7. The resistivity of pure silicon at 20°C is 2.5×10³ Ω·m, and the temperature coefficient of resistivity is −70×10⁻³ K⁻¹. What's the resistivity of pure silicon at 30°C? (A) 1.8×10³ Ω·m (B) −4.25×10³ Ω·m (C) −0.75×10³ Ω·m (D) 4.25×10³ Ω·m (E) 0.75×10³ Ω·m

11. As shown in Fig. 6, an electrical hot plate inside a chamber generates heat in a rate of P = 100 W. One of the chamber walls is made of copper with thermal conductivity kCu = 401 W/m·K. The other walls are fiber wool which are nearly thermal insulated with kfiber = 0.048 W/m·K. The copper wall thickness is 3 mm, and the cross-section area is 0.1 cm². If the temperature outside the chamber is TL = 20 °C, in condition of thermal equilibrium, the temperature TH inside the chamber is (A) 24.8 °C (B) 34.8 °C (C) 58.8 °C (D) 84.8 °C (E) 94.8 °C

7. The resistivity of pure silicon at 20°C is 2.5×10³ Ω·m, and the temperature coefficient of resistivity is −70×10⁻³ K⁻¹. What's the resistivity of pure silicon at 30°C?
(A) 1.8×10³ Ω·m
(B) −4.25×10³ Ω·m
(C) −0.75×10³ Ω·m
(D) 4.25×10³ Ω·m
(E) 0.75×10³ Ω·m

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