(a)(i) Mention two modes of heat transfer other than convection. (ii) Explain land and sea breezes. (b) An iron rod of length 30cm is heated through 50 kelv...

Answer Details

(a)(i) Two modes of heat transfer other than convection: conduction and radiation.

(a)(ii) Land and sea breezes. During the day the land heats up faster than the sea (land has a lower specific heat capacity). The air over the land becomes hotter, rises, and cooler air from over the sea flows in to replace it, giving a sea breeze (from sea to land). At night the land cools faster than the sea, so the air over the sea is now warmer and rises, and cooler air from the land flows out to sea, giving a land breeze (from land to sea).

(b) Increase in length \( \Delta L = L\,\alpha\,\Delta\theta \):

\[ \Delta L = 0.30 \times 1.2\times10^{-5} \times 50 = 1.8\times10^{-4}\ \text{m} = 0.18\ \text{mm}. \]

(c) The heater delivers energy at a constant rate \(P\).

Heating stage (6 min = 360 s), taking \(c = 4200\ \text{J kg}^{-1}\text{K}^{-1}\), \(\Delta\theta = 100-40 = 60\ \text{K}\):

\[ P = \frac{mc\,\Delta\theta}{t_1} = \frac{m \times 4200 \times 60}{360} = 700m. \]

Boiling stage (25 min = 1500 s), half the water (\(m/2\)) vaporised:

\[ P\,t_2 = \tfrac{m}{2}L \;\Rightarrow\; 700m \times 1500 = \tfrac{m}{2}L. \] \[ L = \frac{2 \times 700 \times 1500 \times m}{m} = 2.1\times10^{6}\ \text{J kg}^{-1}. \]

Specific latent heat of vaporisation \(\approx 2.1\times10^{6}\ \text{J kg}^{-1}\).