Heat Transfer
Heat is energy transferred because of a temperature difference. Conduction, convection, and radiation are different physical routes for that transfer.
What makes thermal energy cross from one system to another?
A temperature difference creates the thermodynamic drive. The mechanism determines how energy crosses the boundary, but the spontaneous net direction is from the hotter region toward the colder one.
heat = transfer across boundary
After energy crosses the boundary, we account for it as part of the receiving system's internal energy rather than saying the object “contains heat.”
Choose the pathway between hot and cold.
Microscopic collisions and interactions transfer energy through matter without bulk transport of the material.
Dominant through direct material contact. Better conductors and larger cross-sectional area increase the transfer rate; greater thickness reduces it.
Requires bulk fluid motion. Warm and cool regions can circulate naturally, or fans and pumps can force the flow.
Requires no medium. Every object with nonzero absolute temperature emits electromagnetic radiation; hotter objects radiate much more strongly.