(a) Define specific latent heat of vaporization.
(b) (i) What are renewable energy sources?
(ii) List four renewable energy sources.
(c) Explain why tomatoes keep longer when kept in a moist jute bag in a clay pot.
(d) A box has a volume of 0.28 m\(^3\) and is 70% filled with iron fillings at 25°C. Calculate the:
(i) total mass of the iron fillings;
(ii) energy required to melt 10% of the iron fillings. [Density of iron = 8.00 x 10\(^3\) kgm\(^{-3}\); specific latent heat of fusion of iron = 1.38 x 10\(^5\) Jkg\(^{-1}\), specific heat capacity of iron = 460 Jkg\(^{-1}\) k\(^{-1}\), melting point of iron = 1500°C] 10.
(a) Specific latent heat of vaporisation is the quantity of heat required to change unit mass of a liquid into vapour at constant temperature (its boiling point).
(b)(i) Renewable energy sources are sources of energy that are naturally replenished and are not used up (do not run out) when used.
(b)(ii) Four renewable sources: solar energy, wind energy, hydro (water/tidal) energy, and biomass (biogas). Geothermal energy is also acceptable.
(c) The moist jute bag round a clay (earthenware) pot stays wet; water continually evaporates from its surface. Evaporation takes latent heat from the pot and its contents, so the tomatoes inside are cooled and kept below the surrounding temperature, slowing ripening and decay, so they keep longer.
(d) Volume of iron fillings \(= 70\% \times 0.28 = 0.196\ \text{m}^3\).
(i) Total mass \(= \rho V = 8.00\times10^{3} \times 0.196 = 1568\ \text{kg} \approx 1.57\times10^{3}\ \text{kg}.\)
(ii) Mass to be melted \(= 10\% \times 1568 = 156.8\ \text{kg}.\) It must first be raised from 25°C to the melting point 1500°C, then melted:
\[ Q = mc\,\Delta\theta + mL_f. \]
\[ mc\,\Delta\theta = 156.8 \times 460 \times (1500-25) = 156.8 \times 460 \times 1475 = 1.06\times10^{8}\ \text{J}. \]
\[ mL_f = 156.8 \times 1.38\times10^{5} = 2.16\times10^{7}\ \text{J}. \]
\[ Q = 1.06\times10^{8} + 2.16\times10^{7} = 1.28\times10^{8}\ \text{J}. \]
Energy required \(\approx 1.28\times10^{8}\ \text{J}\).
(a) Specific latent heat of vaporisation is the quantity of heat required to change unit mass of a liquid into vapour at constant temperature (its boiling point).
(b)(i) Renewable energy sources are sources of energy that are naturally replenished and are not used up (do not run out) when used.
(b)(ii) Four renewable sources: solar energy, wind energy, hydro (water/tidal) energy, and biomass (biogas). Geothermal energy is also acceptable.
(c) The moist jute bag round a clay (earthenware) pot stays wet; water continually evaporates from its surface. Evaporation takes latent heat from the pot and its contents, so the tomatoes inside are cooled and kept below the surrounding temperature, slowing ripening and decay, so they keep longer.
(d) Volume of iron fillings \(= 70\% \times 0.28 = 0.196\ \text{m}^3\).
(i) Total mass \(= \rho V = 8.00\times10^{3} \times 0.196 = 1568\ \text{kg} \approx 1.57\times10^{3}\ \text{kg}.\)
(ii) Mass to be melted \(= 10\% \times 1568 = 156.8\ \text{kg}.\) It must first be raised from 25°C to the melting point 1500°C, then melted:
\[ Q = mc\,\Delta\theta + mL_f. \]
\[ mc\,\Delta\theta = 156.8 \times 460 \times (1500-25) = 156.8 \times 460 \times 1475 = 1.06\times10^{8}\ \text{J}. \]
\[ mL_f = 156.8 \times 1.38\times10^{5} = 2.16\times10^{7}\ \text{J}. \]
\[ Q = 1.06\times10^{8} + 2.16\times10^{7} = 1.28\times10^{8}\ \text{J}. \]
Energy required \(\approx 1.28\times10^{8}\ \text{J}\).
