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A cylindrical steel rod, 0.01 m in diameter and 0.2 m in length is first heated to 750°C and then immersed in a water bath at 100°C. The heat transfer coefficient is 250 W/m2K. The density, specific heat and thermal conductivity of steel are ρ = 7801 kg/m3, c = 473 J/kgK, and k = 43 W/mK, respectively. The time required for the rod to reach 300°C is ______ seconds.
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- 413.49
- 34.49
- 43.49
- None of these
Correct Option: C
d = 0.01 m
L = 0.2 m, R = 7801 kg/m3
ti = 750 °C, ta = 100 °C
C = 473 J/Kg K, K = 43 W/mK
h = 250 W/m2K
| Lc = | = | = | |||
| A | πdl | 4 |
 |  | = exp |  |  | = exp | | | ||||
| Ti - Ta | ρVC | ρCLc |
| = exp | | | |||
| 750 - 100 | 7801 × 473 × 0.01 |
⇒ τ = 43.49
