Thermal Properties of Matter — Practice

Question 1

A 20 m steel rail at 20 C is heated to 45 C (alpha = 1.2e-5 /C). Find the expansion.

Accepted Answer

delta-L = alpha L0 delta-T = 1.2e-5 x 20 x 25 = 6e-3 m = 6 mm.

Question 2

A 100 g copper block (c = 390 J/kg K) at 200 C is placed in 200 g water at 20 C. Find the final temperature.

Accepted Answer

0.1 x 390 x (200 - T) = 0.2 x 4200 x (T - 20). 39(200 - T) = 840(T - 20). 7800 - 39T = 840T - 16800, so 24600 = 879T, T = 28 C.

Question 3

Find the heat to convert 100 g ice at -10 C to steam at 100 C (c_ice = 2100, L_f = 3.36e5, c_water = 4200, L_v = 22.6e5 J/kg).

Accepted Answer

Q1 = 0.1 x 2100 x 10 = 2100 J; Q2 = 0.1 x 3.36e5 = 33600 J; Q3 = 0.1 x 4200 x 100 = 42000 J; Q4 = 0.1 x 22.6e5 = 226000 J. Total = 303700 J.

Question 4

At what temperature is the Celsius reading equal to the Fahrenheit reading?

Accepted Answer

C = (9/5)C + 32 gives C = -40, so -40 C = -40 F.

Question 5

Why is the bottom of a lake warmer than the top in winter?

Accepted Answer

Water is densest at 4 C. As the surface cools it sinks until the lake reaches 4 C; further cooled surface water (0-4 C) is less dense, so it stays on top and freezes, leaving the bottom water at about 4 C.

Practice — Thermal Properties of Matter

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