Thermal Properties of Matter | Class 11 Physics Notes

10.9 Heat Transfer

Heat transfer is the process by which thermal energy moves from one body or part of a system to another due to temperature difference. There are three modes of heat transfer: conduction, convection, and radiation. Conduction is heat transfer through a material without the movement of the material itself, occurring via molecular collisions. Metals are good conductors, gases are poor conductors. The rate of heat conduction through a bar of length L and cross-sectional area A with ends at temperatures T_C and T_D is given by H = K A (T_C - T_D)/L, where K is the thermal conductivity. Thermal conductivity varies among materials, with metals having high K and insulators like wood and glass wool having low K. Convection involves heat transfer by actual movement of fluid, either natural (due to buoyancy) or forced (using pumps). Natural convection causes phenomena like sea breeze and trade winds. Radiation is transfer of heat by electromagnetic waves and does not require a medium. All bodies emit thermal radiation depending on temperature and emissivity. Blackbodies are perfect absorbers and emitters of radiation. Wien's displacement law relates the wavelength of maximum emission to temperature. Stefan-Boltzmann law states total energy radiated per unit time is proportional to T⁴. Newton's law of cooling describes the rate of heat loss proportional to temperature difference with surroundings.

📊 Diagram: Fig. 10.13 Heating by conduction, convection and radiation; Fig. 10.14 Steady state heat flow by conduction in a bar with its two ends maintained at temperatures T_C and T_D; Table 10.6 Thermal conductivities of some material; Fig. 10.15; Fig. 10.16; Fig. 10.17 Convection cycles; Fig. 10.18: Energy emitted versus wavelength for a blackbody at different temperatures; Fig. 10.19 Curve showing cooling of hot water with time; Fig. 10.20 Verification of Newton's Law of cooling.

🧪 Activity: Verification of Newton's law of cooling using a double-walled vessel with calorimeter and thermometers to record temperature changes over time and plotting ln(T₂ - T₁) versus time.

🔗 Connection: This section concludes with Newton's law of cooling, summarizing heat transfer processes and preparing for exercises and applications.

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