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Heat transfer occurs when a temperature gradient exists within a system. Thermodynamics states that energy cannot be created or destroyed, only transformed from one state to another and transferred between different media. In basic heat exchange, there are three modes of heat transfer: conduction, convection and radiation. One or more of these conditions may occur simultaneously. Following are brief descriptions, mathematical equations of these different modes of heat transfer and a sample problem.

Heat transfer by conduction 

Heat transfer by conduction occurs when a temperature gradient exists between different media across a defined boundary. This mode of heat transfer is sustained by diffusion at the atomic and molecular levels due through the interaction of particles at different energy levels. Kinetic energy (energy of motion) transfer during these collisions is perceived as changes in temperature or the flow of heat. Conduction may be observed in all states of matter (solids, liquids, and gases). Due to the mean free path or the spacing between the molecules in these three states, the extent of conductive heat transfer is most pronounced in tightly spaced solids and then liquids. Gases have the lowest level of conductive heat transfer. A pot of boiling water on an electric hot plate illustrates conduction heat transfer. Heat is conducted from the hot plate to the pot placed on it (solid to solid conduction). The heated pot in turn transfers heat to the water in contact with it (solid to liquid conduction).

Heat transfer by convection occurs by two coupled mechanisms: interaction of particles at the microscopic level and bulk motion of particles at the macroscopic level when a temperature differential exists. This mode of heat transfer is observed in liquids and gases because movement is very restricted in solids. The bulk transport of particles is the predominant

Mechanism compared to the diffusive transfer of energy which is only present close to the boundary of a solid surface in contact with the fluid.