This document discusses heat transfer and its various modes. It defines heat transfer as the transmission of energy due to temperature differences. There are three main modes of heat transfer: conduction, convection, and radiation. Conduction involves the transfer of energy through direct contact of particles in a material without bulk motion. Convection involves the transfer of energy by the bulk motion of fluid, either through natural convection driven by buoyancy or forced convection by external means. Radiation involves the transmission of energy by electromagnetic waves and does not require a medium. The document provides equations to calculate the rate of heat transfer by each mode and examples of applications of heat transfer in various fields.