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.
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.
When particles of matter are in direct contact, heat is transferred 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.
Heat transfer between a surface and a liquid or gas in motion is known as convection. As the fluid or gas travels faster, the convective heat transfer increases. Two types of convection are natural convection and forced convection. Natural convection is when 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. Forced convection is where the fluid is forced to travel over the surface by a fan or pump or some other external source.
The transfer of heat through empty space is known as radiation (not to be confused with thermal radiation). There is no medium needed in this form of heat transfer; 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 is an integral part of education, 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.