Thermal Behavior of Matter and Heat Engines - Department of ...

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Answer: As the water or steam comes into contact with skin, it cools down to the temperature of skin, e.g. 39 o C. For the case of steam, it needs to cool down to 100 o C water first and then further cool down to 39 o C. That means, the body skin absorb more heat from the steam than water, and the steam burn is worse. 11.5 Conduction Perhaps the most familiar form of heat exchange is conduction, which is the flow of heat directly through a physical material. For example, if you hold one end of a metal rod and put the other end in a fire, it doesn’t take long before you begin to feel warmth on your end. The heat you feel is transported along the rod by conduction. In microscopic point of view, when you placed one end of the rod into the fire the high temperature at that location caused the molecules to vibrate with an increased amplitude. These molecules in turn jostle their neighbors, and cause them to vibrate with greater amplitude as well. Eventually the effect travels from molecule to molecule across the length of the rod, resulting in the macroscopic phenomenon of conduction. Consider now, how much heat flows as a result of conduction? To answer this question we consider the simple system. Here we show a rod of length L and crosssectional area A, with one end at the temperature T 1 and the other at the temperature T 2 > T 1 . Experiments show that the amount of heat Q that flows through this rod: Increases in proportion to the rod’s cross-sectional area, A; Increases in proportion to the temperature difference, ∆ T = T 2 − T1 ; Increases steadily with time, t; Decreases with the length of the rod, L. Combining these observations in a mathematical expression gives: ⎛ ∆T ⎞ Q = kA⎜ ⎟t . ⎝ L ⎠ The constant k is referred to as the thermal conductivity of the rod. 10
Substance Thermal conductivity, k, [W/(m⋅K)] Silver 417 Copper 395 Gold 291 Aluminum 217 Steel, low carbon 66.9 Lead 34.3 Stainless steel 16.3 Ice 1.6 Concrete 1.3 Glass 0.84 Water 0.60 Wood 0.10 Wool 0.040 Air 0.0234 Table: Thermal conductivities Example One of the windows in a house has the shape of a square 1.0 m on a side. The glass in the window is 0.50 cm thick. How much heat is lost through this window in one day if the temperature in the house is 21 o C and the temperature outside is 0.0 o C? Answer: Since the thermal conductivity of glass is 0.84 W/(m⋅K) and the ⎛ ∆T ⎞ formula for heat flow is Q = kA⎜ ⎟t , hence, ⎝ L ⎠ 2 ⎛ 21K ⎞ 8 Q = [ 0.84W /( m ⋅ K)](1.0 m) ⎜ (24 × 60 × 60 s) = 3.0 × 10 J 0.0050 m ⎟ . ⎝ ⎠ 11
Page 1 and 2: Unit 11 Thermal Behavior of Matter
Page 3 and 4: pipelines typically include loops t
Page 5 and 6: Answer: The change in temperature
Page 7 and 8: Example The heat capacity of 1.00 k
Page 9: 11.5 Latent heats The latent heat,
Page 13 and 14: Q c ⎛ ∆T ⎞ = kc A⎜ ⎟t ⎝
Page 15 and 16: 11.7 Radiation All objects give off
Page 17 and 18: In addition, though not shown in th
Page 19: Answer: (a) Since W ⎛ T ⎞ = ⎜

Thermal Behavior of Matter and Heat Engines - Department of ...

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