋求高人指點

Let L be the length of he brass bar at 20'C

Expansion of the brass bar when its temperature is raised to 150'C
= L.a(brass).(150-20) = 130L.a(brass)
where a(brass) is the coefficient of linear expansion of brass

Similarly, expansion of the steel wire when its temperature is raised to 150'C
= L.a(steel).(150-20) = 130L.a(steel)
where a(steel) is the coefficient of linear expansion of steel

The difference in length = 130.L[a(brass)-a(steel)]

From the web, it can be found the the values of coefficient of expansion are:
a(brass) = 19x10^-6 /C and a(steel) = 12x10^-2 /C

Hence, difference in length = 130x7x10^-6.L = (9.1x10^-4)L

Since Young's modulus E = stress/strain
but strain = 9.1x10^-4L/L = 9.1x10^-4

Thus, stress = (9.1x10^-4)E
where E is the Young's Modulus of steel (try to look up its value from your physics textbook or from the web and use it to calculate the stress).