Heat transfer occupies a field which comprises a wide range of functions, from the simple processes of objects heating and cooling to advanced thermodynamic concepts in thermal physics. In order to understand how a drink cools in the summer or how heat travels from the sun to the Earth, you must grasp these basic principles of heat transfer on a fundamental level.
Second Law of Thermodynamics
The Second Law of Thermodynamics states that heat transfers from an object of a higher temperature to that of a lower temperature. The higher energy atoms (and thus higher temperature) move toward the lower energy atoms (lower temperature) in order to maintain equilibrium (known as thermal equilibrium). Heat transfer occurs in order to maintain this principle when an object is at a different temperature from another object or its surroundings.
Heat Transfer by Conduction
When particles of matter are in direct contact, heat transfers by means of conduction. The adjacent atoms of higher energy vibrate against one another, which transfers the higher energy to the lower energy, or higher temperature to lower temperature. That is, atoms of higher intensity and higher heat will vibrate, thereby moving the electrons to areas of lower intensity and lower heat. Fluids and gases are less conductive than solids (metals are the best conductors) due to the fact that they are less dense, meaning that there is a larger distance between atoms.
Convection Heat Transfer
Convection describes heat transfer between a surface and a liquid or gas in motion. As the fluid or gas travels faster, the convective heat transfer increases. Two types of convection are natural convection and forced convection. In natural convection, fluid motion results from the hot atoms in the fluid, where the hot atoms move upwards toward the cooler atoms in the air--the fluid moves under the influence of gravity. Examples of this include the rising clouds of cigarette smoke, or heat from the hood of a car that rises upwards. In forced convection, the fluid is forced to travel over the surface by a fan or pump or some other external source.
Heat Transfer and Radiation
Radiation (not to be confused with thermal radiation) refers to the transfer of heat through empty space. This form of heat transfer occurs without an intervening medium; radiation works even in and through a perfect vacuum. For instance, energy from the sun travels through the vacuum of space before the transfer of heat warms the Earth.
Heat transfer forms an integral part of education in relevant subjects, such as in the curriculum of chemical or mechanical engineering. Manufacturing and HVAC (heating, ventilating and air cooling) are examples of industries that rely heavily on thermodynamics and principles of heat transfer. Thermal science and thermal physics are higher fields of education that deal with heat transfer.