Page 278 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 22] HEAT TRANSFER 263

SOLVED PROBLEM 22.10
7
2
The sun radiates energy at the rate of 6.5 × 10 W/m from its surface. Assuming that the sun radiates
as a blackbody (which is approximately true), find its surface temperature.
4
The emissivity of a blackbody is e = 1. From R = σ T we have

7 2
4 R 4 6.5 × 10 W/m
T = = = 5800 K
2
4
σ 5.67 × 10 −8 W/(m ·K )
SOLVED PROBLEM 22.11
In the operation of a thermograph, the radiation from each small area of a person’s skin is measured and
shown by different shades of gray or by different colors in a thermogram. Because the skin over a tumor
is warmer than elsewhere, thermograms are used in screening for various cancers. What is the percentage
difference between the radiation rates from skin at 34 and 35 C?
◦
4
4
The emissivity of skin is the same at both temperatures, so R 1 /T = R 2 /T . Here T 1 = 34 C + 273 = 307 K
◦
1 2
and T 2 = 35 C + 273 = 308 K. Hence
◦
4
4
T − T 4 (308 K) − (307 K) 4
R 2 − R 1 2 1
= 4 = = 0.013 = 1.3%
R 1 T (307 K) 4
1
which is large enough to detect.
Multiple-Choice Questions

22.1. Heat transfer by conduction occurs
(a) only in liquids (c) only in liquids and solids
(b) only in solids (d) in solids, liquids, and gases

22.2. Heat transfer by convection occurs
(a) only in liquids (c) only in liquids and gases
(b) only in gases (d) in solids, liquids, and gases

22.3. Radiation occurs
(a) only from liquids
(b) only from solids
(c) only from liquids and solids
(d) from solids, liquids, and gases

22.4. Heat flows through a slab of some material at a rate that depends on which one or more of the following?
(a) the thickness of the slab
(b) the area of the slab
(c) the specific heat capacity of the material
(d) the temperature difference between the faces of the slab

22.5. The temperature of an object that emits electromagnetic radiation must be
◦
(a) higher than 0 C
(b) higher than 0 K
(c) higher than that of its surroundings
(d) high enough for it to glow