Question

Find thermal resistance in SI units.

• A. 40 K/W
• B. 20 K/W
• C. 80 K/W
• D. 10 K/W

Answer 1

Answer: (A)

40 K/W

Answer 2

The thermal resistance of a hollow sphere is given by the formula: $R_{th} = \frac{r_2 - r_1}{4 \pi k r_1 r_2}$ Given: $k = \frac{1}{32\pi}$ W/mK $r_1 = 10$ cm $= 0.1$ m $r_2 = 20$ cm $= 0.2$ m

Substituting the values: $R_{th} = \frac{0.2 \text{ m} - 0.1 \text{ m}}{4 \pi \left(\frac{1}{32\pi} \text{ W/mK}\right) (0.1 \text{ m})(0.2 \text{ m})}$ $R_{th} = \frac{0.1 \text{ m}}{\frac{1}{8} \times 0.02 \text{ m}^2 \text{ W/mK}}$ $R_{th} = \frac{0.1}{0.0025} \text{ K/W}$ $R_{th} = 40 \text{ K/W}$