17. Material Time to reach equilibrium A 5 minutes B 30 minutes C 110 minutes In

Question

17. Material Time to reach equilibrium A 5 minutes B 30 minutes C 110 minutes In an experiment, a thin sheet of material A, B, or C was used to divide two objects of different temperature. In each trial, the conditions were identical except for the material of the thin sheet. The data table shows the times in which the objects reached thermal equilibrium. Which material, if any, is the best thermal insulator?

All three are equal insulators.

material B

material A

material C

410 K

690 K

157 K

378 K

19.Rod X and rod Y are cylindrical rods made of the same metal. Rod X has radius r and length L. Rod Y conducts heat at the same rate as Rod X. Which could be the dimensions of Rod Y?

radius r; length 2L

radius 4r; length L

radius 2r; length 4L

radius 2r; length 2L

410 K

690 K

157 K

378 K

Explanation / Answer

17. According to the definition of an insulator, its internal structure blocks the heat passing so heat takes a longer time to pass through and reach equilibrium. In this case, the material C takes the highest material time to reach equilibrium, therefore, C is the best thermal insulator.

18. The mercury was at the liquid condition, therefore the initial temperature was 234K. The final temperature we suppose is T2. Now energy liberated is 157KJ

We know energy H= M*S* (T2-T1) where M= 5Kg, S= .140KJ/Kg.K, T1= 234K, T2=?

for changing phase at same temperature energy E = M* latent heat

In this case E= 5* 11.3KJ= 56.5KJ

Therefore M*S* (T2-T1) = 157-56.5KJ

or 5* .14* (T2- 234) = 100.5

or T2= 100.5/[5 *.14 ] +234 = 377.89 =378K

19. The rate of heat transfer is directly proportional to the surface area of the metal. Thermal conductivity

K = Q* W/ A * T where Q = heat applied, W = thickness, A= surface area and T= temperature difference

Now heat conductivity rate is same for X and Y

therefore dimensions of Y is R= 2r, L=2L

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